Microalgae cultivation and value-based products from wastewater: insights and applications

Abstract

Global water scarcity demands innovative wastewater treatment and nutrient recycling methods; wherein microalgae have emerged as a promising solution for efficiently treating wastewater, while yielding valuable biomass for a variety of products. In this context, current focus is based on key aspects involved, e.g. microalgal cultivation utilizing wastewater from diverse sources (industries, agriculture, aquaculture, domestic, urban wastes, etc.) and symbiotic relationships with microbes. It delves into understanding the formation of high-value products from microalgae-based wastewater and estimates the potential benefits of microalgae cultivation through quantitative data analysis. Despite its promising potential the commercialization of microalgae, is hindered by high cultivation cost. The recent emphasis is on phycoremediation of wastewater with simultaneous feedstock generation for biofuels and nutraceuticals, which redirects nutrients to microalgal biomass, offering a sustainable biorefinery approach. This review provides a novel synthesis with recent advancements in microalgae-based wastewater treatment and biorefinery processes, emphasizing new methodologies and integrated approaches that address key challenges and highlight potential for transformative impacts in sustainable resource management and bio-product development.

Graphical Abstract

Introduction

There is a shortage of water resources existing in the world due to drastic population growth and rapid economic development [266]. Therefore, bioremediation of effluents has become a critical matter of concern across the globe. Wastewater is a byproduct of a variety of domestic, municipal, industrial discharge (Soyabean, brewery, tannery, textile, dairy, molasses, starch, slaughter, etc.) commercial, agricultural and aquaculture activities [34, 61, 165]. The presence of nitrogen, phosphorus (organic and inorganic), metal ions and chemical oxygen demand in wastewater is particularly of concern from an environmental standpoint and extensive research has been directed towards their removal [30, 106]. Conventional processes have contributed to environmental and wastewater purification, having limitations (greenhouse emissions, required high-energy and high-nutrient discharge) [9]. "To create a sustainable future and support the circular economy, waste should be converted into valuable resources for recycle/ reuse, rather than simply discard/discharge into the environment. Microalgae have a wide range of applications in wastewater treatment, including the efficient removal of all types of pollutants from water (bioremediation) [53, 71]. This is in addition to their ability to fix and effectively control carbon dioxide emissions, resulting in negative carbon emissions. An effective way for uptake of nutrients from wastewater is cultivation of microalgae, which additionally produce bio-products, biofuels and co-products [10, 23, 38, 175]. However, comprehensive review is needed on the recent updates on the existing strategies being employed for microalgal cultivation and value added products/byproducts to improve the knowledge for researchers, scientist, students, related companies and other sectors.

Many studies are performed in order to develop new and modified greener approaches for production of microalgal biomass, biofuels and microalgal based high-value products utilizing wastewater. Chlorellaspp. is widely used to treat municipal wastewater at different stages, including before and after primary settling and after the activated sludge tank. Further, some of the reports demonstrated utilization of microalgae in bioremediation of toxic metals [30, 48], carbon, nitrogen, phosphorus [237] and chemical oxygen demand [177] from the waste water. Zhang et al. [263] reported no effect in the growth ofChlorella pyrenoidosa grown in waste water from the soya bean production. However, bacteria coexisting with C. pyrenoidosaimproved the biodegradation of glucose, nitrogen, phosphorus and chemical oxygen demand and reduced the total lipid content in microalgae [263]. Some microalgae symbiotically associate with aerobic and anaerobic microorganisms to recover nutrients from wastewater [16, 106, 201]. There are studies showing complicated relationships between bacteria and microalgae during co-cultivation, which probably include mutualism and antagonism [72, 204].

Wastewater-cultivated microalgae produce hydrocarbons in bulk, which can be converted into biofuels such as biogas, biodiesel, bio-hydrogen, bioethanol and bio-butanol through thermochemical and biological methods [185]. Biofuel prepared by microalgae is also known as third generation biofuel [180], whereas pyrolysis of microalgae also produce co-products such as astaxanthin, carotenoids, phycocyanin, fatty acids, lectins and polysaccharides [134]. Despite the high cost of cultivation and processing, the production of biofuels along with integrated co-products is commercially beneficial from an industrial perspective [49]. Countries like China, Germany, Japan and Taiwan yearly yield approx. 19,000 tones of dehydrated microalgal biomass which produce an annual turnover of 5.7 billion USD by the derived high value products (HVPs) [94]. Various microalgae-based products, including antioxidants, fatty acids, pigments, polysaccharides, proteins, sterols and vitamins, are gaining market consolidation due to higher demand compared to dried biomass [232]. Some recently emerging biologically active compounds such as ꞵ-glucan, fucoxanthin, lutein, phycoerythrin and some polysaccharides needs further attention for effective uses and marketing [94]. We aim to (i) structure a systematic review on wastewater's ability to support microalgae biomass and pinpoint areas that require future advancements, and (ii) highlight recent updates on popular bio-products from wastewater-based microalgal biorefinery.

Microalgae

Microalgae encompass both prokaryotic blue-green algae commonly referred to as cyanobacteria (belonging to the Kingdom Monera and Division Cyanophyta), as well as eukaryotic diatoms and other microalgae (classified under the Kingdom Protista) which are photosynthetic, oxygen evolving, non-cohesive organism with polyphyletic origin [82, 117, 202]. Microalgae includes 72,500 species and 16 classes with both prokaryotic (blue green algae or cyanobacteria) and eukaryotic organism (diatoms, euglenoids and green algae) (Kang et al. [110]). Microalgae are found in varied forms such as, microscopic unicellular and multicellular [2]. According to Norton et al. [170], there are 200,000 to several million microalgal species on Earth, with 40,000 to 60,000 identified. Out of which, about fifteen strains of microalgal species are known to be cultivated for larger scale utilization in industry. Salinity is an important factor for the growth and development of microalgae [253]. Subdivisions in microalgae can be done on the basis of their salinity tolerance,Oligohaline (salinity tolerance: 0.5-5.0 gL^-1), Mesohaline (salinity tolerance: 5.0-18.0 gL^-1) and Euryhaline (salinity tolerance: 18.0-30.0 gL^-1) [158, 160]. According to Oliveira et al. [174] marine algae can be classified as macroalgae consisting Rhodophyta (red algae), Chlorophyta (green algae), Phaeophyta (brown algae) and microalgae consisting Cyanophyta (blue-green algae), Pyrrophyta (dinoflagellates), Chrysophyta (diatoms and golden brown algae), Chlorophyta (microscopic green algae).

