44 DECEMBER 2016 • WORLD AQUACULTURE • WWW.WAS.ORG especially essential metal ions like iron, manganese, and copper, from their microenvironment. Their ability to ensure bioavailability of these nutrients is important for bacterial proliferation and for other organisms via evolution of mutually beneficial commensal relationships. For instance, photosynthetic microalgae can provide bacteria with suitable carbohydrates or lipids in exchange for iron or other metals that are essential as constituents of enzymes and their cofactors. Nutrient availability from these types of relationships is a critical factor in zooplankton-phytoplankton community structure (Hassler et al. 2011). A high abundance of bacteria in the water column increases remineralization of organic nitrogen and phosphorous into usable inorganic forms. Ultimately growth rates and productivity (passed up the food chain through rotifers and copepods used as larval feeds), depends on adequate supplies of these macro- and micronutrients (Natrah et al. 2013). It is therefore of value to pay closer attention to the biophysical characteristics and nutrient composition of EPS and to its contribution to feed quality at various trophic levels. Selection of Particles for Food One of the most beneficial aspects of EPS is its aggregating properties, such that colloidal microgels coalesce into particulate organic matter (POM) and create free-floating, buoyant aggregates that in aquaculture are useful as bioflocs. These particles can entrap bacteria, protozoans, living and dead organic matter and thus become a key feed source (Hargreaves 2013). As is the case with biofilms, there is flux between DOM and POM because there are only weak physicochemical interactions (e.g. hydrogen bonding, electrostatic and ionic forces). Abiotic environmental conditions such as shear from mixing, pH, salinity, oxygenation, and relative ion concentrations can significantly influence whether biofloc aggregates grow or diminish in size. In turn, these factors are affected by metabolic activity of microbial assemblages within the biofloc. As certain bacteria come to dominate within the floc, extracellular enzymatic activity increases, resulting in release of cells from the matrix and their dispersion into the culture medium. Fish and shellfish larvae have different modes of particle capture and transport at different stages of maturity. For filter or detrital feeders, the size, stickiness, and chemical composition of food particles are important factors in selection and palatability. Thus, in an aqueous environment, the rejection or selection of food particles for ingestion, digestion, and assimilation can depend largely on the extent to which EPS affects the physical characteristics of those particles (for shellfish see Ward and Kach 2009, for fish see Rønnestad et al. 2013). Food particle selection and thus feeding rates are influenced by microscale fluid dynamics, such that the relative viscosities, hydrostatic pressure and shear factors affecting particle surface properties become very important. Schematic illustration showing the involvement of organic polymers and colloids, TEPs and protobiofilm in the initial stages of aquatic biofilm formation. Credit: Edo Bar-Zeev, used with permission.
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