Microalgae impart growth-promoting properties in three different modes, namely as biofertilizers, biostimulants, and biopesticides. These properties could be attributed to the presence of a variety of biomolecules such as soluble AAs, phenolic compounds, phytohormone-like compounds, terpenoids, and polysaccharides (Lee and Ryu, 2021). The most common mode of utilization of microalgae biomass is biofertilizers. Biofertilizers are live microorganisms or compounds derived from microbes that enhance or augment plant nutrition by mobilizing or enhancing the nutrient availability in soils by colonizing the rhizosphere, rhizoplane, or root interiors (Mitter et al., 2021). Based on the characteristic functions, biofertilizers can be mainly categorized into a) N-fixing fertilizers b) potassium solubilizing fertilizers c) potassium mobilizing fertilizers, d) phosphate mobilizing and solubilizing fertilizers (Win et al., 2018; Gonçalves, 2021).

Microalgae and cyanobacteria along with fungi and bacteria constitute the uppermost layers of soil collectively called biological soil crust (BSC) which plays a critical role in enhancing soil fertility and crop productivity (Abinandan et al., 2019). Several reports have been published on the presence of microalgae and cyanobacteria in the formation of BSC, especially in a variety of soil types ranging from clay loams, desert soils, semi-arid, silt loam to sandstone and granite (Acea et al., 2003; Malam Issa et al., 2007; Nisha et al., 2007; Wang et al., 2009; Lichner et al., 2013; Manjunath et al., 2016; Renuka et al., 2016; Dineshkumar et al., 2018). These soils are characterized by poor organic (carbon (C) and N and micronutrient content owing to higher surface temperature, Ultra-voilet (UV) irradiation, and elevated carbon dioxide (CO₂) levels. Despite such harsh environmental conditions and lack of moisture on the soil surface, microalgae and cyanobacteria initiate BSC formation and survive these harsh conditions through adaptive mechanisms such as the formation of heterocysts, secretion of hydrophobic AAs, and extracellular polysaccharides (EPS) and specialty molecules known as phytochelatins that prevent desiccation of intracellular contents, protection from UV-B radiations and degradation of nucleic acids (Garcia-Pichel et al., 2001; Bhargava et al., 2005). A survey of various agroecosystems and soil microbial communities revealed a significant presence of cyanobacteria groups such as Oscillatoriales, Nostocales, Chroococcales, Synechococcales, Chroococcidiopsidales, Pleurocapsales, Microcoleaceae, Chlorellales (Trivedi et al., 2016). Experimental inoculation of microalgae/cyanobacteria in different soil types improved soil nutrients (organic C, N, P, and other minerals) concentration, soil stability, soil moisture content, and water penetration to soil (Acea et al., 2003; Malam Issa et al., 2007; Nisha et al., 2007; Wang et al., 2009; Lichner et al., 2013).

Biostimulants are compounds other than fertilizers that enhance the crop productivity by acting directly on the plants regardless of its nutrient content (Du Jardin, 2015). These are group of organic compounds that enhance the crop productivity by increasing the nutrient uptake in plants, imparting resistance to various biotic and abiotic stresses, improve soil water use efficiency, reinforcement of root system, and maintenance of physiological processes such as respiration, photosynthetic activity, Fe uptake and nucleic acid synthesis (Du Jardin, 2015; Lee and Ryu, 2021; Kumar et al., 2022d). Microalgae have been identified with several biostimulatory compounds such as phenolics, phytohormones mimicking compounds, terpenoids, polysaccharides and AAs (Ronga et al., 2019; Gonçalves, 2021). Extracts and metabolites obtained from microalgae species such as Chlorella spp., Spirulina platensis, Acutodesmus spp., Scenedesmus spp., Dunaliella spp., Calothrix elenkini etc. are commonly used as biostimulants (Ronga et al., 2019; Colla and Rouphael, 2020).

Seed treatment with biostimulants generally involves three different processes, namely seed priming, seed coating and seed dipping (Gupta et al., 2022). Seed priming is a pre-sowing treatment where the seeds are hydrated in a controlled manner such that the radicle does not protrude. This technique enhances seed germination rate and enhances root formation in plants (Sharma et al., 2014). In seed coating, the seeds are sprayed or coated with biostimulants to form a uniform layer while in seed dipping or soaking treatments, the seeds are soaked for a definite time (18-24 h) prior to sowing. The major advantage with these methods is that it gives a head start in germination and consequently enhances germination index, seedling vigor, increased shoot and radicle length and reduction in harmful seed micro flora (Rocha et al., 2019).

The use of microalgae extracts and live cell suspension in seed treatments resulted in enhanced growth of plants in a variety of crops such as cereals, vegetables and spices. In a recent study, priming of spinach seeds with whole cell extracts and cell lysates of green microalgae Chlorococcum spp., Micractinium spp., Scenedesmus spp., and Chlorella spp., resulted in enhanced seed germination, faster cotyledon emergence and seedlings weight (Rupawalla et al., 2022). Similarly, priming of seeds of vegetable crops such as tomato, lettuce and cucumber with cellular extracts of Spirulina spp., Chlorella spp., Chlorella vulgaris, Scendesmus spp., Synechocystis spp., and Acutodesmus obliquus resulted in higher germination rate compared to untreated seeds (Garcia-Gonzalez and Sommerfeld, 2016; Bumandalai and Tserennadmid, 2019; Supraja et al., 2020a). In addition to enhancing the germination rate and seedling vigour, hydration of seeds with extracts obtained from Chlorella vulgaris and Scenedesmus quadricauda resulted in increased root length, root diameter and root surface area and the number of root tips in sugar beet (Puglisi et al., 2022). Similar observations were made with cereal crops namely maize and wheat when extracts and cell-free supernatants of cyanobacteria such as Anabaena sp. PCC 7120, Calothrix spp., Hapalosiphon spp., Nostoc spp., and Westiellopsis spp were applied to the seeds (Karthikeyan et al., 2009; Grzesik and Romanowska-Duda, 2014). Apart from the application of microalgae extracts, inoculation of live cyanobacteria suspensions like Anabaena laxa and Calothrix elenkinii with the seeds of spice crops such as pepper, coriander, fennel and cumin promoted germination and enhanced root and shoot formation (Guzmán-Murillo et al., 2013; Kumar et al., 2013). The ability of microalgae cell suspensions and cell-free supernatants in enhancing the growth of seedlings, germination rate and root formation could be attributed to the presence of EPS in the supernatant while that of microalgal extracts could be attributed to the presence of phytohormones such as cytokinins (trans-Zeatin, dihydrozeatin, isopentyladenine and kinetin), gibberellins (GA1, GA3, GA4, GA20 and GA29), auxin (IAA) and ABA (Rupawalla et al., 2022).

