Multifunctional Roles of Microalgae in Sustainable Aquaculture: Nutrition, Live Feed, Bioremediation, and Carbon Sequestration

Introduction

Global aquaculture has expanded dramatically over the past few decades, growing from 20 million tons in 1970 to 88 million tons in 2020, and it is projected to supply 62% of all fish from human consumption by 2030 (1, 2). This rapid intensification is driven by a growing global population’s need for nutrient-rich functional foods to improve health and secure food supplies (3). However, the industry’s long-term sustainability is severely threatened by its heavy reliance on expensive and ecologically unsustainable commercial aquafeeds (4). Furthermore, aquatic ecosystems face escalating pressure from industrial, agricultural, and municipal pollution which introduces hazardous heavy metals, hydrocarbons, and chemical contaminants into the environment (5). To maintain global food security without competing with traditional agriculture for arable land, the aquaculture sector must transition toward innovative, circular-economy solutions that simultaneously address nutritional shortages and environmental degradation (3).

Microalgae represent a highly adaptable, diverse group of photosynthetic microorganisms ubiquitous in aquatic ecosystems that serve as primary producers and sustainable biomass sources to bridge these industry gaps (6, 7). Utilizing versatile metabolic modes including photoautotrophy, heterotrophy, and mixotrophy microalgae accumulate profound nutritional profiles containing up to 60% protein, 60% carbohydrates, or 70% lipids depending on species and growth conditions (6, 8). Because they consistently yield dry cell weight concentrations of 12-65% protein, 2-23% lipid, and 5-26% carbohydrate, they function as essential direct or indirect feeds for wild fish, larval aquatic organisms, and live prey vectors such as rotifers, Artemia, and Copepods (6, 9). Beyond their primary nutritional value and rich array of bioactive secondary metabolites (such as pigments, PUFAs, and immunostimulants (3, 8), microalgae offer vital ecological services. Their photosynthetic efficiency exceeds that of terrestrial crops by 40-50%, enabling them to reclaim wastewater, mitigate pollution, and drive substantial carbon capture, with 1 kg of microalgal biomass absorbing approximately 1.83 kg of CO₂ (3, 10).

Despite this profound dual potential as both a high value aquafeed alternative and an environmental remediation agent, widespread commercial integration remains constrained. The ultimate economic viability and production effectiveness of microalgae depend heavily on establishing optimized cultivation approaches whether photoautotrophic, heterotrophic, or mixed and developing cost-effective harvesting systems (6, 11). While past research has documented their widespread applications across the food, pharmaceutical, and biofuel sectors (3, 9), a consolidated evaluation targeting their integrated roles in modern aquaculture is required.

Despite this profound dual potential as both a high value aquafeed alternative and an environmental remediation agent, widespread commercial integration remains constrained. Whilst past literature reviews have documented the isolated applications of microalgae within independent sectors such as standalone biofuel production, basic human nutraceuticals, or generic wastewater treatment (3, 9) prior reviews have overlooked the operational, biological, and technical integration of these roles within a closed loop aquaculture circular economy. Specifically, the field lacks an assessment that bridges real world biological performance (e. g. specific nutritional yields and phycoremediation capacities) with concrete techno-economic realities (CAPEX/OPEX constraints and processing bottlenecks). To bridge this research gap, this systematic review establishes an integrated framework that address how microalgal biomass can simultaneously function as a nutritional feedstock and a bioremediation agent without compromising economic viability.

The specific objectives of this review are to: (i) quantify and map specific microalgal species to their dual roles as larval feed/enrichers and wastewater remediation agents; (ii) evaluate regulatory frameworks governing microalgae in the global food chain; and (iii) synthesize techno-economic data to deliver an optimized structural framework for viable, closed-loop microalgal aquaculture systems.

Methodology

Search Strategy and information Sources

To ensure a comprehensive and reproducible collection of relevant literature, a systematic search was conducted across the PubMed and Google Scholar databases, covering the time frame from 2000 to 2025. The search strategy targeted literature tracking the multifunctional applications of microalgae within aquaculture frameworks following (12). Specific Boolean operators and search strings were deployed, including microalgae role in aquaculture, aquaculture, live feed and aquafeed, phycoremediation, wastewater treatment, carbon sequestration and carbon dioxide fixation of microalgae. To minimize publication bias, reference lists of retrieved review articles were manually screened to identify additional relevant primary studies.

