Periphytons are communities of photosynthetic and heterotrophic, single-celled organisms that form on many submerged marine and freshwater surfaces. They secrete complex extracellular polymeric substances (EPSs) that aid in numerous roles including adhesion, cohesion, and motility. In some species, they are used as carbon sinks for photosynthesis (when other nutrients are in short supply) and in others they serve as carbon reservoirs for feeding. They can be synthesized in a diverse variety of forms since they contain carbohydrates, lipids, proteins, and in some cases mineral adjuncts.
"[D]iffering amounts of calcium carbonate appeared to be deposited within each periphyton type; metaphyton and epipelon were heavily encrusted with calcium carbonate, whereas in the epiphyton, noncalcified filaments were more prevalent." (Bellinger et al. 2010:487)
"The EPS content in freshwater periphyton tends to be greater relative to estuarine and marine algal mats, and studies have shown the environmental significance of these polymers in the latter systems (e.g., sediment stabilization." (Bellinger et al. 2010:489)
"Production of EPS is typically greater when a nutrient is limiting (N or P)...in nutrient-depleted conditions, it has been shown that there is an increased need for a photosynthetic overflow mechanism in the form of EPS production to protect the photosystems from damage." (Bellinger et al. 2010:490)
"The extracellular sheaths, capsules, and cell walls of cyanobacteria have been associated with calcium carbonate deposition...EPS may also inhibit precipitation through binding of calcium ions by carboxylated sugars, thus preventing crystal growth." (Bellinger et al. 2010:493)
"Polysaccharides found in the extracellular matrices of photosynthetic organisms...represent the most abundant and complex biopolymers found on the planet...exceptional monomer diversity and linkage complexity...myriad structural and functional aspects of EPS in ecosystem dynamics." (Bellinger et al. 2010:494)