Foliar spray of biostimulants is a commonly employed method for enhancing crop productivity in several crops owing to the faster response of plants to nutrients supplemented compared to other treatments (Arahou et al., 2022). FS of biostimulants enhanced the water use efficiency and stomatal functioning of the plants (Renuka et al., 2018). Although widely used method, the mechanism of uptake of biostimulants and nutrients through FS is still not clearly understood. Four major pathways of entry have been hypothesized namely through cuticle cracks, stomata, aqueous and ectodesmatal pores (Ishfaq et al., 2022). The generally accepted phenomenon is the cuticular route of nutrient entry (Oosterhuis, 2009). A typical nutrient absorption pathway through foliage involves foliar adsorption, followed by cuticular penetration, uptake and absorption into cellular compartments of leaf, followed by translocation and utilization. Another possible mechanism is through stomatal penetration through the process of diffusion along the stomatal pores (Fernández et al., 2013).

The efficacy of nutrient and biostimulants uptake in plants depends on the physicochemical properties of spray formulation such as pH, the surface tension of the spray liquids, and retention of spray fluid on the leaf surface. Additionally, the inherent nature of the spray formulation such as the molecular size of the nutrients or stimulatory compounds, ionic charge and solubility determines the success of penetration into the leaf (Pandey et al., 2013). FS is generally applied either through fertigation (addition of stimulants/nutrients in irrigation systems) process or through aerial sprays which are effective in improving nutrient use efficiency (Renuka et al., 2018). Foliar application of microalgae extracts has been proven successful in enhancing the growth of several crop plants. Cellular extracts of Chlorella vulgaris, Spirulina platensis, Scenedesmus spp., Nostoc spp., Anabaena spp., Dunaliella salina have been reported to enhance the growth, biomass yield (shoot weight), fruit yield, leaf pigment content, tolerance to abiotic stress such as drought, salinity and temperature stress in horticultural crops such as tomato (Gitau et al., 2022), lettuce (La Bella et al., 2021; Puglisi et al., 2022), capsicum (Elarroussia et al., 2016), onion (Gemin et al., 2022) and beans (Li et al., 2014; Elarroussia et al., 2016). In addition to the horticulture crops, FS of cellular extracts of Spirulina platensis and Scenedesmus spp., resulted in enhanced root weight, plant growth, number of flowers per plant, earliness of flowering and flower diameter in Petunia x hybrid plant (Plaza et al., 2018). Evaluation of the composition of microalgae cell hydrolysates revealed the presence of plant growth-promoting substances such as phytohormones (auxins and cytokinins), signaling molecules such as betaines, AAs, vitamins, polyamines, (spermine and spermidine), and polysaccharides mainly beta-glucan apart from micronutrients (González-Pérez et al., 2022). Among the various microalgae species, extracts and hydrolysates derived from Spirulina platensis, Chlorella vulgaris and Scendesmus spp., are most utilized for FS application (Arahou et al., 2022; González-Pérez et al., 2022).

Roots are the interface between soil and plant that sustain plant growth mainly by mobilizing nutrients, conducting external stimuli and initiating plant defence response to stressors while the soil is a finite nonrenewable resource that forms the basis of agriculture (Ma et al., 2022). Maintaining the health of roots and soil is essential for sustainable agriculture and this is achieved through the application of soil conditioners and fertilizers that replenish soil health and provide nutrients to roots. However, in intensive agricultural practices, the use of chemical fertilizers and conditioners has resulted in soil compaction, acidification, decreased fertility and imbalance of soil microflora aggravating soil diseases (Ye et al., 2020). This necessitates the use of biodegradable, less harmful soil conditioners and fertilizers. Soil drenching involves proportionate mixing of biofertilizers or stimulants during sowing that enhances plant growth. The mechanism of action of biostimulants/biofertilizers by soil drenching method is by agglomeration of soil particles with organic molecules such as polysaccharides, promotion of biological mineralization of complex nutrients and restoration of soil microflora (Karthikeyan et al., 2009; Alvarez et al., 2021).

Microalgae-based soil drenching applications involve the addition of live microalgae suspensions or dried biomass with suitable carrier materials for the application. Soil is the most economically viable carrier for agriculture applications. However, aerial contamination is the major issue with soil, necessitating alternative carrier materials. Agricultural and agro-industrial wastes such as bagasse, peat, wheat straw, vermiculite and animal manure are effective carriers in the application of cyanobacteria and microalgae in soil drenching applications (Renuka et al., 2018). Application of algae as biofertilizer in soil drenching is done as consortia of cyanobacteria/microalgae or as a consortium consisting of a combination of cyanobacteria and rhizobial bacterial species in a suitable carrier (Alvarez et al., 2021). As discussed earlier, the main application of these consortia is for soil amendment and crop productivity enhancement. The most commonly used cyanobacterial combination is Anabaena spp., and Nostoc spp., finding application in a variety of crops such as corn, rice, wheat, cotton and chickpea. The use of cyanobacterial consortium resulted in enhanced availability of soil N and P, increased soil enzyme activity such as nitrogenase, dehydrogenases, and proliferation of soil microflora leading to enhanced biomass and crop productivity (Karthikeyan et al., 2009; Prasanna et al., 2015a; Prasanna et al., 2015b). Further, a synergistic effect of cyanobacteria with rhizobial bacteria such as Brevundimonas diminuta, Mesorhizobium ciceri, Azotobacter spp., Pseudomonas putida etc., resulted in enhanced N₂ fixation, P solubilization, soil micronutrient content and microbial activity (Alvarez et al., 2021). Several authors reported that the use of synergistic bacterial and cyanobacterial consortium in pot or field experiments resulted in the enhancement of macro and micronutrients in cereal crops such as rice (Rana et al., 2015). Similarly, there was an significant increase in the increased grain yield and leghaemoglobin content in nodules of chickpea (Prasanna et al., 2017). The beneficial effects were observed in flower crops such as chrysanthemum with increased flower diameter (Kanchan et al., 2019).

