Potential of Microalgae as Feed Supplements for Sustainable Aquaculture

Aquaculture is a fast-growing industry mainly depends on the key feedstuffs, fishmeal (FM) and fish oil (FO) that will be limited with the passage of time due to the insubstantial resources available for wild fish harvesting. Therefore, other sources of feedstuffs need to be investigated to substitute FM and FO in aquafeeds. Terrestrial crops can be used to substitutes a portion of the FM however; they can result in changes in the nutritional quality of the fish produced. Microalgae can be considered as a favorable alternative that can substitute FM and FO ensuring the principles of sustainability in aquaculture. Microalgae are reasonably rich in proteins, lipids, carbohydrates, vitamins, minerals, pigments, etc., which are essential for not only sustaining fish health but also its unique array of bioactive compounds can improve coloration and quality of fillet. The aim of this review is to provide an update of the current knowledge of microalgae as a supplement or feed additive to substitute FM and FO in aquafeeds. This review will provide a platform to highlight the potential of microalgae-based aquafeeds for a sustainable aquaculture industry.


INTRODUCTION
A quaculture is an important and fast-growing food sector in the world playing a vital role in human's life. It has undoubtedly contributed significantly to improve the nutritional value of human's diet in the form of seafood and improved the standard of living due to financial gain from the rapidly developing aquaculture industry. Since 2016, worldwide fishery and aquaculture foods have shown tremendous growth of ~ 171 million tons that costs 194.78 billion euros through aquaculture to give 54.5% of the overall production. Statics shown that 19.3 million people have obtained employment in the fisheries and aquaculture sector that accounts 30% of the total jobs, playing a significant role in uprising their socio-economic status.
The entire fish production without marine floras is likely to upsurge ~ 204 million tons in 2030 that accounts an overall increase of 15 % over 2018 (FAO, 2018(FAO, , 2020. Aquaculture is a vital source of animal protein, which is nearly half of the total production that need outward feed contributions to overcome the food consumption demand by the growing human population. Aquafeeds normally contain fishmeal and oil take out from trivial pelagic forage fish, for instance sardines, herrings and anchovies, and little amount after fish embellishments and castoffs. Fish meal (FM) and fish oil (FO) mainly used to fulfil the protein and fatty acid requirements of farmed aquatic species as palatable and inexpensive feed components. Aquafeeds generally increases fish productivity, but an alternative to FM and FO must be find out for sustainable fish farming (FAO, 2016;Turchini et al., 2020). Many fodder fisheries are fully or over exploited, and reports have portrayed the existing trend of fishmeal and oil utilization as a big challenge for marine biodiversity and human food safety (Froehlich et al., 2018). The current fishmeal and oil are very costly, expected to upswing than that of plant oils and protein meals over the following decade. Keeping in mind all issues alternative aquafeed resources are required with high digestibility and nutritional cost analogous to FM and FO that needs to be developed through eco-friendly O n l i n e