Inclusion and Exclusion Criteria

Studies were screened based on strict, predefined inclusion and exclusion criteria to ensure data integrity and alignment with the review’s core objectives. To be included, studies had to: (i) be peer-reviewed, primary research articles or book chapters published in English between 2000 and 2025; (ii) explicitly quantify microalgae dynamics such as nutritional yields and pollutant removal rates within aquaculture systems; and (iii) evaluate at least one primary functional mode, specifically live feed/supplements, phycoremediation or carbon capture. Studies were excluded if they: (i) focused exclusively on terrestrial agricultural applications, wild marine ecosystems without aquaculture context, and standalone biofuel engineering lacking a circular aquaculture component; (ii) consisted of grey literature, conference abstracts and non-peer-reviewed reports; (iii) lacked quantifiable quantitative metrics.

Screening Process and Study Selection (PRISMA Framework)

The study selection process strictly followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (12, 71). The initial database search yielded a total of 1240 records. After removing duplicates (n= 410), the remaining 830 record were screened by title and abstract, resulting in the exclusion of 690 record that did not meet the core topic criteria. The full texts of the remaining 140 articles were thoroughly assessed for eligibility. Out of these, 69 articles were excluded due to missing quantitative data, irrelevance to aquaculture circular economics, and structural baseline deviations. Ultimately, 71 primary papers met all criteria and were synthesized into the final review.

Data Extraction, Synthesis, and Quality Assessment

Data extraction was systematically organized into standardized matrices focusing on technical components, microalgae species, application benefits, and techno-economic implications (CAPEX/OPEX). For each included study, scientific metrics were extracted: microalgae taxonomic classification, nutritional dry weight configurations (protein and lipid percentages), pollutant removal efficiencies (N and P removal percentages), and carbon dioxide fixation capacities.

Concurrently, a qualitative risk-of-bias and quality and quality assessment was conducted on the primary studies, evaluating baseline parameters such as experimental control validity, replication adequacy, and method transparency. Studies exhibiting high methodological ambiguity were omitted. The final data synthesis employed a narrative and tabular synthesis approach, grouping findings into four core thematic quadrants: direct/indirect larval nutrition, greenwater cultivation dynamics, wastewater phycoremediation mechanisms, and techno-economic boundary constraints.

Taxonomic Classification and Global Regulatory Frameworks

Microalgal groups possess unique structural, evolutionary, and biochemical profiles that dictate their suitability for commercial aquaculture. Eukaryotic Chlorophyta (Chlorella, Dunaliella, and Haematococcus) are widely utilized due to their high accumulation of pigments and antioxidants. Conversely, Diatoms (Bacillariophyta) feature distinct silica-based cell walls which, while offering valuable omega-3 fatty acids, introduce mechanical processing limitations that restrict direct whole cell ingestion for certain delicate larval classes. This starkly contrasts with prokaryotic cyanobacteria like Spirulina, whose peptidoglycan-based cell walls lack rigid cellulose, providing an easily digestible texture ideal for direct, unprocessed whole-biomass utilization (2).

Despite their profound biological benefits, widespread entry into the global food chain remains highly dependent on stringent institutional frameworks (Table 1). The U. S. Food and Drug Administration (FDA) has granted Generally Recognized as Safe (GRAS) status to primarily clusters like Arthrospira sp., Chlorella sp., Dunaliella sp., and Haematococcus pluvialis (13). Under European safety baselines, the European Food Safety Authority (EFSA) regulates specific microalgae under the Novel Food Regulation (EU 2015/2283), approving Aphanizomenon flos-aquae, Arthrospira platensis, Chlorella vulgaris, Tetraselmis chuii, and Haematococcus pluvialis (13).

However, a critical regulatory and safety gap persists: current approvals are heavily skewed toward human nutraceutical baselines, leaving a clear deficiency in standardized, international regulatory frameworks tailored explicitly to closed-loop aquaculture feeds and recycled wastewater cultivation media. This absence creates significant biosecurity and compliance risks for commercial implementation, potentially delaying sustainable global adoption.