Microalgae have a unique chemical composition of extracellular and intracellular compounds e.g. lipids, proteins, pigments, carbohydrates, organic compounds, vitamins and minerals, etc. [35, 240]. These compounds in microalgae are responsible for the global interest and biomedical potential such as anti-allergic [62], anti-inflammatory [197], anti-bacterial [98], anti-tumoral [265], anti-viral [212], neuro-protective [63], and cardio-vascular protective [118]. Microalgae like cyanobacteria, diatoms, and green algae is considered as a new model organism for production of biofuel, biochemical, fertilizers, nutraceuticals, pharmaceutical, CO₂bio-fixation and waste bioremediation [28, 155, 194].

Only few strains are used in industry, despite the huge number of microalgae species existing in nature [170], of which ~73,000 species have been isolated and published or are undergoing the process of being given names [74]. Spirulina and Chlorella are two of these rare algal strains that dominate the global microalgae industry for food linked to health and nutrition [115]. According to Borowitzka and Vonshak [28], the primary causes of the majority of strain’s failure to reach large-scale production is due to their poor productivity, slow growth rate, lack of robustness, or lack of value added from the biomass contributing to the high cost. Another explanation is the continued reliance on conventional technology in the commercial manufacturing of items obtained from microalgae. Hence, improved sustainable technology with optimized upstream and downstream processing is also required.

Recent updates and advances on microalgal cultivation in wastewater:

Wastewater is majorly released from sources like domestic, municipal, industrial discharge (soyabean, brewery, tannery, textile. molasses, starch, slaughter, commercial, agricultural and aquaculture activities) [61, 165]. The presence of nitrogen, phosphorus, heavy metals (elements with relative density > 4.5 kg/cm³, atomic weight 63.50-200.60) and chemical oxygen demand in wastewater is concerning [46]. Microalgal cultivation in wastewater is a novel environmental friendly strategy aimed at bolstering the circular economy for building a sustainable future. Instead of being thrown away, wastes could be transformed into useful resources. Cultivating microalgae utilizing wastewater, which also yield bio-products, biofuels and co-products, is an efficient method of obtaining nutrients from waste water [38, 175, 199] (Fig. 1).

Fig. 1

Various applications of microalgal based products in industry

Microalgae are cultivated in open raceway ponds (OPRs) and closed photo -bioreactors (PBRs) [167]. Microalgal cultivation generally utilizes water, atmospheric carbon dioxide and a culture-rich medium with appropriate nutrients and salts [5, 18]. However, in many cases, the amount of water contributes significantly to the operational cost of microalgal cultivation, making the process more costly and energy-intensive. Culture conditions such as bioreactor conditioning, light, airflow, temperature, culture stirring, culture timing, pH are essential for generating the microalgal biomass [36, 78, 97]. Since, microalgae are photosynthetic organisms, increasing light intensity also increases the production ofC. vulgarisbiomass proteins, pigments and lipids in poultry wastewater [78]. Pigment development was influenced by air flow and correlated with the carbon source supplying the pigments [78]. Mixotrophic cultivation of microalgae in textile wastewater increased the biomass ofC. vulgaris, increase the fatty acid methyl ester content (FAME) and decrease the chemical oxygen demand (COD) as compared to the cultivation in photo-autotrophic and heterotrophic conditions [97].

Pretreatment of pollutants or dilution of wastewater is recommended before inoculating microalgae, as it can promote better growth of the microalgae. Cheng et al. [36], reported that pretreatment of swine wastewater using titanium oxide and intense pulse light (TiO2+IPL) increased the growth ofTribonema sp. and Synechocystissp. compared to microalgae grown without pretreatment. Javed et al. [97] reported that 50% dilution of the textile wastewater can increase the biomass ofChlorella vulgariswhich can be utilized further for biofuel production. Pan et al. [182] performed acid pretreatment for the wastewater from potato juice. Higher biomass yields ofHaematococcus pluvialis was obtained from the effluents of both pretreatments when compared to cultivation in standard culture media (control) of potato based wastewater.Under tropical conditions, Arthrospiraachieved 84-96% of NH4-N removal and 72-87% of P removal in outdoor raceway ponds. Furthermore, the removal of 82.19% COD and 60.92% Cr (VI) can be achieved in chromium-containing wastewater [139]. Markou et al. [144] described 73.18% COD removal, 91.19%, carbohydrate removal along with complete removal of phenols, phosphorus, and nitrates using olive oil mill wastewater. In terms of economy, a concentration of 25% complex wastewater is good for protein, and 50% for lipid and carbohydrate production [104]. Decolorization of colored wastewater resulted in an almost two-fold increase in growth rate and a 50% increase in biomass yield ofArthrospira.

Cultivation using carbon-rich wastewater arising from the sago starch industry improves the biomass productivity along with the crude protein, carbohydrate, and lipid content of the biomass [195] adapted a high rate algal pond system in this study and achieved 98%, 99.9% and 99.4% reduction in chemical oxygen demand, ammoniacal-nitrogen, and phosphate levels, respectively

Microalgae work in symbiotic association with both aerobic and anaerobic microorganisms, such as bacteria and fungi, to recover nutrients from wastewater. Maintaining pure microalgal cultures is very difficult when open reactors are used for mass cultivation. However, use of better quality of water, surrounding environment, and operational circumstances affect the composition of microalgal biomass [243]. Bacterial groups synthesize antibiotics, growth stimulants, micronutrients and siderophores that promote growth and protect microalgae from pathogenic microorganisms [204]. Bacteria often produce acyl-homoserine, dimethyl-L-sulfoniopropionate, lactone, nitrogen, symbiotic factors, virbioferrin, and vitamin B₁₂ (cyanocobalamine), to support microalgal growth [162]. Several microalgal genera, such as Chlorella, Scenedesmus, Chlamydomonas and indigenous microalgal consortia, are thoroughly studied for bioremediation (shown in Table 1) and biorefinery processes due to their high waste tolerance and valuable byproducts. Similar mutualistic relationships between microalgae and bacteria have been observed in previous studies (He et al. [81]; , Su et al. [238]). Bacterial synergy withChlorella sorokinianaincreased the microalgae growth, lipid content and biodiesel production from the wastewater of brewery (He et al. [81]). Removal of antibiotics from wastewater before microalgal cultivation is also suggested by Michelon et al. [150] observing significant increase in the growth of microalgae with increased biomass and carbohydrate content in antibiotic less wastewater conditions as compared to antibiotic rich medium. Gutiérrez-Casiano et al. [78] reportedC. vulgaris grown in poultry waste water as an effective means to reduce the concentrations of nitrogen and organic matter, whereas, the bacteria in poultry wastewater reduced the nitrogen and organic matter, producing CO₂, which the microalgae used as food and released dissolved oxygen. Xu et al. [261] reported that the symbiotic association ofC. vulgaris with the endophytic bacterium S395-2-Clonostachys rosea (microalgae-bacteria-fungi) performs best when mixed LED wavelengths of Red (5) and Blue (5) are used to remove CO₂ and nutrients from algae without losing methane in aquaculture wastewater. This sets the promising stage for future symbionts cultivation in wastewater treatment.

