WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2016 41 Transparent Exopolymer Particles (TEP). TEP form the dominant gel matrix (‘glue’) for larger-sized particle aggregates commonly known as ‘marine snow,’ which are known in the aquaculture context as bioflocs. Even when not aggregated, the constituents of these flocs are always present in the water column (Verdugo 2012). A few drops of inexpensive dyes such as Alcian Blue for TEP (negatively charged polysaccharides) and Coomassie Brilliant Blue (proteinaceous particles) can be added to a water sample to allow some of the components of the EPS to become visible with the naked eye. More sophisticated techniques (e.g. various fluorescent staining techniques) can render other constituents of EPS visible (Flemming and Wingender 2010) and various imaging methods (e.g. field-flow fractionation coupled with mass spectrometry, laser in situ scattering transmissometry, digital holographic microscopy) allow for more comprehensive visualization of the components of EPS, but these are not required to gain a basic understanding of how EPS affects aquaculture productivity. Equilibrium Between Soluble and Gel States In hatcheries, accumulations of biofilm contribute to the need for frequent cleaning of tanks, pipes, and filters to prevent overgrowth and flow restrictions in pipes. In grow-out settings, biofilms are also the first step in the fouling of nets and equipment and this determines which larger organisms, such as barnacle larvae or seaweeds, will settle on surfaces. The economic costs of biofouling to the aquaculture industry can be considerable given that fouling interferes with water flow, waste removal, and in the case of shellfish farming, affects product quality. It is therefore advantageous to better understand the physical and chemical characteristics of biofilms to control its accumulation. Important stages of biofilm formation occur even before slime becomes visible. Formation is intimately connected to the reversible equilibrium of EPS, which fluctuates between dissolved and microgel states (Verdugo 2012). Understanding this reversible (CONTINUED ON PAGE 42) Simple staining elucidates a world of particles that is not visible to the naked eye in these images of oyster larvae in a hatchery tank (here stained with Bismark Brown and Alcian Blue). Photos: Alyssa Joyce. EPS are the building blocks of biofilms and biofloc, which are the most commonly known manifestations of EPS. Bacteria are trapped within the EPS matrix but generally constitute less than 50 percent of a biofilm. EPS in fluids and biofilms protect microorganisms from susceptibility to sterilization processes and antibiotics. The EPS microenvironment protects bacteria from UV, ozone or chlorine disinfection.
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