Nutritional Optimization and Micro-Ecosystem Applications in Aquafeeds

Global aquaculture production is overwhelmingly dominated by Asia, which accounts for 92% of global output, with China alone contributing more than 60% of this total (14). Within these production systems, the highly sensitive larval stages of marine mollusks, echinoderms, crustaceans, and fish, filter-feeding organisms rely fundamentally on the 3-50 μm cell size of microalgal cells as their primary nutrient delivery system (1, 15). Microalgae anchor aquatic food webs, delivering structural proteins, essential amino acids, carbohydrates, polyunsaturated fatty acids (PUFASs like DHA and EPA that aquatic animals cannot newly synthesize), carotenoids, antioxidants, immunostimulants, and antimicrobials (11, 16).

Cultivating microalgae enhances aquaculture system sustainability vi a multi-pronged strategy: serving directly as live nourishment for larvae and filter- feeders (such as raising bivalves, clams, and mussels from hatchings to adults, as well as feeding juvenile abalone, shrimp, and fish) and indirectly as nutrient vectors to “gut-load” or enrich intermediate zooplankton vectors like rotifers, Artemia, and copepods (1, 16, 18, 27) (see Table 2).

Methodologically, the selection of specific strains directly determines target species growth metrics, survival rates, seedling development, body length, weight, and environmental tolerance (16, 17, 18) (see Table 3). Crucial species like Arthrospira platensis, Chlorella sp., Pavlova sp., Dunaliella sp., Haematococcus sp., and Nannochloropsis sp. are highly prized for their nutritional profile and aquaculture compatibility (17). Other species, including Scenedesmus sp., Schizochytrium sp., Haematococcus pluvialis, Spirulina platensis, Isochrysis sp., and Chlorella vulgaris, actively promote the growth of shellfish, larval fish, and zooplankton. For example, Nannochloropsis sp. significantly improves broodstock egg quality, fertilization rates, and subsequent larval survival (16).

Furthermore, combining specific microalgal species provides complementary nutrient ratios that can act as a financially insight alternative for ocean-based incidents. For instance, a combination of Nannochloropsis oculate and Schizochytrium sp. provides a complementary lipid-protein ratio that successfully replaces conventional fish meal and fish oil in the diets of globally critical model species like Nile tilapia (Oreochromis niloticus) the second largest farmed fish in the world and rainbow trout (Oncorhynchus mykiss), a key model species for salmonid aquaculture (19, 33). This specific microalgae-based formulation enhances final nutritional quality, boosts fish growth metrics, and yields significantly higher apparent digestibility coefficients for crude proteins, amino acids, and essential polyunsaturated fatty acids (PUFAs like DHA AND EPA) compared to standard fishmeal and fish oil baselines (3). For bivalve farming, critical microalgae identified for direct feeding include Isochrysis galbana, Isochrysis sp. (T. ISO), Pavlova lutheri, Tetraselmis suecica, Pseudoisochrysis paradoxa, Chaetoceros calcitrans, and Skeletonema costatum (19, 27).

However, a recurring limitation in existing literature is the tendency to treat nutritional success as a universal metric. In reality, performance varies dramatically depending on the target host physiology; while certain flagellates significantly boost growth in bivalve hatcheries, the same strains fail to support consistent weight gain in carnivorous finfish larvae without secondary enrichment, highlighting a clear need for host-specific baseline nutritional mapping (1, 11, 27).

Integrated Ecological Services: Phycoremediation and Carbon Capture

The continuous expansion of global aquaculture creates a severe environmental paradox, generating high nutrient effluents (ammonia, nitrates, and phosphates) that drive the eutrophication of receiving water bodies (5, 53). Microalgae actively mitigate this environmental load via interconnected autotrophic, heterotrophic, and mixotrophic pathways (6, 11) (see Table 4).

