Bioflocs as a Nutritious Feed Source
One of the major challenges facing aquaculture producers is the high cost of aquaculture feeds. Protein levels and adequate amino acid balance are critical in aquaculture feeds due to their essential role in maintaining the growth and the general wellbeing of aquatic organisms. However, these nutrients are an expensive component of the feeds and hence influence their market price 
. In tilapia, for example, feeding can account for 50% of the operational costs and could even reach higher levels with high-protein diets and/or inadequate protein 
. However, this could be mitigated by feeding tilapia on alternative feed sources such as phytoplankton, zooplankton, and algae, whose nutritive content would enhance the growth, survival, and production of fish 
. Avnimelech and Kochba 
found that tilapia can uptake 240 mg N kg−1
of biofloc, which is equivalent to 25% of the protein in fish diets. Moreover, bioflocs can contain 20%–<40% crude protein, <1%–>8% lipids, <1%–>15% fiber, <18–>35% total carbohydrates, and <15%–>60% ash, thus providing an alternative feed source to the reared aquatic species 
It is worth noting that the nutritional value of bioflocs is highly dependent on the microbial community that encompasses it and, as mentioned in the previous section, certain factors such as carbon sources and C/N ratio influence the biochemical composition of bioflocs. For example, Moreno-Arias et al. 
reported that the fatty acid and amino acid composition of both biofloc and shrimp cultivated in BFT systems depends on the composition of the aquaculture feed used. The use of plant-based protein sources in the feed is more favorable for biofloc systems and is considered to be more eco-friendly and sustainable. This is because their use reduces the release of phosphorous and nitrogenous wastes in the aquatic ecosystem as well as the dependency on overexploited marine sources 
. The effect of biofloc feed on the general wellbeing and sustainable production of aquatic species is discussed below.
Emerenciano et al. 
investigated the influence of BFT as a food source in a limited water exchange nursery system on the growth performance of pink shrimp (Farfantepenaeus brasiliensis
) post-larvae. The authors reported that rearing post-larvae in the BFT system without commercial food supply did not affect the growth performance of the animals. Moreover, no significant differences in final biomass and weight gain were noted between shrimp reared in BFT with or without commercial diet supplementation. The good growth performance of the larvae was attributed to the diverse microbial community that consisted of protozoa grazers, rotifers, cyanobacteria, and diatoms, which were utilized as a food source. In another study, Emerenciano et al. 
found no significant differences in the final biomass and survival of early post-larvae pink shrimp (Farfantepenaeus paulensis
) reared in BFT with or without commercial feed supplementation. Emerenciano et al. 
also observed no significant difference in spawning performance among females reared in BFT with or without feed supplementation. Zhang et al. 
found that culturing gibel carp (C. auratus gibelio
♀ × C. carpio
♂, 6.4 ± 0.5 g) in BFT without feed addition for 30 days did not affect the growth performance (weight gain, specific growth, and survival) of fish. The fish were able to utilize the bioflocs as a feed, with increased digestive enzyme activity of pepsin and amylase noted in fish reared in water containing high TSS (300, 600, 800, and 1000 mg L−1
TSS). Furthermore, bioflocs enhanced the fish’s innate immunity, as indicated by increased superoxide dismutase (SOD) and total antioxidant capacity (TAOC) activity in the skin and mucus. Upregulated immune-related genes included intelectin (ITLN), dual-specificity phosphatase 1 (DUSP 1), keratin 8 (KRT 8), myeloid-specific-peroxidase (MPO), c-type lysozyme (c-lys), and interleukin-11 (IL-11).
The nutritive content and quality of bioflocs are rich and, as such, bioflocs have been used as a cheaper and sustainable alternative to the highly expensive fishmeal. For example, in shrimp culture, 15% to 30% of conventional protein sources can be replaced by biofloc meal without negatively affecting the general wellbeing of the species 
. The incorporation of biofloc meal in aquaculture indeed reduces the costs of production whilst permitting an intensive culture of species, hence maximizing profits. Several studies have shown that replacing fishmeal with biofloc meal alone or in combination with certain dietary sources such as lysine, soy protein concentrate, and protein hydrolysate improves the growth performance, survival, digestive enzyme activity, and immunity of the reared aquatic species 
Currently, more research studies in the field of pro- and prebiotic bioflocs are ongoing. Probiotics are beneficial microbes that are either added or naturally developed in the BFT system to stimulate the immune system for the reared aquatic species against biotic and abiotic stress. Several beneficial microorganisms, such as those from the Bacillaceae family, have been previously identified and isolated from the shrimp culture BFT system 
. These bacteria have been used in the biocontrol of disease outbreaks caused by pathogenic microbes as well as immunostimulants for enhancing the general wellbeing of aquatic species. Table 1
shows some of the conducted studies on probiotics in BFT systems included in animal diets or added directly into the rearing water for enhancement of the general wellbeing of the reared aquatic species.
Table 1. Some of the conducted studies on probiotics in BFT systems included in animal diets or added directly into the rearing water for enhancement of the general wellbeing of the reared aquatic species.
||Dosage and Duration of Study
||Altai™, Providencia, Santiago, Chile (Bacillus subtilis, Bacillus natto, Bacillus megaterium, Lactobacillus acidophilus, Lactobacillus plantarum, Lactobacillus brevis, Lactobacillus casei, and Saccharomyces cerevisiae)
||109 CFU g−1–45 days
||↑ Growth and survival. ↓ Severe lesions in shrimp tissues. ↓ Abundance of pathogenic bacteria.
||Aguilera-Rivera et al. 
||5.4 × 109 CFU mL−1–90 days
||Panigrahi et al. 
||1 × 104 CFU mL−1–42 days
||↓ Abundance of pathogenic bacteria Vibrio alginolyticus (BCCM 2068). ↑ Immunity.
||Ferreira et al. 
||1.5 × 108 CFU L−1–95 days
||↑ Microbial diversity of beneficial bacteria. ↓ Abundance of pathogenic bacteria.
||Hu et al. 
||5 × 1010 CFU–60 days
||↑ Growth performance, survival rate, and feed utilization.
||Putra et al. 
||5 mg L−1–35 days
||↑ Growth performance.
|1 × 107 CFU mL−1–60 days
||Kathia et al. 
||Multi strain probiotics (B. subtilis, L. plantarum, L. Rhamnosus, L. acidophilus, L. delbrueckii)
||108 CFU g−1–112 days
||↑ Immune response (serum protease, SOD, CAT, AP, MPO, and RBA activities). ↓ Mortality against Aeromonas hydrophila infection challenge.
||Mohammadi et al. 
|107 bacteria mL−1–8 weeks
||↑ Survival percent and weight in fish fed on Bacillus sp. alone or probiotic mixture. ↑ Resistance against pathogenic bacteria.
||Aly et al. 
|0.014 g L−1–12 days
||↔ Growth performance. ↑ Immune response.
||Jung et al. 
|12.8 × 108 cells ml−1 and 13.5 × 108 cells mL−1–60 days
||↑ Development of suspended biomass in the BFT system. ↑ Immunity and disease resistance.
||Dash et al.