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strategies as extraordinary and economical feedstuffs (Cottrell et al., 2020). Marine-based FO contain high quantity of omega-3 (n-3) long-chain (≥C20) polyunsaturated fatty acids (LC-PUFAs), namely eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3). Biological conversion of the PUFA, alpha-linolenic acid (ALA; 18:3n-3), normally found in terrestrial oil kernels of canola used in marketable salmonid nourishments, to EPA and DHA in fish and humans is not sufficient to fulfil their nutritional rations, therefore it is necessary to take in the n-3 LC-PUFA in food (Saini and Keum, 2018;Bou et al., 2017). DHA play an essential role in cell signaling as well as the structure, function, and fluidity of cell membranes while EPA triggers anti-inflammatory response via eicosanoid production. Addition of EPA and DHA in salmonid feed not only safeguard ample growth and development of the fish, but also a carrier to deliver these EFAs to humans, which have significant health benefits in preventing rheumatoid arthritis, cardiovascular, and neurological ailments (Hart et al., 2021;Siscovick et al., 2017;Laye et al., 2018).
The current aquaculture is over relied on terrestrial crops and animal-based materials like soybean meal, canola oil, poultry fat, and blood meal, which involves worries about deviation of crops and animals from human feeding towards aquafeed (Colombo, 2020). As crops growing sector appearances a universal task to nourish nearly a billion of hungry folks, and risks turning the rapidly expanding aquaculture sector into an environmentally unsustainable agrarian practice for the world's grains and oils consumption. Their use in aquafeed has many shortcomings e.g., unbalanced essential amino acid, high levels of antinutritional elements and insufficient level of EAA and EFA cannot fulfil the requirements of fish and human health (Fry et al., 2016;Sprague et al., 2016).
Microalgae are eukaryotic photosynthetic microorganisms that use solar energy, nutrients, and carbon dioxide (CO 2 ) to produce proteins, carbohydrates, lipids, and other valuable organic compounds. Recently an increasing attention focused all over the world on commercial-scale production of microalgae for aquaculture feeds due to their better fatty acid profiles. Compared to terrestrial plant proteins and oils, microalgae have reasonable quantity of DHA and EPA (Acquah et al., 2020). They can propagate under different conditions (autotrophic, heterotrophic and mixotrophic) by assimilating simple nutrients and accumulate useful metabolites like n-3 LC-PUFA and carotenoids (Hardwood, 2019).
Microalgae can deliver many vitamins specially vitamins D and K produced in little quantity in the landdwelling plants. This insufficiency can be fulfilled by adding microalgae in the aqafeed that can also provide other vitamins (A, B, C, D, and E) (Del Mondo et al., 2020;Kiran and Mohan, 2021). In a study, Arthrospira platensis and Chlorella vulgaris were used in the aquafeed to replace fishmeal given to post-larvae of freshwater prawn (Macrobrachium rosenbergii) to investigate its influence on vitamin C and E, antioxidant potential, catalase, and lipid peroxidation activities. After 3 months, a 50% substitute of the fishmeal with A. platensis has significantly enhanced the growth of M. rosenbergii (Radhakrishnan et al., 2016). Recently the total folate content was determined in different species of marine microalgae. The marine microalgae Picochlorum sp. showed the highest folate content (6,470 ± 167 µg/100 g dry biomass), followed by Chlorella vulgaris (3,460 ± 134 µg/100 g dry biomass), and other tested strains (Woortman et al., 2020). In another study among seven microalgae species cyanobacterium (Anabaena cylindrical) was find out as a rich source of vitamin K1 producing 200 μg g −1 on a dry-weight basis, which is about six-fold greater than its rich dietary sources (spinach and parsley) and can be further increased by optimizing the growth conditions (Tarento et al., 2018).
Microalgae strains display a tremendous variation in the inorganic content (ash) or mineral composition due to their existence in diverse habitats, wide-ranging environmental factors, and different genetic composition. The minerals content in microalgae varies from 20-40% that play a significant role in the structural, physiological, catalytic, and regulatory functions of the aquatic organisms (Fox and Zimba, 2018). In a study, the micro and macro minerals of five maritime microalgae strains showed capricious ranges in calcium, phosphorus, magnesium, potassium, sodium, and sulfur as 0. 26-2.99, 0.73-1.46, 0.26-0.71, 0.67-2.39, 0.81-2.66, and 0.41-1.38%, respectively. The chlorophyte (Tetraselmis chuii) showed the highest level of calcium and phosphorus as 2.99 and 1.46%, respectively. Similarly, the bacillariophyte (Phaeodactylum tricornutum) showed highest level of magnesium, potassium, sodium, and sulfur content respectively. Recently the mineral conformation was investigated in maritime microalgae comprised 26 chemical elements (Al, B, Ba, Be, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Sn, Sr, Ti, Tl, V, and Zn). The tested strains showed a high inorganic content ranges from 12.9-36.3% mass of the examined elements per dry biomass (Tibbetts et al., 2015;Silva et al. 2015). It is also useful for a sustainable aquaculture industry through blue revolution by the minimum use of water, farming land, nutrient recycling, CO 2 conversion, and remediation of wastewater (Tibbetts, 2018;Yarnold et al., 2019). Consequently, microalgae-aquaculture association is an emerging paradigm for sustainable aquaculture production that will ultimately shift the aquaculture industry into an O n l i n e F i r s t A r t i c l e ecofriendly circular bioeconomy.