Table 1 Recent updates on microalgae used for the treatment of different wastewater

Microalgal biomass has an efficiency of up to 100% utilization of wastewater [27]. It can also grow in acidic pH (4.0–5.0) environment and simultaneously be utilized for generating the biomass for high value products through biorefinery model [123]. The above mentioned processes are also helpful in industrialization of microalgae based high value product as it leads to high CAPEX and OPEX through microalgal treatment, particularly when combining biomass production with wastewater treatment systems. To improve the likelihood of industrializing microalgal-based treatments with biomass generation, this crucial point should be addressed in future research. Padri et al. [179] reported that after entering the stationary phase of total phosphorus (TP) removal in the monoculture ofC. vulgaris TISTR 8580, the addition of Aspergillus niger, F5 pellets showed a promising result with 70% adsorption efficiency for harvesting and enhancing the nutrient removal purposes from cassava wastewater by microalgal-fungal pellet application.

Michelon et al. [150] highlighted the efficiency of microalgae in bioremediation and simultaneous removal of antibiotics like tetracycline, oxytetracycline, chlorotetracycline and doxycycline from swine wastewater. The study performed by Michelon et al. [149] onChlorella spp. opened new approach for the use of microalgae developed on wastewater against multidrug resistant pathogenic bacteria i.e. Staphylococcus hyicus and Streptococcus suis.

Recently, there has been interest in using renewable raw materials to prepare metal ions for reduction to nanoparticles (NPs) [226]. For greener synthesis of NPs, a variety of biological elements can be employed, including bacteria, actinomycetes, fungi, cyanobacteria, macro-algae, and plants. Green synthesis is preferred over chemical and physical methods due to its biocompatibility, cost-effectiveness, ease of handling and eco-friendliness. Microalgae and cyanobacteria are extensively used for the removal of heavy metals, the removal of dyes from industrial effluents, pollutant bioremediation and the synthesis of molecules of commercial significance [8]. Mansour et al. [143] recorded the potential of nanoparticles form ofArthrospira platensisin removing the pollutants from the textile dyeing industry wastewater. Mitra et al. [159] developed the strategies for upstream and downstream processing ofArthrospira biorefinery process for both high- and low-value products upon valorization.

Recent updates on value based products by microalgae grown in wastewater

According to the report of Oswald et al. [176], microalgae have been utilized for wastewater treatment for several decades. Additionally, the USDOE aquatic species programme has reported that combining wastewater treatment with microalgae-based biofuel and coproducts is a promising way to reduce the cost of producing diesel and other pharmaceuticals [228]. Utilization of wastewater - cultivated microalgae to produce a range of coproducts and bio-products has increased recently due to the growing popularity of the microalgal biotechnology and circular bio-economy [1, 33, 89]. Microalgae are a promising alternative because of their versatile biochemical composition, rapid growth rate and high tolerance to nutrients and salt stress [210]. This makes microalgae a potential feedstock for commercially important bio-based products, such as animal feed, chemicals, bioplastics, biochar, fertilizers, and biofuels [138, 188].

Microalgal biomass is a suitable feedstock for the biorefinery concept because, it can produce bioactive compounds that support a range of food, pharmaceuticals, chemicals and nutraceuticals, etc. [232]. The biorefinery concept is based on oil refineries, which can convert biomass into a range of high-value products [37, 64]. Numerous fields, including nutraceuticals (polyunsaturated fatty acids (PUFA), carotenoids, vitamins, phytosterols, or polyphenols), can benefit from the by-products produced [129, 147]. For increasing the production of value based products and attain circular economy, strategy for cultivation of microalgae in wastewater is now widely used. However, complete use of biotechnological and molecular tools can also increase the production of value based products and thus, further study in these fields is suggested. Numerous genetic components and engineering tools, including promoters, gene vectors, selection markers and gene editors, have been made available for the modification of metabolic pathways in microalgae by the ongoing advancements in metabolic engineering and synthetic biology [73]. The swift advancement of CRISPR gene editing and Omics technologies has presented unparalleled prospects for the creation of modified microalgae strains exhibiting elevated pigment production [99].

Value based products were initially extracted from various sources such as animal, vegetables, plants or artificially synthesized [76]. The synthetic biomolecules are less effective and have many side effects [113]. Microalgae-based byproducts have a wide range of biological activities and industrial applications [45]. The primary and secondary metabolites of microalgae can be used as a feedstock for a variety of bio-products [245]. Proteins can be transformed into biopolymers, such as bioplastics [59], carbohydrates can serve as prospective sources of bio-hydrogen (bio-H₂). Lipids can also be converted into high-value polyunsaturated fatty acids (PUFA), such omega-3 [248] and pigments which contain high concentrations of important carotenoids [11]. Industries have recently made investments in value based products (proteins, fatty acids, astaxanthin, phycocyanins, β-carotene, lipids, eicosapentaenoic acid (EPA), docosa-hexanoic acid (DHA) by microalgae cultivated in wastewater is turning waste into wealth [25].

Pigments- Phycobiliproteins, Carotenoids, etc.

Primary photosynthetic pigments extracted from microalgae are phycobilins, carotenoids, and chlorophylls [109, 175]. Phycobilins and carotenoids are described below:

Phycobiliproteins

Are highly fluorescent and soluble in water. They have specific cysteine residues linked to a linear prosthetic group (bilins) [100]. These additional pigments are unique to red algae, cyanobacteria and cryptomonads, such as Porphyridium, Nostoc and Arthrospira. A significant function of phycobiliproteins is pigmentat metabolism in microalgae; they also exhibit anti-aging, anti-carcinogenic, anti-inflammatory, anti-oxidative, hepato-protective, and neuro-protective properties [250]. As natural colourant, phycobiliproteins have applications in industries especially in food, cosmetic, pharmaceutical and textile sector, biomedical research and clinical diagnostics [254]. Leftover biomass from the phycobiliprotein production processes are converted into biofertilizer, which is used to produce biogas and recover bioenergy. Arashiro et al. [13] reported the biomass ofNostoc, Phormidium, and Geitlerinemain municipal wastewater, and extracted pigments like phycocyanin, phycoerythrin from microalgae. Combining the extraction of pigments and generation of biogas from residual biomass would result in approximately 5–10% more energy recovery and circular economy [13]. Based on chromophores and composition, phycobiliproteins can be categorized as allophycocyanins (λmax = 650–660 nm), phycoerythrins (λmax = 490–570 nm), and phycocyanins (λmax = 610–625 nm) [69, 137] (Fig. 3).