The major benefit of inoculating cyanobacteria was a reduction in the requirement for chemical NPK fertilizers. Most of the above-mentioned reports utilized live cyanobacteria or consortia partially replacing the chemical NPK fertilizers. The available reports uniformly suggest that the use of cyanobacterial consortia resulted in grain yield in crops such as corn and rice similar to chemical fertilizers albeit with a savings of N fertilizer to the tune of 50% (Prasanna et al., 2015a; Prasanna et al., 2015b; Prasanna et al., 2015c). In the case of green microalgae, the application of Chlorella vulgaris in synergy with Pseudomonas putida resulted in enhanced soil P mobilization to rice plants and prevented arsenic translocation in plant tissues suggesting the dual role of microalgae as biofertilizer and heavy metal sequestration (Srivastava et al., 2018). In the case of rice, treatment with cyanobacteria species such as Anabaena variablis and Nostoc spp., by root drench method enhanced the plant height, leaf length and grain yield compared to the inorganic fertilizers (Singh and Datta, 2007; Innok et al., 2009). Similarly, the use of eukaryotic microalgae biomass, specifically green algae Chlorella spp., enhanced the growth and yield of vegetable plants. For example, the application of Chlorella pyrenoidosa by root drench method enhanced the grain yield and shoot weight in soybean (Dubey and Dubey, 2010). Extracts of Chlorella vulgaris enhanced the growth of wheat, maize and tomato (Shaaban, 2001b; Coppens et al., 2016; Barone et al., 2018). The other microalgae species that have been reported to possess biofertilizer properties are Scenedesmus spp., Dunaliella spp., Spirulina spp., Scenedesmus quadricauda, and Nannochloropsis spp. (Ammar et al., 2022). A summary of different modes of application of biostimulants derived from microalgae is presented in Table 1 .

As discussed earlier, the method of bio stimulant application is mainly dependent on the type of crop and the need of the crop, i.e., whether for nutrient enhancement or for tolerance against biotic and abiotic stresses. The method of biostimulant application and timing of application significantly affect the response of plants to stimulants. For example, in a study with lettuce, the response to Chlorella vulgaris extracts was different between root drenching and FS applications (Puglisi et al., 2022). In the case of root drenching, the microalgae extract exerted significant influence on carbon metabolism compared to a foliar application as observed with enhanced activities of malate dehydrogenase and citrate synthase, key enzymes involved in carbon fixation (Kreb’s cycle) supported with enhanced carbon content in the biomass and root and shoot weight (Puglisi et al., 2022). However, the N metabolism was influenced both by root drenching and FS as evidenced by increased activity of enzymes such as glutamate synthase and glutamine synthetase involved in N metabolism with both the treatments (Puglisi et al., 2022). This was morphologically validated with enhanced protein content in the shoot, increased leaf pigments and biomass weight. Similar observations of enhanced N metabolism and increased shoot and root N and biomass protein contents were observed with foliar application of Scendesmus quadricauda extracts in lettuce, commercial algae-based biostimulants on spinach (Fan et al., 2013; Ronga et al., 2019; Puglisi et al., 2020).

In addition to supporting plant growth, bio stimulant application promotes stress tolerance in plants by means of activation of secondary metabolism. Several reports have been published with enhanced abiotic/biotic stress tolerance ( Table 1 ). This observation can be attributed to the increased activity of the enzyme PAL involved in phenylpropanoid pathway involved in the synthesis of phenolics and flavonoids which function as plant defense molecules (Dehghanian et al., 2022). Microalgae extracts derived from Chlorella vulgaris, Scenedesmus quadricauda significantly enhanced the PAL activity in crops such as lettuce and sugar beet (Barone et al., 2018; Puglisi et al., 2020; Puglisi et al., 2022). Among the different methods of application, FS acted instantly in increasing the enzymatic activity compared to soil drenching. This was evidenced by time dependent expression of key enzymes (Puglisi et al., 2022). The PAL enzyme showed immediate increase in its activity after FS while the root drenched samples showed delayed expression of the enzymes at 4^th day indicating influence of application method on the time of physiological response (Puglisi et al., 2022). The immediate physiological response to FS as compared to root drenching applications could be attributed to the faster rate of absorption through stomata compared to root cells (Hong et al., 2021; Arahou et al., 2022). This has been earlier validated with nano-fertilizer applications of micronutrients where foliar application of minerals modulated plant growth better and faster compared to soil application (Alshaal and El-Ramady, 2017). For commercial and large scale applications, root drench and foliar applications seem to be best options of bio stimulant application. However, for faster physiological response in plants, FS of biostimulants is generally preferred. It is reiterated again that the mode of bio stimulant application is case dependent and varies with respect to the crops requirement. A summary of functions attributed with different methods of bio stimulant application is presented in Figure 2 .