The core efficiency of phycoremediation relies on three concurrent biological phenomena:

1. Bioassimilation: Direct enzymatic uptake of inorganic nitrogen and phosphorous for cellular protein and nucleic acid synthesis, achieving up to 99% total pollutant clearance (9, 57). Beyond aquatic waste streams, microalgal biostimulants, such as Chlorella vulgaris suspensions, show parallel efficiency in alternative agricultural configurations like hydroponics. Utilizing specialized suspensions can enhance growth, nutrient uptake, and total biomass yield in crops like lettuce, where specific concentrations achieve maximum increases in leaf area (67.4%), root fresh weight (138.7%), phosphorus (96.4%), potassium (59.5%), and iron (43.8%) assimilation (4).

2. Bioadsorption: The immediate, passive adhesion of positively charged toxins to the highly functionalized, negatively charged outer surfaces of the algal cell wall (54, 56).

3. Bioaccumulation: The active, metabolic influx of persistent contaminants (heavy metals, agricultural pesticides) across the cellular membrane (52, 55).

A significant critical variance exists in remediation kinetics between target pollutants. While the removal of inorganic fertilizer fractions and the passive biadsorption of toxic heavy metals are highly efficient, the bioremediation of long-chain hydrocarbons (C₁₆-C₄₀) remains far less reliable (9, 57, 68). Strains like Tetradesmus sp. and Arthrospira maxima demonstrates localized, synergistic success in oily wastewaters, yet real-world degradation rates fluctuate widely based on seasonal temperature shifts and hydraulic retentions times (67, 68).

Concurrently, this metabolic activity drives substantial biological carbon mitigation (32). The combustion of fossil fuels, which accounts for more than 80% of global energy consumption, releases over 24 gigatons of CO₂ into the atmospheric CO₂ concentrations from approximately 295 ppm to over 380 ppm, leading to global warming, severe climate shifts, and ecological extinctions (58). Physical, chemical or biological carbon sequestration techniques aim to reduce this atmospheric burden (28).

Human activities continue to exacerbate pollution, releasing detrimental pollutants like heavy metals, chemicals, and pesticides into the environment through municipal, industrial, and agricultural sources (8). Heavy metals represent a group of metallic elements known for high toxicity, which poses risks to both environmental systems and human health (9). The contamination of water bodies by heavy metals poses a serious threat because these substances are both long-lasting and harmful to living organisms (9).

Microalgae are highly versatile microorganisms capable of growing under diverse environmental conditions while fixing carbon 10 to 50 times more efficiently than terrestrial crops (29). Due to a photosynthetic efficiency that exceeds terrestrial crops by 40-50%, microalgae actively recycle waste CO₂ into stable organic molecules. This process is highly productive with 1kg of cellular biomass absorbing approximately 1.83 kg of CO₂ (3, 10, 29). Optimal carbon dioxide fixation rates regularly exceed 1.5 kg/m²/year, while concurrently producing intracellular lipids rich in valuable unsaturated fatty acids (18). Microalgae display high tolerance to diverse carbo inputs, efficiently utilizing atmospheric CO2, soluble carbonates (such as sodium bicarbonate and sodium carbonate), or concentrated industrial flue gases (29). According to circular economy concepts, carbon mitigation via microalgal capture is doubly advantageous, as the resulting biomass can be directly converted into biofuels, plastics, biochar, and nutrient-dense animal feeds (31, 32).

Systemic Health, Coloration, and Bioaccessibility Dynamics

Beyond macro- nutritional delivery, microalgal biomass introduces specialized secondary metabolites that modulate aquatic animal immunity, cellular protection, and market value (3, 8).

Immunological Modulation and Gut Health

Supplementingaquaculture diets with microalgae improves growth and weight gain, decreases relative feed consumption, enhances overall disease resistance, and exerts antibacterial and antiviral activities (3). Incorporating low-dose (0.5%) Haematococcus sp. and Anabaena sp. fractions into feed formulations significantly upregulates lysozyme activity and immune responses. This reduces mortality when animals are challenged by virulent aquatic pathogens like Vibrio anguillarum (1, 36, 37). For example, specific bioactive compounds produced by Anabaena sp. have shown the capacity to trigger targeted cell death in myeloid leukemia cell lines (36). Similarly, juvenile rainbow trout feed a 0.5% Haematococcus sp. diet exhibit enhanced morphological parameters, strengthened immune profiles, and robust disease resistance against Vibrio anguillarum (37).