Phycocyanin

Is a water-soluble protein extracted mainly from red algae and cyanobacteria [95]. Khan et al. [112] reported the presence of phycocyanin and phycoerythrin from newly reported cyanobacteriumTrichocholeus desertorum. Similarly, other robust species should be explored for the presence of value based products and as feedstock for biorefinery. A complex mixture of monomers, trimers, hexamers and other oligomers makes phycocyanins [151, 190]. Applications of phycocyanins are limited by its low stability to light exposure, high pH and elevated temperatures [172]. Stability has been improved and properties have been preserved through the use of additives such as glucose or citric acid [22, 155, 163]. Mastropetros et al. [146] recorded the high growth of cyanobacteriumPhormidium spp. in low organic effluent medium with high phycocyanin production. Global market of phycocyanin is projected to grow at year 2027 to a value of 245.5 million dollars [14]. Arthrospira platensis – Spirulina- single cell protein (SCP), is the primary source of the industrialized phycocyanin products, which are available in powder or aqueous form for cosmetics, food and analytical applications. As synthetic colors have many adverse effects, there is an increasing demand for natural colors which are applicable in confectionaries, beverages, drugs, molecular markers, cosmetics, etc. It is also reported to have therapeutic values like immunomodulating activity, anti-oxidant activity, anti-nephrolith activity, anti-cancer activity, etc. [181, 190, 191]. Owing to its fluorescence properties the development of phycofluor probes for immunodiagnostics as a substitute for synthetic dyes was explored Paswan et al. [189]. The fluorescence quenching ability of C-PC with heavy metals can be explored as a selective and sensitive biosensor for heavy metals like Hg⁺²even in low concentrations [24]. C-PC could also protect against ethanol-induced sub-acute liver injury and improve immunity in mice [260]. Phycocyanin belongs to the family of phycobiliproteins, which also contains two other proteins, allophycocyanin and phycoerythrin (Fig. 3).

Phycoerythrin (PE),

Composed of α, β, and γ subunits, is a primary phycobiliprotein found in Porphyridiumspecies [125,126,127]. With a molecular weight of 263 kDa, PE holds significant economic value and has a wide range of industrial applications. It has garnered considerable attention from the scientific community. However, mechanical processes such as centrifugation and filtration can compromise the integrity of phycobilisomes during harvesting, causing the release of phycobiliproteins into the media and leading to resource loss [50]. PE is found in waste media, but in very small amounts. Balaraman et al. [20] concluded slaughter wastewater is effective in the cultivation of microalgaePorphyrium cruentiumwith large quantity of phycoerythrin. Liang et al. [128] conducted an efficiency recovery of PE by a chitosan based flocculation technique from wastewater with low concentration of phycobilin. Ammonium sulphate precipitation, ultrafiltration, chromatography and aqueous two-phase systems are used in the extraction of phycobiliproteins from the phycobilisome solution [103]. All these methods are applicable to high concentration of phycobilin in wastewater [128]. Recovery method for phycoerythrin is suggested and should be developed from a low concentration of phycobilin in wastewater (Fig. 2).

Fig. 2

Structures of different phycobiliproteins

Carotenoids

Carotenoids have a stable molecular structure and a 40-carbon polyene chain [213]. Critical chemical characteristics like light absorption, which is required for photosynthesis and oxygen-dependent life, are produced by this structure. A carotenoid may contain oxygenate groups or cyclone groups, which are rings [213]. Carotene is the general term for hydrocarbon carotenoids,however, xanthophyll is the specific term for oxygen derivatives like lutein (which is OH-functional) and canthaxanthin (which is oxy-functional, a keto-carotenoid) [217]. Secondary carotenoids are hydrocarbon carotenoids, also referred to as carotenes, and these carotenoids are named after the primary carotenoids. Primary carotenoids are located on thylakoid membranes. The key distinction between the two is that the microalgae contains photosynthetic apparatus which is primarily made up of structural and functional components, and the microalgae depend on the primary carotenoids to survive. But, the microalgae only produce large amounts of secondary carotenoids when they are subjected to a carotenoid breeding process, involving specific stimulant conditions [132]. There are currently 600 known varieties of carotenoids, 50 of which are found in the diets of humans [208]. Astaxanthin, ß-carotene, lutein, lycopene, zeaxanthin, violaxanthin, and fucoxanthin are the principal carotenoids found in microalgae. Among all, capsanthin, astaxanthin, ß-carotene, fucoxanthin, lutein, lycopene, canthaxanthin, and zeaxanthin are the main carotenoids of market interest [214]. The production of carotenoids by microalgae has several health benefits (Novoveská et al. [171]). Despite the apparent benefits, there is lack of large-scale biomanufacturing for microalgal based carotenoids production. According to Barkia et al. [21], there was a rise in the demand for carotenoids from 1.24 billion USD in 2016 to approximately 1.53 billion USD in 2021. Different types of stress-induced carotenoids in microalgae is described below (Table2).

Table 2 Stress induced carotenoids in microalgae (Adapted from Chokshi et al. [41])

ß-carotene,

A class of fat-soluble tarpon pigments (tarpon: hydrocarbons in general, formula C₁₀H₁₆), is a type of secondary carotenoid that is generally found in plants, microalgae, and photosynthetic bacteria in a range of colors from yellow to red. The principal sources of ꞵ-carotene are Chlamydomonas, Chlorella, Dunaliella, Muriellopsis, and Haematococcusspecies [19, 246]. Typically, microalgae contains mostly all-trans and 9-cisisomers of ß-carotene [80]. ꞵ-carotene is regarded as a precursor to vitamin A (retinol) and an orange pigment. The most important applications of ꞵ-carotene pigments are as antioxidants, edible colors in foods, cosmetics, and cancer therapy [255, 257]. Han et al. [79] extractedß -carotene from Dunaliella spp. grown in poultry litter wastewater. Symbiotic microalgae and yeast (Chlorella sorokiniana and Rhodotorula gracilis) were used as feedstock for production of polyhydroxybutyrate and ß-carotene from dairy wastewater [120] (Fig. 3).