Cyanobacteria are known to secrete EPS and anti-microbial compounds that inhibit the growth of plant pathogens. Nostoc spp., Nodularia harveyana, produce cyclic compounds such as 4,4’ –dihydroxybiphenyl, norharmane and diterpenoids that show anti-bacterial activity (Volk and Furkert, 2006). Norharmane is indole alkaloids mostly secreted by cyanobacterial species belonging to the order Nostacales that have mutagenic properties and inhibit key metabolic enzymes such as monoamine oxidase, indoleamine 2,3-dioxygenase, nitric oxide synthase (Volk, 2008). These antimicrobials were effective against several fungal pathogens. For example, Abdel-Hafez et al. (2015) reported the fungicidal activity of extracts obtained from Nostoc muscorum and Oscillatoria against Alternaria porri that causes a purple blotch of onion. The group analyzed the fungicidal extracts revealing the presence of inhibitory compounds such as β-ionone, norharmane, and α-isomethyl ionone. Along with these, a variety of antimicrobial compounds such as nostocyclyne A, nostocin A, ambigol A and B, hapalindoles and scytophycins, tjipanazoles, fischerellin-A have been identified and applied as biocontrol agents obtained from cyanobacteria such as Nostoc spp., Scytonema spp., Tolypothrix spp., Fischerella spp. respectively (Singh et al., 2016). These antimicrobials inhibit protein synthesis, enzymatic functions and cell division in pathogens. In another mechanism, some anti-microbial compounds especially siderophore type molecules competitively bind to nutrients such as Fe, Cu and deprive the nutrients to pathogens as observed in the case of biocidal activities of Pseudomonas spp., (Lee and Ryu, 2021). Further, cyanobacteria such as Anabaena laxa, Anabaena variabilis, Nostoc spp., Calothrix elenkinii produce hydrolytic enzymes, mainly cell wall-degrading enzymes such as chitosanase, β-1,4-glucanase, and β-1,3-glucanase that hydrolyse the cell wall of pathogens such as Pythium aphanidermatum (Prasanna et al., 2008; Gupta et al., 2011; Natarajan et al., 2013). Apart from directly acting on fungal pathogens, microalgal and cyanobacterial extracts enhance plant defence mechanisms by enhancing the activity of hydrolytic enzymes (endochitinases such as β-N-acetylhexosamindase, chitin-1,4-β-chitobiosidase, Endoglucanase – β-1,3 glucanase), polyphenol oxidase and POD (Roberti et al., 2015). Further, cyanobacterial extracts enhance the activity of PAL enzyme that catalyzes phenylpropanoid pathway involved in biosynthesis of SA attributed to systemic resistance to infections in plants and phenolics and phytoalexins (Singh et al., 2011; Babu et al., 2015). Application of cyanobacterial biomass and cell-free extracts to the soil resulted in reduction in disease symptoms. For example, Chaudhary et al. (2012) and Prasanna et al. (2013) reported that biopesticide formulation of Anabaena spp., led to a 10% to 15% reduction in damping-off disease in tomato seedlings and significant increase in plant growth when challenged with pathogenic fungi. Similarly, the application of Spirulina platensis resulted in a reduction of root rot, root wilt and damping off symptoms in Moringa plant (Imara et al., 2021).

In addition to the antimicrobial properties, microalgae possess insecticidal properties, specifically nematicidal properties owing to the secretion of neurotoxins such as anatoxin-A, microcystin, nodularins inhibiting the growth of nematode pests. Anatoxin-A, mimics neurotransmitter acetylcholine and bind irreversibly to acetylcholine receptors leading to continuous muscle contraction in nematode pests leading to their immobility (Mankiewicz et al., 2003). Number of cyanobacterial species such as Oscillatoria chlorina, Aulosira fertilissima, Spirulina platensis, Amphora cofeaeformis have been reported to exhibit nematicidal properties against soil borne nematodes such as root knot nematode (Meloidogyne arenaria, Meloidogyne incognita), Xiphinema index that affect the root formation (Khan et al., 2007; Chandel, 2009; Bileva, 2013; El-Eslamboly et al., 2019). Root knot nematodes forms gall like structure in the roots leading to hardening of root surface ultimately affecting nutrient uptake in plants. Soil application of cyanobacteria and microalgae such as Chlorella vulgaris inhibited the nematode growth and reduced the gall formation in roots and promoted root and shoot growth in plants (Khan et al., 2007). Microalgae derived secondary metabolites such as hexamethyl phenol, methoxyphenyl and flavonoids were shown to inhibit nematode growth and reduction of gall formation in roots of Cucumis sativus plants and concomitant enhancement in fruit yield and quality (El-Eslamboly et al., 2019). Additionally, cyanobacteria such as Anabaena spp., and Scytonema spp., produce peptide toxins that act as repellents to insect pests. (Sathiyamoorthy and Shanmugasundaram, 1996) reported presence of low molecular weight (<12 kDa) peptides in Scytonema spp., that have strong odour repelling cotton leaf chewing insects such as Helicoverpa armigera and Stylepta derogate. Similarly, Abdel-Rahim and Hamed (2013) reported insecticidal properties of extracts obtained from Anabaena flos aquae against Spodoptera littoralis larvae (Cotton leaf worm) that destroys cotton crops. The authors reported that the cyanobacterial extracts suppressed the oviposition in the insects and caused sterility in insects. A list of bio pesticide and crop protection properties of cyanobacteria and microalgae is presented in Table 3 and the mode of action is presented in Figure 3 .

Review of literature clearly indicates that microalgae and cyanobacteria have excellent plant growth promoting properties. However, their utilization in large scale has not seen the commercial light. Some of the important challenges associated with commercial utilization of microalgae in agro-chemicals sector are high cost of cultivation, varying biomass productivities, high cost and energy intensive downstream processes in obtaining purified metabolites demonstrating plant growth promoting applications (Sharma et al., 2013; Smetana et al., 2017). This warrants identification of processes and technologies that reduce the cost of biomass production and improve energy efficiency in downstream processes. Following section describes the various steps involved in microalgae biomass generation and discuss strategies for low cost biomass production and multi product utilization to achieve environmental sustainability along with economic viability.

Microalgae have been considered as a potential industrial source of food, chemicals and bio-products owing to their higher photosynthetic efficiency and high unit dry matter per yield/land area in comparison to land plants (Chen et al., 2022). The presence of myriad of chemical compounds comes from their ability to adapt to wide environmental conditions and adopt different modes of nutrition such as autotrophic (completely photosynthetic), mixotrophic (ability to utilize both reduced organic carbon and light sources) and heterotrophic (fermentative approach with utilization of organic carbon source) (Udayan et al., 2022). Microalgae biomass is the functional material and the primary basis for biofertilizer or biostimulants production. Systematic evaluation of biomass production processes and understanding their suitability is very critical in mass production of biofertilizers and agro-chemicals. A typical microalgae biomass production or derived products involves three major steps, viz., (i) cultivation, (ii) harvesting and (iii) downstream processing consisting dewatering, extraction, and purification or formulation. The production strategies vary with respect to intended application such as for food, feed, fuel or agriculture applications. Microalgae are generally mass cultivated either in open raceway ponds (ORP) or closed PBRs utilizing the phototrophic mode of nutrition exploiting solar energy, carbon dioxide and inorganic minerals. Amongst the two systems, ORP are commercially preferred over PBRs owing to their low capital and operational costs while PBRs are preferred for accurate control of culture parameters and production of high value products such as proteins, pigments (carotenoids and xanthophylls), and polyunsaturated fatty acids (Udayan et al., 2022). The choice of cultivation depends on the species/strains and further downstream processes selected. Although ORP is widely preferred, major disadvantages include contamination of culture, low biomass productivity and susceptibility to predators and environmental conditions (Tan et al., 2020). Photo bioreactors offer better control the over the cultures with high biomass productivity, reduced contamination risks and higher metabolite yield; however, the major disadvantages are high capital investment, higher energy requirements in circulating culture and maintenance of temperature and risk of bio fouling and increased dissolved oxygen in cultures leading to cell death (Tan et al., 2020; Udayan et al., 2021; Udayan et al., 2022).

