WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2016 45 Given that these properties are intimately associated with the flux between sol and gel states of EPS, there is potential for production gains if studies can lead to a better understanding of how EPS characteristics are involved in food particle selection of different organisms at appropriate developmental stages. In this regard, comparatively few studies have examined whether bioflocs serve as a food source for aquatic organisms in natural settings (Wotton 2011). There are a handful of studies demonstrating that copepods obtain nutrients from bacterial assemblages concentrated in biofloc due to greater feeding efficiency than if they rely only on individual free-floating microbes. In natural environments, similar feeding patterns based on preferences for aggregated particles are observed in some shellfish and shrimp. The question of whether fish larvae in their endogenous and mixed feeding periods can metabolize biofloc and its various components has also not been exhaustively studied, despite its importance for aquaculture production. Fish larval metabolism of carbohydrates for instance, has not received nearly as much attention as metabolism of proteins and lipids, despite the fact that regulation of enzyme synthesis for carbohydrates evidently precedes that of protein and lipids in some post-hatching fish larvae (Rønnestad et al. 2013). Techniques for tracking gene regulation in relation to maturation of digestive capabilities in larval fish could reveal important information about the nutritional value of bioflocs, and how enhancements at particular stages potentially affect health and growth rates. There is certainly evidence that exposure to particular microbes during larval development can prepare organisms for favorable metabolic and immune responses in juvenile and adult life stages of fish and crustaceans — a kind of metabolic programming that has stimulated advanced sequencing studies of microbial communities for their effects (Conceição et al. 2010). It has also stimulated interest in using bioflocs to administer immunostimulants or bacterial strains important for digestion and absorption of nutrients. Establishing a healthy gut microbiome at appropriate developmental stages can inhibit the growth of pathogens and increase the bioavailability of amino acids, vitamins, and micronutrients, although comparatively little is yet known about gut microbial ecology and its role in growth and survival (Conceição et al. 2010). As a result, there is a focus in aquaculture research on gaining a better understanding of biofloc in terms of its microbiota and the nutritional value of its constituents. In some settings, biofloc is a cost-effective means of treating wastes and recycling nutrients, and some studies have shown improved feed conversion ratios with biofloc for filter-feeding species like Pacific white shrimp and tilapia. Of course there are tradeoffs that need to be managed, but techniques are constantly being refined to increase the feasibility and efficiency of biofloc systems (Hargreaves 2013). Better technologies for monitoring floc characteristics and composition in hatchery settings — indeed based on a better understanding of EPS — have the potential to improve its palatability and nutritional quality. EPS and Aquaculture Productivity One of the most important points in this discussion about EPS is the overlooked role that EPS plays in aquaculture productivity. Current hatchery practices do not often monitor this ‘mystery factor X’ that is ubiquitously present but largely undetected. Simple staining methods can reveal the invisible undiscovered world of EPS that influences everything from hygiene practices and water treatment to feed choices. Research in this area can lead to new sensors and management practices that have the potential to improve aquaculture productivity. Note Alyssa Joyce, Assistant Professor, Department of Marine Sciences, University of Gothenburg; Physical Address: Carl Skottsbergs gata 22B, 41319 Göteborg, Sweden; Postal Address: Hattebacksvagen 7, 45296 Stömstad, Sweden; Tel: +46 31 786 9653, Fax: +46 31 786 1333, Mobile +46 766 229653; alyssa.joyce@gu.se References Bar-Zeev, E., I. Berman-Frank, O. Girshevitz and T. Berman. 2012. Revised paradigm of aquatic biofilm formation facilitated by microgel transparent exopolymer particles. Proceedings of the National Academy of Sciences 109:9119-9124. Conceição, L.E.C., C. Aragão, R. Nadège, S. Engrola, P. Gavaia, S. Mira and J. Dias. 2010. Novel methodologies in marine fish larval nutrition. Fish Physiology and Biochemistry 36:1-16. Flemming, H-C. and J. Wingender. 2010. The biofilm matrix. Nature Reviews Microbiology 8:623-633. Hargreaves, J.A. 2013. Biofloc production systems for aquaculture. Southern Region Aquaculture Center Publication No. 4503:1-12. Hassler, C. S., E. Alasonati and C. Mancuso Nichols. 2011. Exopolysaccharides produced by bacteria isolated from the pelagic Southern Ocean—Role in Fe binding, chemical reactivity, and bioavailability. Marine Chemistry 123(1-4):88-98. Natrah, F.M.I., P. Bossier, P. Sorgeloos, F.M. Yusoff and T. Defoirdt. 2013. Significance of microalgal–bacterial interactions for aquaculture. Reviews in Aquaculture 6:48-61. Rønnestad, I., M. Yúfera, B. Ueberschär, L. Ribeiro, O. Saele and C. Boglione. 2013. Feeding behaviour and digestive physiology in larval fish: current knowledge, and gaps and bottlenecks in research Reviews in Aquaculture 5(Suppl. 1):S59-S98. Verdugo, P. 2012. Marine microgels. Annual Review of Marine Science 4:375-400. Ward, J.E. and D.J. Kach. 2009. Marine aggregates facilitate ingestion of nanoparticles by suspension-feeding bivalves. Marine Environmental Research 68:137-142. Wotton, R.S. 2011. EPS (Extracellular Polymeric Substances), silk, and chitin: vitally important exudates in aquatic ecosystems. Journal of the North American Benthological Society 30(3):762-769. For filter or detrital feeders, the size, stickiness, and chemical composition of food particles are important factors in selection and palatability. Food particle selection and feeding rates are influenced by microscale fluid dynamics. There is potential for production gains if studies can lead to a better understanding of how EPS characteristics are involved in food particle selection of different organisms at appropriate developmental stages.
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