Fig. 3

Biorefinery model for microalgal high value products

Astaxanthin

Is a naturally occurring keto-carotenoid known for its exceptional antioxidant properties, which are approximately 1000 times greater than vitamin E (ɑ-tocopherol) and 10 times greater than other microalgal carotenoids, such as canthaxanthin, β-carotene, lutein and zeaxanthin [192]. This pigment is widely distributed in aquatic species like crabs, salmon and prawns [125,126,127]. Astaxanthin is valued for its anti-oxidative, anti-aging, anti-inflammatory, and anti-atherosclerotic properties and is commonly used as an active ingredient in pharmaceutical products for human consumption (Mokhtari et al. [161]). The annual market value for natural astaxanthin has surpassed US$550 million and continues to rise [233]. The two main producers of astaxanthin areChlorella zofingiensis and Haematococcus pluvialis, while smaller natural producers are Tetraselmis sp., Scenedesmus sp., and Chlorella sorokiniana [134]. Haematococcus pluvialis and Chromochloris zofingiensiswere grown in palm oil mill effluent (POME) to produce astaxanthin and perform altogether phycoremediation [60]. Cheirslip et al. [31] reported improved astaxanthin and co-bioproduct from Haematococcus spp. microalgae combined with industrial wastewater valorization using a two-stage LED lighting scheme.

Fucoxanthin

Is a type of xanthophyll pigment commonly found in brown algae and diatoms. South-east Asia and European nations consume a variety of edible brown algae, such as Sargassam fusiforme, Laminaria japonica, Undaria pinnatifida, and Padina tetrastromatica [256]. While diatoms such asPhaeodactylum tricornutumare preferred sources of fucoxanthin in the food industry due to their higher content, extraction efficiency, and shorter growth cycles compared to macroalgae, brown algae remain valuable sources of this nutrient [196]. Reportedly, fucoxanthin possesses anti-oxidant, anti-diabetic, anti-inflammatory, anti-cancer, and anti-obesity properties. Jiang et al. [105] reportedPhaeodactylum tricornutuma diatom removing toxic nutrients from the diluted swine wastewater yielded high amount of fucoxanthin. Numerous studies have indicated that fucoxanthin consumption can support various essential biological functions and exert therapeutic effects on health conditions. Research has demonstrated that fucoxanthin may aid in the management of several chronic medical conditions, including cardiovascular disease, type -2 diabetes, hyper-cholesterolemia, hypertension, obesity, osteoporosis, metabolic syndrome, hepatic disorders, cancer, as well as promoting ocular and skeletal health. This compound functions as an antioxidant and scavenger of reactive oxygen species (ROS), addresses inflammation-related disorders, and exhibits antibacterial properties [17]. As of the latest data, the price for pure fucoxanthin varies between USD 40,000 and 80,000 per kilogram, contingent upon factors such as quality and specific criteria [107]. In 2020, the global market for fucoxanthin was estimated to be approximately USD 600 million and is projected to expand at a compound annual growth rate (CAGR) of 6% from 2018 to 2025 [70]. The initial commercial report on microalgal fucoxanthin production emerged in 2018 from Algatechnologies Ltd., an Israeli microalgal biotechnology firm. The company introduced a patented natural oleoresin containing 3% fucoxanthin, marketed under the trademark Fucovital®. This product is derived fromPhaeodactylum tricornutum, cultivated in a fully controlled, closed tubular photobioreactor exposed to sunlight [6].

Lipid content in microalgae grown in different types of wastewaters:

One of the main biochemical constituents of microalgae is lipid. There are reports by various researchers suggesting that, under different stress scenarios, microalgae can accumulate about 60% of TAG, thus providing a viable source for its conversion to biodiesel [183]. The accumulation of lipid in microalgae is often enhanced by culture instances such as nutrient shortage or salinity stress [42, 43, 184], temperature changes [44], or light intensity [67]. The use of diverse wastewater types as a source of organic and inorganic nutrients is a viable strategy for the sustainable production of microalgal biomass. According to a research by Chinnasamy et al. [39, 40], the generation of microalgal biodiesel derived from wastewater can reach 0.40–0.78 t/ha/year. The primary cause of the decreased biodiesel output is the microalgae's reduced lipid content, which can be enhanced by growing the algae in wastewater that contains more organic ingredients.

The lipid content of different microalgae cultured in different wastewaters is displayed in Table 3. According to Ledda et al. [122], in tubular raceway ponds, microalgae Nannochloropsis gaditana when cultured in centrate from wastewater treatment yielded biomass of 0.1 gL^-1. However, microalgal development is inhibited and their production of biomass and chlorophyll content is reduced at greater concentrations of centrate (above 30%). This shows that for a larger biomass output, selection and the dilution or pre-treatment of wastewater prior to microalgae growing are also significant. Another study conducted by Gupta et al. [77] showed that when Chlorella sorokiniana is used to treat raw sewage, it has a strong ability to remove nutrients and create lipid content of about 22% w/w of biomass, in comparison to microalgae Scenedesmus obliquus. Nonetheless, Ji et al. [102]'s study revealed that Scenedesmus obliquus can use the nutrients in food wastewater to extract 38.9 mgL^-1TN and 12.1 mgL^-1TP from 1% of food wastewater in addition to accumulating more lipid and carbohydrates in the cells, which can then be utilized to produce biodiesel and bioethanol. This implies that different types of microalgae may react differently or that the origin of the microalgae may have a major impact on the remediation of wastewater and simultaneously yield some potential high value products. Cyanobacterial lipids (especially GLA) are also known to possess several bioactivities (like antiradical and antimicrobial activities) owing to the essential fatty acids (mainly GLA) rich profile [206]. The most utilized source of γ-linolenic acid is borage oil, but cyanobacteria too have, GLA inS. platensisranged 18– 21% (w/w), of the total fatty acids [221]. The γ-linolenic acid is present in someSpirulina/Arthrospira strains which are with distinguishable characters among different genera [164].

Table 3 Lipid content of various microalgae grown in various wastewaters (Adapted from Pancha et al. [185])

Sometimes wastewater needs to be pre-treated or diluted before being used as a growing medium for microalgae since, a high organic or ammonia load may hinder the growth of the microalgae. Zhu et al. ([267]) demonstrated unequivocally that microalgae Chlorella zofingiensis uses piggery wastewater to make 30 mgL^-1 of biodiesel per day; nevertheless, this strain exhibits strong growth and nutrient removal efficiency when the wastewater's COD reaches 1900 mgL^-1. Microalgae typically collect a considerable amount of lipid during the nutrient shortage phase, as previously documented. Hemalatha and Mohan [83] reported on an interesting two-stage wastewater treatment method in which they cultured microalgae in a mixotrophic ecosystem utilizing pharmaceutical wastewater to enhance biomass production. The culture was then moved to tap water to increase lipid accumulation.

In conclusion, we can state that the dual role of phycoremediation in conjunction with the production of microalgal biodiesel along with co-products is one of the promising approaches; however, the choice of microalgae, the method of cultivation, and the wastewater pre-treatment are crucial factors that need to be taken into account prior to the full-scale operation.