NUTRITIONAL POTENTIAL OF MICROALGAE IN AQUACULTURE
Microalgae are very diverse group of eukaryotic photosynthetic organisms usually existed in marine and freshwater environments (Daneshvar et al., 2018). They can propagate in different forms such as single cells or in chains or in the form of trivial clusters (Postma et al., 2016), and play a vital role in marine environment by utilizing the sunlight and CO 2 for the synthesis of different biomolecules like proteins, polysaccharides, lipids, vitamins, or pigments (Ibrahim et al., 2020;Moha-Leon et al., 2018). Consequently, they play an important role in nourishing the trophic chains in the aquatic environments and world widely distributed with >7000 species in diverse environments (Bellou et al., 2014;Shah et al., 2016).
Microalgae are the principal food source by providing necessary nutrients to the zooplankton and lower to higher trophic fish in the food chain (Yarnold et al., 2019).
Different microalgae species comprise up to 60% protein, 60% carbohydrates or 70% oils based on the specificity of the strain and respective growth conditions (Draaisma et al., 2013) and contain valued pigments, growth hormones and secondary metabolites with substantial antimicrobial, antioxidant, anti-inflammatory and immunostimulant characteristics that are very beneficial for aquatic organisms (Garcıa-Chavarrıa and Lara-Flores, 2013;Shah et al., 2017). Consequently, microalgae can be incorporated in aqafeeds to nourish the fish larvae, mollusks, and crustaceans. It can be also used as live food to feed the zooplanktonic organisms like rotifers and micro-crustaceans (Copepod, Cladocera and Artemia sp.) that are live prey of maritime and crustacean larvae (Conceicao et al., 2010;Hemaiswarya et al., 2011;Perez-Legaspi et al., 2018;Yarnold et al., 2019). Microalgae is a balanced feed source of protein, lipid, and carbohydrate suitable to protect fish health. Table I shows the nutritional content of microalgae compared with other alternative feed ingredients.

MICROALGAE AS A POTENTIAL PROTEIN SOURCE IN AQUAFEED
The microalgae biomass contain protein as an important component and its yield is dependent on many aspects e.g. type of specie, growth circumstances (pH, light, and temperature), nutritive value and environmental conditions. Nitrogen is an essential element to increase the protein yield of microalgae. A higher quantity of protein has been reported in microalgae when grown in high nitrogen concentration. The aquaculture industry uses ~70% of high-protein aquafeeds for the enhanced growth of aquatic organisms (Ansari and Gupta, 2019;Hua et al., 2019). The aquafeeds usually contain high quantity of protein with all the required amino acids however, majority of the current aquafeeds based on terrestrial plant protein are missing in some of the essential amino acids for instance lysine, methionine, threonine etc. It has been reported that microalgae contain virtually all the required amino acids therefore instead of terrestrial plants inclusion of microalgae in the aqafeed can produce more nutritious aquatic organisms that will be beneficial for human health (Chrapusta et al., 2017).
According to the recommendations of WHO/FAO/ UNU vis-a-vis humans body need for essential amino acids the microalgae species (Chlorella and Arthrospira) contain high quality proteins and their amino acids profiles are almost similar to the protein sources (eggs and soybean) (Chronakis and Madsen, 2011). Microalgae can be added as feed or feed additives in the prospective aquafeed formulation of fish, shrimp, crab, shellfish, sea cucumber and other aquatic organisms. Microalgae was grown in an outside raceway reactor provided with digestate that was partially substituted (10% of the diet) in aquafeed of the Acipenser baerii. The outcomes of the experiments have confirmed the practicability to grow microalgae on digestate shown higher yield (6.2 gDM m −2 d −1 ) with enhanced nutrient removal and reducing the chemical oxygen demand. The feeding test of the experiment compared with control groups (p > 0.05) shown better growth recital, somatic directories, fillet nutritional configuration and celiac function point out the significance of microalgae as protein source could be used in Siberian sturgeon aquafeed (Bongiorno et al., 2020).