The second important step in the biomass production process is harvesting, which involves the separation of microalgae cells from the growth medium and the recycling of the spent medium. A typical microalgae culture has asolids content of less than 1%, (approximately 0.6 to 1 g biomass per litre culture) and needs to be concentrated between 15% and 25% solids content for further downstream processing (Milledge and Heaven, 2013). Several processes such as chemical -induced flocculation, bio-flocculation (coagulation induced by bacterial/fungal species), electrocoagulation, flotation, and membrane filtration have been developed and evaluated for harvesting microalgae biomass (Milledge and Heaven, 2013; Vandamme et al., 2013; Barros et al., 2015; Udayan et al., 2022). Despite the availability of enormous literature on the harvesting of microalgae, it is a general consensus that harvesting is the major bottleneck in the commercial production of microalgae biomass. This could be attributed to the wide range of cellular properties such as size, shape, the surface charge of microalgae cells necessitating customized solutions limiting commercial scalability (Kumar et al., 2022a). However, commercially a combination of flocculation followed by pressure filtration or centrifugation is considered economically viable with lower energy inputs (less than 0.1 kWh·kg⁻¹ algae) (Fasaei et al., 2018). The success of a harvesting process is entirely dependent on the cultivation step where the cell density of the culture determines the energy expenditure incurred during harvesting. Cultivation in ORP is generally performed at low cell densities to avoid the shading effect and consequent dark fermentation at the bottom of the ponds (Shekh et al., 2022). This necessitates the deployment of multiple harvesting steps such as two-stage process of initial concentration to 2% to 3% solids content followed by dewatering to 15% solids content (Milledge and Heaven, 2013). However, this problem can be avoided with microalgae cultivation in flat plate PBRs at higher cell density reducing the energy and cost requirements during the harvesting stage (Milledge and Heaven, 2013).

The final step involved in typical microalgae biomass production is dewatering and downstream processing for further production of commercial products. Dehydration is the most energy-intensive process and has been a major area of concern and a limiting factor in the commercialization of microalgae products. The choice of dehydration is dependent on the end product application and earlier reports indicate that convective air drying of harvested biomass paste is the most economical option for large-scale production (Sirohi et al., 2022). However, it has been observed that heating of biomass beyond 70°C resulted in the loss of heat-labile compounds such as phycobiliproteins, antioxidants and secondary metabolites (Phong et al., 2017). For the biofertilizer application, mainly as a source of macro and micronutrients, the dewatering process does not influence biomass stability. However, for use as biostimulants such as phytohormone-rich extracts or pesticidal extracts, dehydration plays a crucial role as many of these compounds are heat sensitive necessitating optimization of dewatering and further downstream processes.

In order to select a particular cultivation, harvesting and downstream strategy, a thorough evaluation of sustainability aspects involved in various stages of biomass production is required. For commodity products such as food, animal feed, biofuel or biofertilizers the choice of cultivation method must be cheaper with minimal energy expenditure. However, the choice of process varies between species/strains necessitating a detailed sustainability analysis. This study is known as life cycle analysis (LCA). A typical LCA involves assessment of process against six parameters namely global warming potential expressed as g CO₂ eq. per kg biomass, acidification potential expressed as g SO₂ eq. per kg biomass, eutrophication potential expressed as g PO₄ ^- eq. per kg biomass, cumulative energy demand expressed as MJ per kg biomass, land use expressed as m² per kg biomass and water usage expressed as L per kg biomass. These parameters could also be tested against other functional units that are potentially generated from microalgae biomass such as energy produced (in Kcal) in case of biofuels, or proteins/polysaccharides/pigments/or metabolite of interest (expressed in grams/kg biomass) (Kumar et al., 2022a).

Life cycle analysis of microalgae based products have been studied for various scenarios such as biofuels (Huang et al., 2022; Kim et al., 2022), food and nutraceuticals (Schade and Meier, 2020; Schade et al., 2020), multiproduct refineries (Arashiro et al., 2022; Ubando et al., 2022) biofertilizers (de Siqueira Castro et al., 2020; Arashiro et al., 2022). The major consensus among most of these studies is that the cultivation of microalgae in ORP is cheaper and less energy intensive compared to PBRs. However, the biomass productivity is roughly 12 times higher with PBRs (1.5 kg dry biomass m^-3 day^-1) compared to ORP (0.117 kg dry biomass m^-3 day^-1) (Silva et al., 2015). The lower sustainability quotient with respect to PBRs based cultivation could be attributed to the high capital costs involved in construction of PBRs which contribute to almost 85% of the total energy consumed in the process (Silva et al., 2015). Calculation of net energy returns, a simple ratio of energy produced over energy consumed for various cultivation steps indicated that biomass production using horizontal tubular PBRs had NER <1 while biomass produced with ORP or flat plate PBRs had NER >1, suggesting commercial feasibility of ORP based biomass production (Jorquera et al., 2010).

Energy required for heating cultures in ORP and the energy required for injecting CO₂ and maintaining the flow of cultures in PBRs and supply of reduced organic carbon sources in mixotrophic or heterotrophic cultivation are some critical factors that negatively influence microalgae based processes (Ighalo et al., 2022). Microalgae based processes had relatively lower land and water use compared to aquaculture processes, however, in comparison to poultry and insect farming the water and nutrient requirements were relatively higher for microalgae biomass production (Maiolo et al., 2020; Schade et al., 2020). The overall energy requirements for production of one metric ton (1 MT) of microalgae biomass was estimated between 12000 and 22000 kWh, which was 4 to 7 folds higher compared to poultry or insect meal based protein production (Maiolo et al., 2020). The capital and operational inputs contributing to the energy consumption as part of the life cycle assessment during microalgae cultivation and biomass processing is presented in Figure 4 .