Fatty acids

Have a hydrocarbon chain that has a methyl group (-CH₃) at one end and a carboxyl group (-COOH) [142]. Different families of fatty acids can be identified based on the double bond in relation to the methyl group: omega-3, omega-5, omega-6, omega-7, omega-9, etc. [15]. At the generic level, fatty acids are used as biomarker to differentiate closely related microalgal species [220] in Phytochemistry reported fattyacids as biological marker for identifying the microalgae Microalgae are primary producers of the food chain and hold prominence towards pharmaceutical and nutraceutical applications. Fatty acids (FAs) are one of the primary metabolites of microalgae, which enrich their utility both in the form of food and fuels. Additionally, the vast structural diversity coupled with taxonomic specificity makes these FAs as potential biomarkers. The determination of lipid and fatty acid profiling of 12 different strains of microalgae has been accomplished in this study and further discussed in respect to their chemotaxonomic perspective in microalgae. Palmitic acid (C16:0) and oleic acid (C18:1n9c) were found to be dominant among the members of Cyanophyceae whereas members of Chlorophyceae were rich in palmitic acid (C16:0), oleic acid (C18:1n9c) and linoleic acid (C18:2n6) [154]. Nutritional value of microalgae is primarily derived from essential fatty acids in microalgae [116]. Among value-added products produced by microalgae, polyunsaturated fatty acids (PUFA) or long-chain fatty acids is well known for their beneficial impact on human health. Distribution of fatty acids in the two strainsS. maxima and S. platensis have been reported [154]. Spirulina maxima (% of total fatty acids) Spirulina platensis (% of total fatty acids) palmitic (16:0) 63 25.8 palmitoleic (16:1 omega-6) 2 3.8 stearic (18:0) 1 1.7 oleic (18:1 omega-6) 4 16.6 linoleic (18:2 omega-6) 9 12 gamma-linolenic (18:3 omega-6) 13 40.1 alpha-linolenic (18:3 omega-3) traces gamma-linolenic acid represents only 10-20% of fatty acids in S. maxima, i.e. 1-2% of dry matter [91, 154], compared to 40% inS. platensis, or some 4% of dry weight. Thus, Spirulina can be considered one of the best-known sources of gamma-linolenic acid, after human milk and some vegetable oils having high nutrient value.

DHA and EPA

Through the trophic levels in marine food web, species of microalgae produce two nutritionally significant PUFAs namely docosahexanoic acid (DHA) and eicosapentaenoic acid (EPA) [209]. EPA and DHA supplements lowers risk of inflammation and prevent cardiovascular disease [47]. DHA is essential for protecting against damage, lowering inflammation and preserving the fluidity of the brain and membrane of retina [216]. Recent improvements in the cost-effectiveness of producing PUFAs have led many producers to focus more on PUFA production. Tossavainen et al. [249] performed the biomass growth of mixed algae namelyEuglena gracilis with Selenastrum in aquaculture wastewater and observed the high amount of DHA and EPA and concluded microalgae enriched with PUFA (DHA and EPA) which can be used in replacement of fish oil as feed. Aurantiochytriumsp. was cultivated for phycoremediation of food based wastewater and production of PUFA for fish aquaculture [92]. Green microalgae,Scenedesmus sp.was used for phycoremediation of wastewater and large amount of fatty acids [251]. High production of both saturated and unsaturated fatty acids suggests a viable potential for biodiesel production.

Sterols

Derived from microalgae are recognized to possess beneficial bioactive properties, including the ability to lower cholesterol, reduce inflammation and potentially inhibit cancer growth. By 2022, the phytosterol market is predicted to generate USD$935 million in sales worldwide [207]. Notably, β-sitosterol, campesterol, brassicasterol, stigmasterol, and ergosterol are commercially significant phytosterols. Currently, phytosterols are primarily sourced from vegetable and tall oils extracted from land plants. By 2030 and beyond, it is expected that the land-based sources will be unable to keep up with the growing demand. Safafar et al. [218] cultivated six microalgae from various classesNannochloropsis sp. (Eustigmatophyceae), Desmodesmus sp. (Chlorophyta), Chlorella sp., and Dunaliella sp. (Bacillariophyceae) for phycoremediation of industrial wastewater and production of value based products. Desmodesmus cultivated well in industrial wastewater and produced large amount of tocopherol, carotenoids and phenolic compounds. Mitra et al, [157] reported EPA production through Nannochloropsis oceanica grown in various waste waters.

The eicosapentaenoic acid rich marine eustigmatophyte Nannochloropsis oceanica was grown in wastewaters sampled from four different industries (i.e. pesticides industry, pharmaceutical industry, activated sludge treatment plant of municipality sewage and petroleum (oil) industry). Under the wastewater based growth conditions used in this study, the biomass productivity ranged from 21.78 ± 0.87 to 27.78 ± 0.22 mg L⁻¹ d⁻¹ in relation to freeze dried biomass, while the lipid productivity varied between 5.59 ± 0.02 and 6.81 ± 0.04 mg L⁻¹ d⁻¹. Although comparatively higher biomass, lipid and EPA productivity was observed in Conway medium, the %EPA content was similarly observed in pesticides industry and municipal effluents.

Polyhydroxyalkanoates

(PHAs) are biopolyesters synthesized by microorganisms under nutrient-limited conditions [54, 153]. Microalgae are easily obtainable living fossil and are expected to be a major source of PHAs ( biodegradable), with low solvency resistance, high purity within cells, resistant to UV light, and possess both thermoplastic and elastomeric qualities [203]. They extracted PHAs from the Chlorella vulgaris. The physicochemical characteristics and biodegradability of PHAs represents one of the most promising prospective alternatives to traditional non-biodegradable plastics [241]. Four different nordic microalgal species were grown in wastewater, as an inexpensive and high-carbohydrate feedstock for microbial fermentation that yields polyhydroxybutyrate (PHBs) [148]. The most PHBs was produced using the hydrolysate ofDesmodesmus sp., followed by Chlorococcum sp. [148]. Pyruvate supplementation, combined with mixed microbial cultures (MMC) and organic wastewater, has enhanced the production, extraction, and characterization of PHAs from swine wastewater [229]. Despite extensive efforts to overcome production barriers by using low-cost, renewable substrates and continuously advancing sustainable biopolymer production [215], only a few companies currently produce PHAs commercially The exploitation ofArthrospirasp. for polyhydroxy butyrate (PHB) accumulation was reported by [231]. A study conducted onA. platensissuggested that high internal phosphate might restrict NADPH accumulation along with PHB biosynthesis even after 30 days of phosphate limitation [186]. In one of the studies,Spirulina-based bioplastics indicated better blending performance which could positively commercialize the product suitable for various applications [262]. Polyhydroxyalkanoates have wide applications in various sectors from household to medical areas like tissue engineering [223].