MICROALGAE AS A NATURAL SOURCE OF POLYUNSATURATED FATTY ACIDS
Microalgae are a rich source of lipids that represents 74% of microalgae's total biomass depends on the species (Bernaerts et al., 2019). The lipids are made of fatty acids by 12-24 carbon atoms that comprise polyunsaturated fatty acids of n-3 PUFAs and n-6 PUFAs families, respectively (Patras et al., 2019). Microalgae produces eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) that are vital for the growth, reproduction, immunity, and nutritional value of aquafeed (Remize et al.2021). Many researchers are currently investigating the effectiveness of microalgae that can be developed as non-FM or -FO diets. In one of such study fish feed and oil was substituted by algae feed (Schizochytrium sp. powder) and plant proteins. The results imply the potential of microalgae as an alternative source of Omega-3 fatty acid proceeding marine fish (Oliver et al., 2020). Currently the nutritional digestibility of a maritime microalga (Schizochytrium spp.) enriched in DHA and LC-PUFA was determined as an alternative lipid source fed to rainbow trout (Oncorhynchus mykiss). The results demonstrated the ADCs of the nutrients, energy, DHA and other fatty acids profiles of Schizochytrium spp. as an excellent alternative of fish oil replacement that may be provided with extra LC-PUFA in fish feed with vegetable oils (Bélanger et al., 2021).
Microalgae embody an auspicious prospect of Omega-3 PUFA yield, and several species have the capability to naturally synthesize EPA in high quantity such as Nannochloropsis, Phaeodactylum tricornutum, Odontella aurita, Monodus subterraneus, and Pophyridium cruentum (Bernaerts et al., 2018). The EPA levels (12.74 ± 1.84% and 10.93 ± 1.84%) have been reported in Nannochloropsis salina CCMP1176 and Nannochloropsis oceanica CCMP537 proceeding total fatty acids (Ma et al., 2014). Similarly, in another study, EPA levels (22.4 -31.4 ± 1.7%) was reported in Phaeodactylum tricornutum (Ryckebosch et al., 2014). At present, numerous enterprises are producing EPA-rich microalgae biomass under photoautotrophic conditions such as Shenzhen Qianhai Xiaozao Technology Co., Ltd. of China harvests EPA rich lipid extract from Nannochloropsis salina. Additional companies are Arizona Algae Products, LLC (US) or Simris Alg AB (Sweden) (Oliver et al., 2020). Currently microalgae (Aurantiochytrium sp.) which as key source of n-3 PUFAs has been investigated to determine its potential on the growth performance and immune response of Trachinotus ovatus, supplied with different microalgae content (1.00-11.00%) for 8 weeks in its diets. The results revealed that adding Aurantiochytrium sp., in the diet has a beneficial paraphernalia on the fish survival, weight gain, and explicit growing rate improved by 1.02, 1.16, and 1.08 times, respectively (Li et al., 2021).

MICROALGAE POLYSACCHARIDES AS IMMUNOSTIMULANTS IN AQUACULTURE
Aquaculture is the rapidly growing industry faces the problem of disease outbreaks usually controlled by traditional methods of using antibiotics and chemical disinfectants; however, they have the problems of resistance O n l i n e

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Microalgae as Feed Supplements for Sustainable Aquaculture development and bioaccumulation of toxic residues in aquatic organisms. Vaccines are an effective way of disease control, but its use is time consuming, expensive, and traumatic to the fishes. Therefore, immunostimulants are natural compounds that trigger the host defense system against infections. Currently microalgae polysaccharides are used as immunostimulants to control the diseases in aquatic organisms have been focused due to its less toxicity, bioactivity, and environment friendly nature (Marudhupandi and Inbakandan, 2015). The bioactivities and applications of sulfated polysaccharides from microalgae has been reported as anti-inflammatory, immunomodulatory, antiviral and antioxidant properties in the aquatic organisms (Raposo et al., 2013;Amna et al., 2018;Mohan et al., 2019;Nastasia et al., 2020). In a recent report, sulfated polysaccharides have been isolated from Codium fragile that have immuno-stimulatory effects on Olive flounder and can be utilized as feed additive to heighten the immunity of fish (Yang et al., 2019). Microalgae can be used as nutritional supplements in the aquafeeds for their potent immuno-stimulatory effects in aquatic organisms. Currently it was observed that supplementing C. vulgaris at 10% in the meal of O. niloticus has protected it beside arsenic-induced immunotoxicity and oxidative stress (Zahran et al., 2018). Similarly, adding C. vulgaris at 6% in the meal of gigantic freshwater prawn (M. rosenbergii) has shown enhanced prophenol oxidase activity with the entire quantity of hematocytes of M. rosenbergii post larvae that might improve the larval survival to Aeromonas hydrophila infection (Maliwat et al., 2017). In another study C. vulgaris was added as dietary supplementation of nile tilapia (O. niloticus) to protect it against sublethal concentrations of penoxsulam herbicide and improve its anti-infective capacity against Aeromonus sobria (Galal et al., 2018). Similarly, the dietary intake of 5% Schizochytrium limacinum has encouraging results in improving the intestinal health and nutrient utilization potential of rainbow trout O. mykiss (Lyons et al., 2017).

