WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2016 43 equilibrium. Indeed, many species of microalgae cannot be grown axenically, given that they require nutrients provided by bacteria in the EPS matrix for their survival. However, with environmental stress, an imbalance in this relationship can develop, where bacteria or the microalgae are stimulated to produce toxins that accumulate within the EPS matrix. A similar process occurs when large microalgal blooms degrade in the environment, leading to bacterial spikes and decomposition processes such that hatchery or aquaculture intake water quality is affected. Larger TEP particles for instance are broken up by mechanical filtration, and colloidal and dissolved forms of EPS, as well as its precursors, easily pass through hatchery filtration systems. Sterilization may have some effect on microbial communities within the EPS, without fundamentally altering its chemical composition. These EPS can play an important role in the health of animals raised in a production system as they can contain toxins and high levels of heavy metals, but go largely undetected even in the most modern hatcheries or production facilities due to our lack of focus on this key element of water quality. Biosensor technologies capable of detecting EPS accumulations that are associated with initial stages of imbalance might allow hatchery managers to make timely adjustments of culture conditions or intake water quality if we simply focused on this aspect in routine assessments. However, the EPS matrix does not necessarily contain high levels of toxins or deleterious bacteria and can also be beneficial. An often overlooked role of EPS is the ability of bacteria embedded in the matrix to increase the bioavailability of macro- and micronutrients (Natrah et al. 2013). Bacteria, for instance, produce siderophores capable of sequestering nutrients, Images of TEPs using Alcian Blue staining (left) and Coomassie staining (right) to stain polysaccharide gels blue. Photos: Eva Maria Zetche. (CONTINUED ON PAGE 44) Particle aggregate as determined by confocal laser scanning microscopy (CLSM) using Concanavalin A and Alexa Fluor® 633 to stain polysaccharide gels (blue), SYTO® 9 to stain bacteria (green) and Chl a autofluorescence (red). Photo: Anja Engel. Transparent exopolymer particle (TEP) as determined by confocal laser scanning microscopy (CLSM) using Concanavalin A and Alexa Fluor® 633 to stain the polysaccharide gelatinous matrix (green, large image). Insert same particle stained with Alcian Blue. Photo: Anja Engel.
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