Carbohydrate

A necessary substance, carbohydrates can be found in the microalgal cell at varying levels as starch and cellulose [88]. Depending on the species, the amount of carbohydrates in microalgal biomass ranges from 10 to 52% [88]. Tan et al. [242] investigated biomass and carbohydrate productivity through the growth ofScenedesmus parvus (S. parvus) both indoors and outdoors. According to the results, in extremely acidic circumstances, S. parvus produced 12640 ± 1.0 mg L^1 biomass and 15.47 ± 0.048 mg L^1d~1. Additionally, the carbohydrates (270 mg g^-1 biomass) from the biomass of Chlorella vulgaris (C. vulgaris) were recovered by Alavijeh et al. [7]. Nzayisenga et al. [173] cultivatedChlorella sp. heterotrophically and mixotrophically to produce carbohydrates, using glycerol, glucose, and wastewater from municipalities as a source of nutrients or a medium for growth. Nevertheless, certain algae's low carbohydrate content raises the price of value products and microalgae-based bio-refineries. The carbohydrate content of Arthrospira constitute approximately 15% of the dry matter, majorly polysaccharides. Although, Arthrospirausually contain 10 – 20 % carbohydrate, however under some stress conditions it accumulates an ample amount of carbohydrates which makes them potent bioethanol feedstock for future applications [145]. The nutrient stress exerted by cultivating using wastewater nutrients and stress caused by CO2 fixation also resulted in the accumulation of carbohydrates in theA. platensis [222].

Proteins

Protein is made from several kinds of amino acids, which are produced by microalgae. Tetraselmis chuii (T. chuii), B. braunii, and Porphyridium aerugineum (P. aerugineum) are microalgae cells that contain tryptophan, lysine, leucine, and arginine; Nannochloropsis granulate(N. granulate) has leucine in its cells [247].

According to Nehme et al. [168], the pharmaceutical, health, and nutrition sectors all heavily rely on critical amino acids for their products. Furthermore, it may be used as dietary supplements and has applications in pigmentation, reproduction, and the control of metabolic activity in both adults and new-borns [259]. Numerous investigations revealed that MSAs are present in the cells ofChlamydomonas nivalis (C. nivalis), C. sorokiniana, and Desmodesmussp. [68]. As a result, microalgae biomass must be taken into account as a rich source of protein that may be used as nutrient supplement in the food and feed, pharmaceutical, and nutritional industries. Certain microalgae strains may have their protein and amino acid production increased by adjusting their metabolic pathways to up-regulate the relevant genes.Arthrospiracontains 70% protein of its total dry weight (mainly composed of Phycocyanin) [190]. Due to the absence of cellulosic walls, the protein ofArthrospira shows very high digestibility (83-90% as compared to 95.1% for pure casein). The use of such biomass has gained attention both as an alternative source of alimentary protein and as a collusive in dietary treatment requiring a reduced caloric intake. Spirulina is a well known source of protein which can be used in functional foods and grown in waste waters sequestering various toxic metals [154].

Vitamins

Arthrospira is claimed to be a rich source of vitamins, especially vitamin B₁₂and vitamin- E [227]. It has been reported that hydrogen peroxide stress induced inA. platensisleads to a 4-fold increase in Vitamin C and an 8-fold increase in α- tocopherol [1]. Many cyanobacteria are reported to produce vitamins that plants do not prepare such as Vitamin B12 [76]. The biomass ofArthrospirawas also found to be rich in pro-vitamin A, B1, B2, B6, D, and E [87]. In a study conducted on Chinese adults, it was found that vitamin A equivalence ofArthrospira β-carotene improves the vitamin A content in the human body upon consumption [244]. Thus, making a ground for its future exploitation in several vitamin deficiency diseases. Spirulina is rich in a wide range of vitamins and minerals essential for maintaining a healthy immune system, like, B6, E, and C. Spirulina is reported also to boost the production of white blood cells and antibodies that fight viruses and bacteria in our body. Dry Spirulinacontains 50-190mg/kg of Vitamin E [55], a level comparable to that of wheat germ, and daily requirements of Vitamin E is estimated at 15 IU (29) or 12mg of free tocopherols.

Enzymes

Plasmids and chloroplasts, which are photosynthetic organelles, were principally obtained by the endosymbiotic absorption of a cyanobacterium. Following rounds of secondary and even tertiary endosymbiosis, these initial plastids (found in chlorophytes and rhodophytes) were subsequently transferred to other eukaryotic clades (BBC Research, Report). Microorganism-derived enzymes have the potential to serve as catalysts in a variety of industrial processes. There is a continuous need for sustainable solutions in the hunt for novel sources of microbial enzymes. Several findings demonstrate the high capacity of microalgal cells to synthesize enzymes, despite the fact that microalgae are not currently used in the commercial manufacturing of enzymes [198]. There have been reports of other groups of enzymes, such as lyases, oxidoreductases, and hydrolases. Holocellulases are a group of enzymes that break down the carbohydrate polymers found in the cell walls of plants and algae. They include cellulases, hemicellulases, and pectinases. It has been discovered that strains of Anabaena may be capable of producing hydrolytic enzymes with fungicidal potential. The marine carbohydrase, or algal polysaccharidases, as classified by CAZymes, include agarases, carrageenases, and alginate lyases [124]. These enzymes have distinct structural and biochemical characteristics and are not closely connected to the glycoside hydrolases that are now understood. α-amylase activity in Chlamydomonas reinhardtii growing photo-autotrophically in 12-h/12-h light/dark cycles was reported by Levis and Gibbs, [124]. After four hours of darkness, amylase attained its peak activity and it continued to be active throughout the whole cell cycle [124]. It has been reported that the unicellular golden-brown algae Pothierochromonas malhamensis which has an osmotic control mechanism, contains intracellular α-galactosidase (α-gal) activity. In cell-free extracts, P. malhamensis α-gal has maximum activity at pH 7.0. There is an increase in intracellular enzyme activity when the external osmotic pressure is high. Within a few minutes this situation results in cell shrinkage (plasmolysis). The cell produces intracellular isofloridoside (α-galactosylglycerol) via biosynthesis as a recuperation mechanism. In response to increased external osmotic pressure, α-galactosidase is produced, which in turn lowers the intracellular concentration of isofloridoside (O-α-D-galactopyranosyl-(1 → 1) glycerol) and releases galactose and glycerol. A portion of the carbon released is integrated into the algal cell's store of β-glucans or as wall components [52]. A calcium-dependent serine protease from the cyanobacterium Anabaena variabilis was described by Lockau et al. [133]. This protease, which resembles trypsin, requires calcium for full activity. Strohmeier et al. [236] demonstrated that A. variabilis possesses a second soluble trypsin-like protease, a prolylendopeptidase, by using the A. variabilis mutant IM 141, which is devoid of this calcium-dependent protease [133, 236]. Compared to lipases found in bacteria, fungi, animals and plants, relatively little research has been done on microalgal lipases or the genes producing them. Some researchers have identified and described lipase from the photosynthetic cyanobacterium Arthrospira platensis [51]. The intracellular proportion of the following four species of thermotolerant blue-green algae that were isolated in Thailand showed phytase activity from Synechococcus SKP50, S. lividus DSK74, Chroococcidiopsis thermalis Gehler, and Synechococcus bigranulatus Skuja. The highest recorded enzyme activity of 1.83 mU mL⁻¹ was attained by S. lividus SKP50. There was no phytotase activity in the extracellular culture broth. Furthermore, Tethacystis aeria and Shewanella xiamenensis, two soil microalgae, have been found to exhibit laccase activity on the substrate 2′-azino-bis-3-ethylbenzthiazoline-6-sulfonic acid (ABTS) [169, 178]. Although prokaryotic and eukaryotic cells have SODs, not much is known about the action of SODs in microalgae. A Mn-SOD from the microalgae Porphyridium cruentum was isolated by Misra and Fridovich, [156]. The enzyme was stable when frozen and thawed, with a molecular weight of 40 kDa [96].