MICROALGAE CAROTENOIDS AS FUNCTIONAL FEED ADDITIVES IN AQUACULTURE
Microalgae produce carotenoids with distinctive antioxidant and coloring characteristics including xanthophylls e.g., zeaxanthin, lutein, antheraxanthin that are found in land-dwelling plants. Moreover, they can also produce other pigments (astaxanthin, fucoxanthin, diatoxanthin, diadinoxanthin) specifically found in algae, cyanobacteria, and some species of yeast (Novoveská et al., 2019;Ambati et al., 2014Ambati et al., , 2019. Numerous carotenoids are used in the aquaculture industry to color farmed fish especially astaxanthin is utilized to augment the pigmentation in farmed salmon. The pigmentation of fish is an important factor that can stimulate the consumer's choice to buy it. Carotenoids are not only important for coloring but also show a significant role in the growth, reproduction, and health care of aquatic organisms (Alfnes et al., 2006;Lehnert et al., 2019;Costa and Miranda-Filho, 2019).
The aquatic animals are unable to synthesize carotenoids therefore, microalgae can be provided as feed additive in their meal which are their naturally producers. An important carotenoid astaxanthin that is commercially used in the aquaculture industry produced by a microalga (Haematococcus pluvialis) at>4 percentage per DW that is a promising yield as compared to other organisms (Butler et al., 2018). Spirulina was added as a carotenoid source (0, 2.5, 5, and 10% of fishmeal weight) in the feed of yellow tail cichlid Pseudotropheus acei. The data shows a significant increase in total eggs production, percentage of eggs hatching, enhanced growth rate and raised carotenoids level in the skin of experimental one as compared to the control group of fishes (Güroy et al., 2012). In a study, four fish meals were supplemented with the carotenoids (astaxanthin, lutein, canthaxanthin and lutein+canthaxanthin) standardized at 50 mg kg -1 in the diet of goldfish juveniles compared to control (without carotenoids). The meal with lutein, astaxanthin and canthaxanthin showed a greater persistence values and increased carotenoid pigmentation if the skin of goldfish juveniles as compared to control treatments (Besen et al., 2019).

MICROALGAE AS POTENTIAL SUBSTITUTION OF THE CONVENTIONAL CONSTITUENTS IN AQUAFEED
The aquaculture production has tremendously increased during the last decade due to the amassed consumer's demand. Hence, this sector needs massive quantities of aquafeed that depends on FM, FO, and terrestrial plants, problems of low nutrients status, less availability, and expansive. To overcome these issues, microalgae is the best economical and alternative feed ingredient in aquafeed. Currently live microalgae strains as a whole or lipid-extracted algae (LEA) have been tried in aquafeed that have significantly enhanced the growth performance, physiological movement, and nutritional status of the aquatic species . Numerous studies have been conducted to determine the potential substitution of conventional constituents in aquafeed with microalgae as shown in Table II   The Nannochloropsis sp. has been used as living food in aquatic species in a high-rate algal pond (HRAP) system on zootechnical basis considering morphometric parameters, and dietary nutrient digestibility of the celiac system of Dicentrarchus labrax. According to the results 10% of terrestrial plant ingredients replaced with microalgae has significantly increased the final body weight of the fish without disturbing its growth O n l i n e