Water and nutrient requirements are the two critical parameters that determine the economics of microalgae biomass production. For example, the estimated nutrient requirement (inorganic salts) for the production of 1 MT of fresh biomass of microalga Tetraselmis suecia was 90 kg and the water requirement was approximately 15000 m³ per m² per month, which is 10 times higher compared to other poultry and insect meal production (Maiolo et al., 2020). This high water requirement makes microalgae biomass-based products and processes unsustainable, necessitating the identification of cheaper sources of water and nutrients. Wastewater obtained from industrial processes could be a potential source of nutrient-rich water for the cultivation of microalgae (Dagnaisser et al., 2022). The concept of utilization of industrial wastewater for the cultivation of microalgae has been practised for quite some time now with several success stories and enormous literature (Carraro et al., 2022; Nagarajan et al., 2022; Satya et al., 2022; Yan et al., 2022). Microalgae have the unique ability to grow in non-potable waters and have excellent bioremediation potential with high removal efficiencies against nitrates, phosphates, and ammonia in waste streams and reduce the chemical (COD) and biological oxygen demand (BOD) of wastewaters (Ahmed et al., 2022; Díaz et al., 2022). Further, the ability to use industrial flue gas as a source of CO₂ and the presence of carbon concentration mechanism in microalgae make them an attractive resource for dual applications of bioremediation and metabolite/biomass production (Çakirsoy et al., 2022; Ighalo et al., 2022; Ma et al., 2022).

The dual application of bioremediation and microalgae biomass production has been successfully exploited previously for biofuel production (Ali et al., 2022; Das et al., 2022). The concept has helped to improve the economics of biomass production with microalgae having the ability to grow in different types of industrial wastewater streams such as that of steel slag, cement, food processing, distillery, aquaculture, leather tannery and anaerobic digestates (You et al., 2021; Rambabu et al., 2022; Venugopal, 2022). As discussed earlier, microalgae consistently showed a nutrient removal efficiency against nitrates, phosphates, organic carbon, ammonia and other nutrients above 70%, and in a few cases above 90% from the initial levels (Abdelfattah et al., 2022; Nagarajan et al., 2022). The ability of microalgae to absorb nutrients in abundance could be attributed to the special physiological mechanisms such as carbon concentration mechanism, lipid remodeling and P allocation in fast-growing microalgae as described earlier (Çakirsoy et al., 2022). Microalgae prefer ammonia over inorganic N sources as their assimilation is less energy intensive and can be directly incorporated into AAs and other N moieties (Sarma et al., 2021). This provides an opportunity for ammonia-rich wastewaters such as agriculture run- offs, agro-industrial residues and anaerobic digestates to act as an excellent N sources in microalgae cultivation (Wang et al., 2018).

A typical microalgae biomass contains relatively higher nutrient content compared to other organic fertilizers, especially N and P. The average N content ranges between 4.9% and 7.1% N while the P content ranges from 1.5% to 2.1% (Khan et al., 2019). Further with the presence of a luxury uptake mechanism, microalgae accumulate nutrients from the surrounding medium and act as a concentrated source of nutrients for plant growth. The use of wastewater-derived microalgae biomass as biofertilizers and source of biostimulants has been a rising trend with several case studies being published in recent years (Álvarez-González et al., 2022; Arashiro et al., 2022; Dagnaisser et al., 2022). Table 4 lists various case studies available on the applications of wastewater-generated biomass as biofertilizer. Domestic and municipal wastewater has been predominantly used as a source of nutrients for microalgae biomass production ( Table 4 ). However, the major disadvantage of this is the varying levels of nutrients as frequent nutrient composition changes affect biomass productivity. Among the various wastewater resources evaluated, food industry effluents are promising for the recovery of various algae-derived bioproducts owing to their better quality compared to other effluents (Arashiro et al., 2022). Further, the group reported that microalgae biomass production utilizing wastewater significantly reduced the energy expenditure and environmental impact thus enhancing the sustainability of the whole process. Further integration of industrial gases, specifically CO₂-rich flue gases along with wastewater utilization enhances biomass productivity and bioproducts recovery (Yadav et al., 2019; Carraro et al., 2022).

Although microalgae show prolific growth and efficiently remediate nutrients in waste effluents, the process of utilizing wastewater treatment derived biomass for agriculture applications has certain inherent challenges. These include scalability of biomass production, presence of xenobiotic residues and heavy metals in biomass and contamination of cultures with bacteria, fungi and virus limiting their widespread application (Abdelfattah et al., 2022; Amin et al., 2022). The first critical challenge in wastewater mediated biomass production is meeting the light requirement for cultures. The industrial effluents derived from food, livestock, aquaculture processing and sewage have high suspended solids, reducing the light penetration into the growth medium consequently affecting photosynthetic rate and biomass productivity (Abdelfattah et al., 2022). This light penetration issue is a major challenge in scalability of wastewater mediated large scale biomass production. The average biomass yield obtained utilizing wastewater ranged between 0.7 g L^-1 to 1.4 g L^-1 with microalgae such as Scendesmsus spp., Chlorella spp., Desmodesmus spp., Tetradesmus spp., Selenastrum spp., predominantly effective in effluent treatment (Rani et al., 2021; Amin et al., 2022). Further the biomass productivity significantly varies with respect to the temperature, pH, effluent nature and nutrient content (Rani et al., 2021). A “one shoe size fits all” approach cannot be applied with respect to biomass production utilizing industrial effluents, thus requiring a case specific standardization and cultivation strategy. The second critical issue is harvesting and dewatering of microalgae from effluent treatment ponds and reactors. As discussed earlier the biomass concentration is very low in large scale algae cultures requiring energy intensive dewatering steps.