Biofertilizers and biostimulants

Since three decades, farmers have been using chemical fertilizers, but repeated and excessive use of these fertilizers gradually deteriorated soil quality resulting in progressively diminishing crop yield [131]. Various agricultural land has drastically become barren and unproductive which could be recovered through biofertilizers/manures. The role of microalgae in the soil ecosystem is neglected, though they contribute to soil health and benefit crops in many ways. The potential benefits of cyanobacteria inrice fields are the degradation of pesticides employed in the crop [239], soil amelioration, soil conditioning and releasing inhibitors of plant pathogens [111], and production of plant growth-promoting substance.Recently microalgae have been used in agriculture as natural biofertilizers to enhance soil properties because of their inherent high concentrations of macro- and micronutrients that are essential for plant growth, health, and development [75]. Microalgal biofertilizers improve nutrient absorption in plants with greater development and agricultural yields, according to recent studies [211]. According to Abinandan et al. [3], biofertilizers are an economical, sustainable, organic substitute for synthetic fertilisers that also pose minimal environmental impact. Furthermore, an increasing body of research indicates that microalgae can enhance soil health, lessen erosion, aid in crust formation, treat agricultural runoff, remove metals from the soil, and aid in nitrogen recovery [29]. As secondary products, biofertilizers range in price from USD 300 to USD 1200 per tonne,the industry was estimated to be worth USD 2.3 billion in 2020 (Biofertilizers industry, [26]. The prediction indicates that throughout the next ten years, the worldwide market for biofertilizers would develop at a faster rate due to a number of initiatives and advantageous rules enacted by governmental bodies [6]. The effect of algalization on paddy yield and its nutrition were analyzed by us. The application of the mat in conjunction with different doses of fertilizer was performed in the paddy fields of Andhra Pradesh, India.Leptolyngbya boryana mat acted as a promising metal sequester and demonstrated an elicited photosynthetic activity (2-fold increase in Chl-a), carbohydrate (2-fold increase), and lipid (1.7-fold increase) content (% w/w) after 2-month cultivation. It also displayed proficient metal (ppm) uptake efficiency in the order Ca > Fe > Mg > K > Na > Mn > Zn > Cr. In grains, higher nutrient accumulation for Fe (~2-fold), Mg (~1.5-fold), and Ca (~5-fold), and a significantly higher translocation factor (Tf) in the order Ca > K > Mg were recorded. In essence, algalization substantially improved the overall nutritive quality of the rice grains, especially their iron content [205].

Recommendations and suggestions

• 1. Bioremediation Integration: Microalgae will be key players in comprehensive bioremediation, tackling pollutants in various sources such as domestic and industrial wastewaters alongside biomass production with high value products. Their role extends beyond biomass; efficient removal of contaminants, contributing to cleaner environments with sustainable practices.

• 2. Biorefinery Expansion: The significance of microalgae in biorefinery will definitely expand, serving as feedstock for diverse high-value products like biofuels, pharmaceuticals and cosmetic ingredients. Beyond lipids and proteins, it drives the production of a wide array of valuable compounds, enhancing the versatility of biorefinery concepts.

• 3. Circular Bio-economy Implementation: Microalgal cultivation will integrate into circular bioeconomy models, emphasizing resource recycling and reuse within closed-loop systems. This integration will not only enhance sustainability but, also minimize waste generation and environmental impact, promoting eco-friendly practices.

• 4. Synergistic Microalgae Systems: The future holds the development of synergistic microalgae systems, co-cultured with bacteria or fungi, enhancing productivity, nutrient production and nutrient cycling. These integrated systems will optimize resource utilization, maximizing biomass and high-value product production while bolstering system resilience.

• 5. Advanced Genetic Engineering: Continued advancements in genetic engineering, like CRISPR, will lead to the creation of genetically modified microalgae strains with enhanced traits i.e. 4^th generation energy feedstock. These strains will be tailored for specific applications, including pigment production, bioremediation, and biorefinery processes, revolutionizing sustainable biotechnologies.

• 6. The envisaged future scope is dependent on metabolic and genetic engineering. The majority of the products have been produced from only a handful of the genera. Moreover, with the onset of various OMICS approaches, they have become attractive candidates for improved secondary metabolites, the intervention of synthetic biology toolssuch as gene editing, metabolic engineering, and a variety of metabolomics/ fluxomics, can be used to have significantly enhanced production of desired high-value secondary metabolites. The uptake rate of technologies being developed is too slow due to general unawareness of the benefits of the algal products.

Conclusion

There is an urgency of addressing water scarcity and means of combating the critical issue through innovative wastewater treatment approaches. Microalgae-based solutions present a promising avenue for both remediation and biomass production for multiple high value products integrated in a biorefinery approach i.e. ‘ a win win situation’ of waste management with the microalgal feedstock generation having lot of potential benefits. Moving forward with focus on refining cultivation techniques and optimizing biorefinery processes to enhance efficiency and reduce costs. Collaborative efforts between researchers, industries, and policymakers will be crucial in driving the implementation of microalgae-based wastewater treatment on a larger scale. By investing in research and development, we can unlock the full potential of microalgae and pave the way for a sustainable future for water, health, environment and energy.