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I. Ahmad et al.
performance, dietary nutrient utilization, and gut enzymatic activities (Pascon et al., 2021). Microalga (Schizochytrium spp.) has been investigated to determine its digestibility, macronutrients availability and individual fatty acids (omega 3-rich) in Oncorhynchus mykiss. Thus, the microalgae were found to be a potential ancillary of FO and LC-PUFA in FM (Bélanger et al., 2021). Similarly, Schizochytrium sp. was included as 30% in the diets of S. salar that could reinstate the fillet n-3 LC-PUFA content as well as increase nutritional quality of the product for consumers (Hart et al., 2021). In another study addition of C. vulgaris (4-8%) in diets of Macrobrachium rosenbergii delivered best growth rates and enhanced immunity of the post larvae (Maliwat et al., 2021). The traditional aquaculture is facing with the problems of eutrophication and depletion of oxygen for the aquatic organisms. In this regard, microalgae can be helpful in providing oxygen during its natural photosynthetic process as well as assimilating the nutrients released from the sludge and providing a conducive environment for the better growth and survival of aquatic organisms (Chen et al., 2020). The Schizochytrium sp. was added in the diet of O. niloticus reared in net cages that has beneficial effects on the red blood cells, lymphocytes, and intestinal microbiota without any contrary effects on the structure and integrity of the intestinal villi (Souza et al., 2020).
Aquaculture companies have focused to reduce the cost of aquafeed by replacing FM and FO by adding sustainable and cost-effective microalgae to produce fish free feed for O. niloticus. Such an effort was carried out by adding two microalgae species to replace FM and FO in the diet of farmed fish, which has beneficial effects such as highest amount of lipid, protein and DHA in the fillet as compared to the conventional feed (Sarker et al., 2020a). Similarly, three microalgae species (Nannochloropsis sp., Isochrysis sp., and Schizochytrium sp.) have been added in the diet of O. niloticus to substitute FM and FO in their feed. A significant improvement in ADCs of the crude protein, amino acids, lipid, and other fatty acids was observed in the farmed fish (Sarker et al., 2020b).
Integrated algae-aquaculture systems provide a suitable platform to develop ecofriendly, economical, and sustainable aquafeeds. Accordingly, different microalgae based FMs, the diet comprising 7.5% of whole and LEA has delivered essential nutrients with significant growth performance indicators (FCR 1.36 g/g, PER 1.84 g/g, and HSI 2.01%) in O. niloticus cultivated in a raceway pond (Ansari et al., 2020). Spotted wolf fish (Anarhichas minor) dominate the cold-water aquaculture. Its FM was substituted up to 15% of the N. oceanica that has resulted in high level of omega 3-fatty level in the muscle, liver, and whole body of all treatment sets, replicating the use of ~ 50% plant-based ingredients in the diets (Knutsen et al., 2019a). Moreover, Scenedesumus obliquus can be used as a substitute and valued feed constituent to partially replace FM to the existence of extraordinary protein content (above 50% of dry matter). Therefore, substituting 4% of FM has tremendously increased the body weight of the fish from 140 to 250 g after 12 weeks with rapid muscle growth and proximate arrangement of muscle suggesting the potential use of microalgae in aquaculture (Knutsen et al., 2019b).
Microalgae can be incorporated in FM with a particular percentage to progress the growth configurations, stress comebacks, liver functions, physiological events, and disease resistances of numerous fish varieties. For example, Lipid-extracted microlagae (LEA) meal mixed by soy protein (10% of the diet) to replace 25% of FM in the tested diet has shown good growth performance with growth indicators (FCR 1.19 g/g, PER 1.76 g/g, and HSI 2.00%) matched to the control diet in Perca flavescens (Jiang et al., 2019). According to Younis et al. (2018) the FM of O. niloticus is substituted by 20% G. arcuate that has significantly increased their body weight from 13.01 to 36.13 g after 12 weeks. Furthermore, Sarker et al. (2018) investigated that 33% LEA substitution of the FM has significant impact on the growth performance, feed utilization, and persistence comparable to control diet. After 12 weeks, the body weight increased from 1.98 to 28.06g with the growth indicators (FCR 1.26 g/g, and PER 2.12 g/g). Kiron et al. (2016) found that increasing the integration of Desmodesmus sp. as of 10% to 20% in FM has no adverse effects on the feed consumption and health of Salmo salar L. Similarly, Norambuena et al. (2015) found that Ulva ohnoi and Entomoneis spp. with inclusion levels of 2.5% and 5.0% could be added in FM of Salmo salar L that shows an enhanced feed efficacy (rich in n-3 LC-PUFA) related to the reference diets.
Digestible protein (DP) content is an important feed ingredient that is necessary for the development of new diet formulations for the acquaculture industry. The Protein degree of hydrolysis (DH) and predicted protein apparent digestibility coefficients (ADCs) of N. granulata algal meals can provided key parameters of its incorporation in the meals of L. vannamei (Tibbetts et al., 2017). In other report microalgae, concentrates have been used as larval feed of T. noae to determine its ingestion and digestion efficiency that was subjective to the type of microalgae and larval age (Southgate et al., 2017). The dietary potential of Schizochytrium as a meal supplement has been accessed in a feeding trial to investigate the survival, growth performance, digestive enzymes, and fatty acid configuration in the larvae of Litopenaeus vannamei. It was observed that addition of 4% Schizochytrium meal in microdiets of shrimps could progress their growth performance and other essential life activities (Wang et al.,