Natural Coloration

The growing demand within the food and aquaculture industries for natural pigments over synthetic alternatives has elevated the commercial value of microalgal production systems (38). Microalgae synthesize structurally diverse, chemically distinct natural pigments including chlorophylls, carotenoids, and phycobiliproteins, which find widespread applications across the food, feed, cosmetics, and medical sectors (such as fluorescent cell imaging technologies) (38,39).

The characteristic pinkish-red color of the flesh in salmonids and the shells of crustaceans is driven by the bioaccumulation of astaxanthin. This pigment is initially synthesized by microalgae and transferred up the aquatic food chain through zooplankton and intermediate crustaceans (40). Targeted accumulation of carotenoids like astaxanthin from Haematococcus pluvialis, or phycocyanin and beta-carotene from Spirulina and Dunaliella, is critical for marketability (1, 17, 38), ultimately boosting commercial value and enhancing visual appeal for consumers worldwide (38). Driven by these demands, the global market size for microalgal pigments is projected to reach $452.4 million, growing at a 4% Compound Annual Growth Rate (CAGR) (38).

Nutritional Deficiencies and Bioaccessibility

Microalgae synthesize highly bioaccessible trace minerals and bioactive molecules (including vitamins and betta-glucans) capable of resolving systematic nutritional deficits and treating metabolic conditions like hyperglycemia, diabetes, obesity, and cardiovascular diseases (1, 35). Crucially, iron deficiency anemia remains a serious global health crisis affecting over one billion people worldwide (34). Microalgae represent a highly sustainable, iron-rich dietary alternative, but total iron bioaccessibility varies fundamentally depending on the cultivation mode. Autotrophic strains exhibit lower iron bioaccessibility profiles due to rigid cell membrane constraints (Arthrospira platensis at 18.4%, Galdieria sulphuraria at 39.8%, and Chlorella vulgaris at 41.8%). Conversely, alternative heterotrophic cultivation strategies drastically alter cellular membrane configurations, boosting iron bioaccessibility up to 66.9% in Galdieria sulphuraria and 76.3% in Chlorella vulgaris, representing a profound mechanism for addressing iron deficiency via aquaculture food webs (34).

Techno-Economic Interdependencies and Production Bottlenecks

The transition of microalgae from a successful laboratory treatment to an industrially viable aquaculture component depends entirely on resolving severe techno-economic constraints (see Table 5) (41).A quantitative Techno-Economic Analysis (TEA) is essential for evaluating the financial viability of using these organisms as biorefinery feedstocks for renewable fuels, feeds, and co-products (41).

The initial choice of cultivation infrastructure dictates the entire downstream financial trajectory of the facility. Open raceway ponds offer clear scalability and low Capital Expenditure (CAPEX), but their open-air nature exposes cultures to environmental fluctuations and severe contamination risks. Conversely, Photobioreactors (PBRs) maximize purity, reduce contamination risks, and optimize growth parameters, yet their extreme CAPEX (5-10x higher than open ponds) and elevated Operating Expenditure (OPEX) driven by constant artificial illumination, precise carbon dosing, and high maintenance costs restrict their output exclusively to high value niche compounds (45, 46).

The primary financial breakdown occurs during the harvesting and dewatering phases of dilute solutions from open cultivation systems (42). Removing microalgal cells from highly dilute cultivation mediums (0.1-1% solids) requires sequential unit operations (including flocculation, centrifugation, filtration, and sedimentation) that consume substantial energy (0.2 t0 5 kWh per kg), resulting in a processing cost of €0.5 to €2 per kilogram of raw biomass (42, 50). While the commercial viability of microalgae based high density fuels for decarbonizing transport remains promising, it is highly contingent on specific local conditions and targeted resource recovery (43). Downstream value transformation techniques, such as utilizing specialized catalysts derived from lipid extracted algal waste, can marginally mitigate production costs for example, dropping biodiesel inputs from $2.03/kg to $1.70/kg, yet the overall process remains economically volatile (43, 44).