Apart from scalability, microalgae cultivated utilizing dye and tannery effluents, and pharmaceutical wastes pose the challenge of presence of xenobiotic residues and heavy metals in the biomass. Microalgae have the ability to degrade organic hydrocarbons such as phenols, azo dyes, and benzene compounds such as naphthalene, antibiotics and hormones residues present in wastewater (Xiong et al., 2021; Touliabah et al., 2022). The mechanisms behind the degradation of organic compounds by microalgae include (i) secretion of extracellular polymeric substances such as polysaccharides, proteins and humic acid that degrade organic molecules, (ii) bioaccumulation of organic compounds followed by intracellular biotransformation through redox reactions, hydrolysis and conjugation with macromolecules and (iii) photolysis (Xiong et al., 2021). A range of antibiotics belonging to classes such as tetracyclines, cephalosporins, macrolides, penicillins, and beta-lactams have been degraded using microalgae with varied efficiency (Chandel et al., 2022; Wang et al., 2022). Similarly, microalgae remove heavy metals from effluents through the phenomenon of bio adsorption and bioaccumulation followed by chelation and vacuolar sequestration or biotransformation utilizing special intracellular chelating agents called ‘phytochelatins’ (Chugh et al., 2022). Although the ability of microalgae to remediate xenobiotics, heavy metals have been demonstrated, there are no pilot scale studies involved on the additive toxicity of microalgae biomass enriched with a heavy metal or a bio transformed molecules. On the other hand, high concentrations of heavy metals or xenobiotics, colored dyes have shown to induce oxidative stress in microalgae causing cell death (Bhatt et al., 2022). This defeats the whole purpose of microalgae mediated bioremediation. Further the coexistence of fungi, bacteria and viruses in the waste effluents pose another challenge in utilization of microalgae biomass. Microalgae and other microbes have been known to interact both mutually and competitively in a polluted environment such as industry effluents and sewage. The interactions have been supported through nutritional reciprocity, chemotaxis, communication through volatile organic compounds and signal transduction between microalgae and microbiome present in the wastewater environment (Ashraf et al., 2022; Astafyeva et al., 2022; Chegukrishnamurthi et al., 2022).

In addition to heavy metals and xenobiotics, co-occurrence of virulent microbes along with microalgae biomass and their consequent application as biofertilizers poses the risk of introduction of virulence into the ecosystem. Further occurrence of horizontal gene transfers between micro biome and microalgae in polluted waters have been demonstrated offering special adaptive mechanisms to microalgae such as tolerance to heavy metals and impart extremophilic properties (You et al., 2021). Though HGTs offer niche adaptive benefits to algae, it poses the risk of transfer of antibiotic resistance genes, disrupting existing transcription mechanisms of biotransformation thereby affecting the bioremediation process and consequently the biomass application as biofertilizer. The major drawback in deployment of microalgae biomass produced from wastewaters and effluents for biofertilizer application is the lack of pilot scale/industrial scale trials and understand the real time effect of environment on microalgae biomass production and its impact on the biomass productivity. Most of the published literature are based on lab scale trials that do not truly reflect real time scenarios necessitating systematic studies. Nevertheless, the concept of biomass generation through bioremediation of wastewater and industrial effluents offers an exciting opportunity in increasing the sustainability of microalgae biomass production; since microalgae biomass is the primary basis of supplying nutrients and stimulants for plant growth.

Microalgae bio refineries involve complete fractionation and utilization of biomass analogous to petrochemical refinery (Vermuë et al., 2018). As discussed earlier, a typical microalgae based ingredient [food/feed/fuel] production would involve cultivation, harvesting, dewatering, extraction and purification steps involving multiple unit processes that are energy and cost intensive. To achieve commercial feasibility, prudent selection of processes followed by generation of high value ingredients is essential. Among the various commodities that can be produced from microalgae, biofuels are supposed to be cheapest. However, with the high process costs, the commercial feasibility of microalgae derived biofuels is questionable. To offset this uncertainty, several scenarios of multiproduct generation and biomass valorization strategies are being suggested such as (i) production of animal feed after biofuel extraction, (ii) gasification of deoiled biomass for generation of syngas, (iii) liquefaction of deoiled biomass for production of bio crude/bio char, (iv) anaerobic digestion of biomass to produce methane, (v) fermentation of deoiled microalgae biomass for bioethanol, (vi) application of deoiled biomass as biofertilizers and soil conditioners (Katiyar et al., 2021). For further economic feasibility and reduced energy footprints, integration of bio refinery with bioremediation of wastewaters and CO₂ could enhance the commercial prospects along with environmental benefits. A typical microalgae biorefinery involving multiproduct utilization is presented in Figure 5 .

Experimental trials have indicated positive outcomes from integrated biorefinery based routes towards biofertilizers application. Nayak et al. (2019) reported the use of deoiled Scenedesmus spp., biomass cultivated using domestic wastewater and coal-fired flue gas [2.5% v/v] as biofertilizers in the cultivation of rice. Deoiled Scenedesmus spp., biomass contained an entire spectrum of AAs and carbohydrates that promoted plant growth and reduced the requirements for chemical fertilizers. Similarly, Chlorella minutissima cultivated using sewage water was used as biofertilizer in field experiments for the cultivation of spinach and baby corn (Sharma et al., 2021). Phycoremediation with Chlorella minutissima resulted in a significant reduction (up to 80%) of both BOD and COD, and >50% of ammonia, nitrate and phosphate levels in the sewage water. The NPK content of microalgae biomass was higher compared to other natural manures such as vermicomposting, oil cake and biogas slurry. Application of microalgae biomass resulted in enhanced soil microbial activity, total organic carbon, available N and P as compared to soils enriched with chemical fertilizers (Sharma et al., 2021). Microalgae biomass obtained from external CO₂ supplementation from flue gas showed enhanced nutrient content and exhibited biofertilizer potential. Sinha et al. (2021) reported that deoiled biomass of CO₂ tolerant strain Tetradesmus obliquus CT02 exhibited biofertilizer properties when evaluated upon tomato plants. The deoiled biomass enhanced the germination percentage and index of tomato seedlings in comparison with commercial NPK fertilizers. In similar lines, cellular extracts of deoiled biomass of Nostoc spp. cultivated using municipal wastewater showed biostimulatory effects and improved the growth of lettuce plants. Foliar application of extracts derived from deoiled Nostoc spp., LS04 resulted in enhanced shoot and root length, fresh weight, DW, leaf chlorophyll and nutrient content compared to control groups (Silambarasan et al., 2021b).