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2017). According to Kousoulaki et al. (2016), adding 5% whole biomass of Schizochtrum sp. in the extruded meal of Salmo salar L effectively replaced FO deprived of any adversarial effect on their growth performance, preservative ability of nutritional value. According to Kissinger et al. (2016) FM replaced by microalgae up to 80% showed no sizable effect on the growth performance or intestinal integrity of S. rivoliana with an improved body weight from 2.5 to 74.0 g after 9 weeks. Sørensen et al. (2016) scrutinized the whole cell microalgae Phaeodactylum tricornutum as an impending feed constituent for Salmo salar. A direct decline in apparent digestibility coefficients (ADC) was detected for protein, lipid, and DM to replace FM with P. tricornutum biomass from 0-12% in the feed. The algae biomass can substitute 6% of the FM devoid of any contrary impact on nutrient digestibility, growth and feed consumption of the fish . The enhanced digestibility of crude protein and many essential amino acids found in Spirulina sp. recommend it as a good contender to be considered as an alternative protein source while Schizochytrium sp., contain maximum quantity of lipid and unsaturated fatty acids is a good candidate of FO substitute in tilapia feed. These microalgae species have been investigated for the apparent digestibility of macronutrients, amino acids and fatty acids in the Nile tilapia (Sarker et al., 2016). Similarly, the potential of Pavlova viridis as a PUFA source was assessed by comparing to Nannochloropsis sp. in the diets of Dicentrarchus labrax L. during 8-week feeding trial. It was observed that 50-100% microalgae could be added to replace FO in the diet of Dicentrarchus sp has no adverse effects on their growth performance and nutrient utilization (Haas et al., 2016). In another study the nutrient digestibility, growth performance, biometry, dressing out parameters, fillet muscle proximate and fatty acid composition of Dicentrarchus labrax L. have been investigated by replacing 20% PR and up to 36% FO with freeze-dried biomass of Isochrysis sp. has no adversative effects, feed intake or growth performance as compared to controls (Tibaldi et al., 2015). Another study has assessed the effects of diets comprising 0, 2.5, 5, 7.5 and 10% of Spirulina platensis on the hematological and serum biochemical factors of Oncorhynchus mykiss. It was found that replacing 10% diet with Arthrospira sp has significantly increased the red and white blood count, hemoglobin, total protein, and albumin levels in Oncorhynchus sp. (Yeganeh et al., 2015).

INTEGRATED MICROALGAE-AQUACULTURE SYSTEM, A SUSTAINABLE BIOREFINERY APPROACH
The current aquaculture industry is facing the problems related to the environmental safety and food security. Therefore, researchers have focused to resolve these problems to develop a sustainable aquaculture. The main problems in the customary aquaculture are water deterioration and antibiotics misuse that are not only responsible for resistance development but also polluting the water body . The water deterioration generally occurs due to depletion of oxygen, detrimental algal bloom, and eutrophication of the water used in aquaculture. These problems possess serious threat for rearing the aquatic species as well as causes environmental pollution Liu et al., 2014). One of the main causes of water deterioration is due to the excessive use of customary aquaculture feed comprised biomass contain protein and lipid that is not fully utilized by the aquatic animals. The remaining feed is changed to soluble nutrients by specific microorganisms ultimately causes eutrophication in water body . Water deterioration also occurs due to the wastes secreted by the aquatic species that in the end can cause diseases in the aquatic animals ultimately their death (Lamb et al., 2017;Bhatnagar and Devi, 2013). To overcome these problems a biorefinery approach can be a cost-effective and sustainable paradigm in which microalgae cultivation and aquaculture, integrated for shared benefits (Shaalan et al., 2018). Figure  1 shows an integrated microlage-aquaculture system for sustainable aquaculture production. Fig. 1. The integrated microlage-aquaculture system for sustainable aquaculture production.
The integrated system of using microalgae in aquaculture system is an emerging paradigm that can be adopted to develop an ecofriendly and sustainable aquaculture. Aquaculture industry still facing the problems of expansive aquafeeds while its conventional ingredients possess environmental issues. Considering these problems microalgae is the best candidate, which comprise essential nutrients, can replace FM and FO in aquafeeds of the aquatic species to produce cost-effective and high-quality nutritious food for the malnourished population of the developing counters. However, the diversity in microalgae strains, their nutrients composition, environmental factors, and formulation in aquafeeds needs further research. Moreover, an integrated and economically sustainable biorefinery approach can be implemented to cultivate microalgae as aquaculture feed whereas the aquaculture wastewater used as a nutrients source to produce useful biomass. Microalgae has significant environmental benefits of fixing carbon dioxide and wastewater remediation, thus reducing the pollution problem caused by aquaculture wastewater. Therefore, considering the environmental and economic facets, microalgae-assisted aquaculture needs to be developed that will open new avenues in aquaculture and environmental sustainability.

