*3.2.3. Microalgae*

Photoautotrophic community (microalgae) also play an important role in the biofloc system. Microalgae assimilate mainly ammonia and nitrate to produce biomass, additionally consume carbon dioxide, and produce oxygen. The divisions of microalgae reported in biofloc cultures are Chlorophyta, Chrysophyta, and Cyanophyta. These microorganisms catch the solar energy, to produce chemical energy (carbohydrates), which is used in their metabolic process.

In biofloc cultures the microalgae can live as free cell into the water column or could form aggregates. In some cases the aggregations of chrysophytes and cyanophytes can measure up to 2 mm in diameter [41]. Their sizes are highly variable, with cells of less than 10 μm to more 50 μm [42].

Chlorophytes are green microalgae that are the most numerous and diverse in the freshwater; they can reproduce massively forming blooms, but, at difference of the cyanophytes, are nontoxic. This division presents a high plasticity and is able to colonize diverse habitats; they are spherical or oblong and may have flagella or not [43].

Chrysophytes are the most representative organisms that correspond into the Bacillariophyceae class (diatoms), which is divided in centric and pennate. The planktonic species are mainly centric; meanwhile, the pennate are commonly benthic. All centric species are marine, while most of the pennate live in freshwater [43]. In aquaculture, diatoms are considered as beneficial algae because they are a source of food and nutrients for most aquatic animals [44].

Cyanophytes are known as the most ancient photosynthetic organisms; they possess a high morphologic and structural variability. During its evolution they have developed various ecophysiological adaptation strategies to survive in extreme environmental conditions [45]. Given its abundance in different environments, the division Cyanophyta is important for nutrient circulation, incorporating nitrogen into the food chain, which makes them primary producers or decomposers.

In aquaculture ponds, excessive concentrations of major nutrients (nitrogen and phosphorus) can lead to uncontrolled microalgae blooms, sometimes are cyanobacteria dominated which is known to produce some toxic compounds to aquatic animals, and can cause unpleasant flavors in cultured species [46]. Several authors have reported the presence of cyanobacteria in biofloc, with concentrations varying according to the biofloc type. Becerra-Dórame et al. [47] reported 2.1 × 104 cells mL−1 in heterotrophic biofloc, while in autotrophic, they found 3.3 × 106 cells mL−1. Although cyanobacteria can become toxic or problematic, Lezama-Cervantes et al. [48] found several species of Cyanobacteria (*Nostoc* sp., *Anabaena* sp., *Phormidium* sp., *Chroococcus* sp., *Oscillatoria* sp., and *Lyngbya* sp.) in a microbial mats used to culture *L*. *vannamei* postlarvae and fund evidenced of active grazing by the shrimp.

Aquaculture microalgae are widely used; their nutritional characteristic has permitted to produce crustaceans, fishes, and mollusk in laboratory. Several factors can contribute to the nutritional value of microalgae, including its size and shape, digestibility, biochemical composition, and bioactive compounds as enzymes, vitamins, antioxidants, etc. Microalgae grown to late-logarithmic growth phase typically contain 30–40% protein, 10–20% lipid, and 5–15% carbohydrate; PUFAs derived from microalgae, i.e., docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and arachidonic acid (AA), are known to be essential for various farmed species [49].

In general, microalgae are common inhabitants in biofloc, even in a bacteria dominated (heterotrophic). Monroy-Dosta et al. [40] evaluated the microorganism composition in biofloc tilapia culture, indicated that the first microalgae appeared were chlorophytes, followed by diatoms and finally cyanobacteria, and also mentioned that diatoms achieved the highest concentration and cyanobacteria the lowest. Ray et al. [50] cultured *L*. *vannamei* in zero-water exchange, and differing from the previous authors, in their study the chlorophytes were dominant over the diatoms. Biofloc systems are highly dynamic; Kuang et al. [51] indicate that in nature, certain species of ciliates and rotifers have a selective consumption of microalgae, and therefore may influence their diversity. The physicochemical parameters also affect the microalgae dominance. Maicá et al. [52] observed in a *L*. *vannamei* biofloc culture greater abundance of chlorophytes in salinity of 2 %, while in 25 % diatoms were the most abundant.
