42 DECEMBER 2016 • WORLD AQUACULTURE • WWW.WAS.ORG equilibrium, its volume transitions and environmental sensitivities can enable aquaculturists to more easily predict where, whether and how EPS will form biofilms or bioflocs. One of the most interesting characteristics of EPS is that there is a constant flux between dissolved and gel states, depending on environmental conditions such as pH, temperature, salinity, O2, CO2 and shear forces. EPS can either be invisible or become noticeable as foam, slime, biofilm or biofloc dependent on whether the cross-linkages in self-assembling gels are strong enough to be altered by these environmental conditions. As a gel, sticky EPS adsorbs to surfaces and forms a scaffold in which other microorganisms become trapped, leading to structural complexity of the film (Flemming and Wingender 2010). The flux between soluble and gel states is particularly important because the volume of EPS can change by as much as 20 times when condensing from the solvated phase, or in reverse when gels dissolve and swell to their initial volume in solution. Factors such as changes in shear, pH, and water temperature can separately or together affect the equilibrium, resulting in tendencies of gels to fragment and/or disperse back into the dissolved state. In some cases, it is desirable to deliberately cause EPS to form aggregates such as biofloc. Therefore, the knowledge of how, when and why gels form and our ability to track what parameters inhibit or stimulate their formation in any particular system is important to control EPS formation. Embedded Microbes in the EPS Matrix One of the key reasons why EPS is important to aquaculture is that the EPS matrix is colonized by a diverse microbial community (Natrah et al. 2013). The intermingled long polysaccharide chains constituting the largest percentage of the microgels develop hydrophilic and hydrophobic microdomains with different charge densities and conformations as they interact with other matrix constituents (DNA, proteins, lipids, other organic molecules, and detritus). These microdomains form pockets and channels that are colonized by bacteria and other microorganisms already suspended in the aqueous medium. The heterogeneous matrix environment is relatively rich in dissolved nutrients and immobilizes microorganisms in close proximity so that they form assemblages capable of intercellular communication (quorum sensing) and proliferation. Surrounded by a well-hydrated microenvironment, the trapped microbes also become more tolerant to desiccation and excess oxygen, and are better protected from disinfection and protozoan grazing. Bacteria are particularly adept at sequestering metal ions and absorbing xenobiotics from an aqueous medium, which can lead to accumulations of noxious chemicals within the EPS matrix. In medical settings, EPS in fluids and biofilms protect microorganisms from susceptibility to sterilization processes and antibiotics. Hence there is continual research in medicine and water treatment on biofilms and the ways in which the EPS microenvironment protects bacteria from UV, ozone or chlorine disinfection. Although the need for hygiene is well known in hatcheries, comparatively insufficient attention has been paid to the role of EPS in aquaculture, particularly the role of EPS in disinfection or intake water quality. EPS in Hatcheries At the water intakes of hatcheries, and indeed in any tankbased aquaculture systems, activated carbon filter media, sand/ drum filters, and protein skimmers are the most useful methods for controlling excess EPS and its resident microbial flora. But due to their small size, EPS and/or their precursors still pass through most if not all hatchery filtration devices (Bar-Zeev et al. 2012) and even sterilization such as pasteurization or UV treatment does little to influence the toxins and heavy metals contained within the EPS matrix. EPS are well below the size of normal physical filters used in aquaculture, and by “hitching a ride” in the EPS matrix, toxins and pathogens can subsequently affect the stability of larval cultures. The EPS composition also influences the algal, rotifer, and copepod cultures used as feed, with further impacts on growth and survival. Indeed, otherwise mysterious mortalities in hatcheries may in some cases be related to the presence of factors carried in the EPS matrix, but because the EPS matrix is largely invisible and hence beyond the awareness of most aquaculturists, we do not fully understand EPS in the context of water treatment. In particular, we do not fully understand how it serves as a medium for transporting a range of substances or provides the framework within which specific bacterial communities can develop (Natrah et al. 2013). Microalgal cultures in hatcheries are a good example of how EPS can be simultaneously beneficial and deleterious. Microalgal cultures used for shellfish feeds invariably have an EPS microenvironment where microbial assemblages and algae release EPS that have stimulatory or inhibitory effects on each other (Natrah et al. 2013). In healthy culture conditions, these assemblages come into balance as nutrient availability and gas exchanges lead to stable Digital holographic microscopy image of a biofilm formed by the diatom Nitzschia capitellata. The biofilm structure is more easily visualized with the false-coloring of the phase image whereby the color scale gives a quantitative measurement of optical path length. The edge of the biofilm (black arrow) and trails (yellow arrows) created from the diatoms moving within the biofilm can be clearly seen. Image: Eva-Maria Zetche.
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