MICROALGAE BASED AQUACULTURE WASTEWATER REMEDIATION
Microalgae have the potential to assimilate nitrate, nitrite, ammonia, phosphate, and organic carbon from aquaculture wastewater for their growth. Aquatic species generate these unwanted compounds in the wastewater body used for rearing. Therefore, microalgae are the best candidate to remove such nutrients and convert into useful biomass that can be used as feed constituent in aquafeeds. Thus, microalgae cultivation in wastewater has exclusive benefits in terms of bio-circular economy e.g., removing nutrients from the aquatic ecosystem and producing cost effective biomass for aquaculture industry. Subsequently biomass harvest, the treated water can be castoff for rearing aquatic organisms or other useful applications to develop a sustainable and ecofriendly environment .
A sustainable biorefinery approach was investigated in aquaculture wastewater of tilapia rearing tanks by growing Chlorella sorokiniana heterotrophically to utilize wastewater substrate for dual benefits (nutrients bioremediation and biomass generation). Microalgae has significantly removed phosphate, ammonia, nitrate, and COD (chemical oxygen demand) as 73. 35, 75.56, 84.51, and 71.88% from the aquaculture wastewater with biomass productivity comprised proteins, lipids, and carbohydrates as 141.57, 150.19 and 172.91 mg/L/day . Similarly, Scenedesmus obliquus, Chlorella sorokiniana and Ankistrodesmus falcatus were cultivated in aquaculture wastewater (AWW) to investigate the biorefinary model to produce biomass with subsequent nutrient removal. A. falcatus generated biomass of 198.46 mg L −1 d −1 with added sodium nitrate (400 mg L −1 ) while C. sorokiniana produced biomass of 157.04 mg L −1 d −1 with supplemented sodium nitrate (600 mg L −1 ) in AWW as compared to the BG11 medium. Microalgae grown in AWW showed significant removal of ammonia, nitrate, phosphate, and COD in the range of 86. 45-98.21, 75.76-80.85, 98.52-100 and 42-69% respectively .
The wastewater of the recirculating aquaculture system (RAS) was used as nutrient medium to cocultivate two different species of microalgae (C. vulgaris and T. obliquus). Both strains grow vigorously than their monoculture with average removal efficiencies of nitrate (98.73±0.06) and phosphate (99.46±0.04%), respectively (Tejido-Nuñez et al., 2020). Numerous strains of microalgae have been grown in aquaculture wastewater for nutrients bioremediation and biomass generation for an ecofriendly and sustainable aquaculture (Peng et al., 2020;Nasir et al., 2019;Gupta et al., 2016). Similarly, consortia of microalgae and associated water-borne bacteria through their extracellular enzymatic activities can potentially remediate the compact wastes of aquatic species. In such a symbiotic relationship, microalgae can effectively assimilate the nutrients from the AWW refining the selfpurification capability of aquaculture system by producing high value biomass (Addy et al., 2017;Fang et al., 2017).

CONCLUSIONS
Aquaculture is a fast-growing sector playing an important role in providing high quality seafood mainly depends on FM and FO in the aquafeeds. Microalgae contain valued source of the essential nutrients required for high quality aquafeeds, comprising omega-3 fatty acids, EPA and DHA, essential amino acids, pigments, and antioxidants. Microalgae is the best source to replace FM and FO in aquafeeds due to their important role in the enhanced growth performance, physiological movement, and nutritional status of the aquatic species. Moreover, the integrated microalgae-aquaculture system provides a sustainable biorefinery approach e.g., removing the wastes of aquatic organisms and converting into cost-effective biomass. Therefore, microalgae-assisted aquaculture is necessary to develop a sustainable circular bioeconomy.

ACKNOWLEDGEMENTS
The author highly acknowledge the facilities provided by the Department of Bioengineering, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, Kingdom of Saudi Arabia for the conduction of this study.

Statement of conflict of interest
The author has declared no conflict of interest.