In addition to the direct utilization of microalgae deoiled biomass and its extracts, bio char obtained through thermochemical conversion processes (hydrothermal carbonization, pyrolysis and Torre faction) of deoiled biomass could be used for agriculture applications (Mona et al., 2021). This integrated bio refinery route typically involves four steps viz., (i) cultivation of microalgae using wastewater; (ii) harvesting and downstream processing of biomass for lipid/hydrocarbon extraction; (iii) thermochemical conversion of deoiled biomass for bio char production and (iv) biofertilizer application of microalgae based bio char. Previous reports on use of bio char obtained from various agro-residues and biomass like straw, coffee husk, bamboo and variety of wood have been reviewed and reported to possess biofertilizer properties (Wang et al., 2020a). Application of bio char obtained from these biomasses in agro-forestry systems have resulted in enhanced soil fertility, improved soil texture especially in sandy and loamy soils through enhanced water retention, improvement in soil cation exchange capacities (CEC) leading to increased soil pH, and nutrient availability for plants in the rhizosphere region (Wang et al., 2020a). Thermochemical conversion of biomass results in oxidation of aromatic carbon groups and other complex organic molecules resulting in availability of carboxyl, carbonyl and other negatively charged functional groups that form complexes with cations thus enhancing the CEC of soil leading to nutrient retention (Laird et al., 2010). Further the porous microstructure and higher surface area of bio char causes slow release of nutrients to the plants thereby acting as slow release fertilizers reducing the problem of leaching and surface runoff (Gwenzi et al., 2015; Yu et al., 2019; Wang et al., 2020a). Thermochemical conversion of deoiled microalgae biomass could yield up to 90% bio char with higher nutritive value compared to deoiled microalgae biomass. Ultimate analysis of bio char and deoiled biomass revealed that thermochemical conversion enhances the net N, carbon, C/N ratio in bio char over the raw biomass (Phusunti et al., 2017). However, this depends on the thermos-conversion process parameters such as pyrolysis temperature and residence time during the process (Gan et al., 2018). The ultimate elemental analysis of biochar obtained from Chlorella spp. and Nannochloropsis spp., consisted 33% -50% w/w carbon, 33% - 57% w/w oxygen, 1% -5% w/w hydrogen and 4%-12% w/w N along with minerals such as Ca, Fe, Mg and K. The chemical composition of biochar, especially the mineralized N content indicated their suitability as biofertilizers especially for nutrient enrichment of soil (Borges et al., 2014; Chang et al., 2015). Biochar obtained from Scenedesmus abundans biomass cultivated using domestic wastewater had bio fertilization properties. The biochar contained 54.23% w/w carbon, 9.8% w/w hydrogen, 4.2% w/w ammoniacal N, 2.7% w/w sulphur and 28% w/w oxygen (Arun et al., 2020). The authors reported that biochar obtained from wastewater grown Scendemsus abundans biomass enhanced the growth, leaf and shoot weight and leaf chlorophyll content in tomato plants compared to chemical fertilizers. Apart from enhancing and mobilizing nutrient content in soil, microalgae derived biochar have been found to improve soil enzyme activity and promote soil microflora and soil microbial carbon content (Palanisamy et al., 2017). In another case study, use of algal biochar promoted defense response to drought stress in maize plants under deficit irrigation conditions. It was observed that algal biochar in consortia with plant growth promoting rhizobacteria Serratia odorifera accelerated plant growth by enhancing the nutrient availability in soil and enhancing photosynthetic activity (Ullah et al., 2020).

Along with the use of municipal and sewage wastewaters, microalgae biomass obtained from wastewater generated from hi-tech agriculture systems such as hydroponics could be utilized used for biofertilizer application in a closed-loop model. Soilless cultivation such as hydroponics hasgained significant attention in recent years as the system can achieve maximum yield under a shorter time duration with excellent quality (Richa et al., 2020). Hydroponics utilizes nutrient solutions consisting of fertilizer salts in a definite ratio as per plant’s growth requirements. Typically, the nutrient solutions used for the cultivation of vegetables and other horticulture plants have electrical conductivity in the range between 0.8 and 2.5 dS m^-1 and pH between 5 and 7 and are generally prepared using salts that have high solubility in water (Hosseinzadeh et al., 2017). The physicochemical properties of nutrient solutions used in hydroponic systems meet the essential nutrient requirements of microalgae suggesting its potential in microalgae cultivation (Zhang et al., 2017). Even the wastewater (hydroponic system drainage) solution obtained after a plant cultivation cycle contains high concentrations of residual N (150–600 mg L^-1) and P (30–100 mg L^-1) along with other nutrients such as sulphates, K, Ca, trace minerals and organic substances like humic acids and root exudates (Saxena and Bassi, 2013). This wastewater solution that when released into the environment results in eutrophication and imbalance in the aquatic ecosystem (Richa et al., 2020). Nutrient recycling of hydroponics wastewater using membrane filtration technologies [reverse osmosis, ultrafiltration], sand filtration, UV treatment and denitrification using biological filters, and chemical treatment has been generally practised (Patel et al., 2020; Rufí-Salís et al., 2020). However, challenges exist in wastewater recycling treatments as organic residues and root exudates present in wastewater require multiple strategies as they promote microbial growth (Richa et al., 2020).

Hydroponics wastewater treatment and nutrient recycling could be effectively achieved by microalgae co-cultivation strategies. Co-cultivation of microalgae such as Chlorella vulgaris and Scenedesmus quadricauda with tomato plants in hydroponic system resulted in enhanced growth of both the plant and algae with biostimulatory properties exhibited by microalgae. This could be attributed to the mutual benefits exchanged by plant and algae where the roots exudates and inorganic nutrients promote algae growth and algal exudates induce stimulatory effects on to the plants (Barone et al., 2019). Zhang et al. (2017) reported that co-cultivation of microalgae Chlorella infusionum and tomato plants in hydroponic systems resulted in higher microalgae and crop biomass productivities when compared to individual monocultures. The enhancement in biomass productivity could be attributed to the aeration induced by microalgal photosynthesis and simultaneous utilization of CO₂ from crop root respiration. Further co-cultivation reduces nutrient load during the drainage of the hydroponic wastewater. Supraja et al. (2020b) observed the above-mentioned phenomena of enhanced plant and microalgae biomass productivities and reduced the nutrient load in drain water during co-cultivation of microalgae consortia consisting Chlorella spp., Scenedesmus spp., Synechocystis spp., and Spirulina spp., with tomato plants. Co-cultivation resulted in the utilization > 80% of initial N, P and K in the nutrient media. All the aforesaid studies suggest that microalgae co-cultivation in hydroponics is a cost-effective solution for simultaneous microalgae biomass production and wastewater treatment. A typical closed loop circular economy model of microalgae integrated biorefinery is presented in Figure 6 .