EPS (Extracellular Polymeric Substances), silk, and chitin: vitally important exudates in aquatic ecosystems

Exudates are ubiquitous in marine and fresh waters. They include Extracellular Polymeric Substances (EPS; including mucus, slimes, and biofilm matrices), silk, and chitin. EPS have many uses for the organisms that produce them-attachment, locomotion, feeding, and protection. They also act as glues to bind other materials, including the egesta within fecal pellets. Silk, an exudate produced only in fresh waters, is used in tube construction, to make nets used in feeding, and as a means of attachment. Chitin is the basis of the peritrophic matrix that surrounds the gut contents of some invertebrates and may act as a binding surrounding their fecal pellets. EPS are found free in the water column and in and on the substratum, as well as in close contact with the organisms that exude them. Free EPS have an essential role in particle formation and aggregation, processes that have been studied primarily in marine systems. Like EPS, silk is highly adsorptive and is found free within substrata, but rarely in the water column. Understanding the role of exudates requires collaboration between marine and freshwater biologists, physical geographers, and chemists, especially those working on surface processes and colloids. The results of these collaborations should expand our understanding of the many vital roles played by exudates in aquatic ecosystems.