40 DECEMBER 2016 • WORLD AQUACULTURE • WWW.WAS.ORG Although largely undetectable by conventional water analyses, extracellular polymeric substances (EPS) play an important role at many levels of aquaculture production, including management of hatcheries and grow-out sites, production of biofloc and the design of aquaculture diets. EPS have been well studied in the medical literature, as well as for their role in drinking water treatment and wastewater activated sludge management. They have also been well-studied by ecologists for their contribution to carbon, nitrogen, and micronutrient cycling in aquatic systems and have been, in their various forms, called extracellular polymeric substances, exopolymeric substances, exopolymers, and exudates. They are invisible under light microscopy, and it is perhaps this invisibility that has meant that they are generally not routinely examined in aquaculture systems. Their properties generally become noticeable in aquaculture only when conditions promote excess production that results in the formation of biofilm, scum or foam. Although these outcomes are often considered undesirable in aquaculture production, the beneficial contributions of EPS to nutrient cycling, feed quality and microbial assemblages are also often overlooked. This article will summarize some important roles of EPS that deserve more consideration because of their key role in aquaculture productivity. EPS and Biofilms In aquaculture, we understand implicitly that the ubiquitous biofilm on tank walls, pipes and filters is related to the growth of microorganisms, and as result, hygiene is often prioritized to control the accumulation of potentially pathogenic bacteria. But in other contexts, the same accumulation of bacterial biofilms is beneficial, particularly in biofilters where microorganisms are instrumental in water conditioning. In recirculating aquaculture systems, for instance, EPS is the key factor in the colonization of biofiltration substrates The Role of Exopolymers in Aquaculture Hatcheries Alyssa Joyce by bacteria that are capable of oxidizing ammonia and other organic compounds in culture water. As a result, in aquaculture there is ongoing confusion regarding the importance of EPS and the microorganisms contained within the EPS matrix. EPS are the building blocks of biofilms and biofloc, which are the most commonly known manifestations of EPS, but EPS is in fact ubiquitous in freshwater and marine aquaculture environments even though it is invisible without staining. EPS are mainly composed of carbohydrates, but also include lipids, proteins, nucleic acids and their respective constituents. These are exuded by bacteria, algae, and dinoflagellates and are present in the feces, pseudofeces, and epidermal mucus of most aquatic organisms (Wotton 2011). Biofilm is commonly understood as being composed of microorganisms but in reality it is a gel matrix of macropolymers that only partially consists of bacteria. Indeed bacteria are trapped within the EPS matrix but generally constitute less than 50 percent of a biofilm (Flemming and Wingender 2010). Characteristics of EPS To better understand the role of EPS, it is first necessary to understand its characteristics. EPS can dissolve in water, but when present in sufficient quantity and under the right conditions, molecules within the EPS will tangle and cross-link to form sticky porous gels. In the presence of various cations, long-chain polymers (composed principally of sugars) with an overall negative charge become entangled with each other to form nanogels that anneal through lowenergy physical processes. Environmental conditions determine the characteristics of these nanogels, which for instance contribute to the mechanical stability of biofilms and provide the means by which they adhere to surfaces. Further aggregation of nanogels results in the formation of micro- and then macrogels and a particulate form of EPS known as Size scale of organic carbon components in seawater and methods for studying them. Modified from Verdugo, 2016. Every diver is familiar with marine snow. Photo: Freya Sommer/ Pierre de Wit.
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