Transparent exopolymer particles (TEPs) are extracellular acidic polysaccharides produced by phytoplankton and bacteria in saltwater, freshwater, and wastewater. They are incredibly abundant and play a significant role in biogeochemical cycling of carbon and other elements in water. Through this, they also play a role in the structure of food webs and trophic levels. TEP production and overall concentration has been observed to be higher in the Pacific Ocean compared to the Atlantic, and is more related to solar radiation in the Pacific. TEP concentration has been found to decrease with depth, having the highest concentration at the surface, especially associated with the SML, either by upward flux or sea surface production. Chlorophyll a has been found to be the best indicator of TEP concentration, rather than heterotrophic grazing abundance, further emphasizing the role of phytoplankton in TEP production. TEP concentration is especially enhanced by haptophyte phytoplanktonic dominance, solar radiation exposure, and close proximity to sea ice. TEPs also do not seem to show any diel cycles. High concentrations of TEPs in the surface ocean slow the sinking of solid particle aggregations, prolonging pelagic residence time. TEPs may provide an upward flux of materials such as bacteria, phytoplankton, carbon, and trace nutrients. High TEP concentrations were found under arctic sea ice, probably released by sympagic algae. TEP is efficiently recycled in the ocean, as heterotrophic grazers such as zooplankton and protists consume TEP and produce new TEP precursors to be reused, further emphasizing the importance of TEPs in marine carbon cycling. TEP abundance tends to be higher in coastal, shallow waters compared to deeper, oceanic waters. Diatom-dominated phytoplankton colonies produce larger, and stickier, TEPs, which may indicate that TEP size distribution and composition may be a useful tool in determining aggregate planktonic community structure.
TEPs are formed from cell surface mucus sloughing, the disintegration of bacterial colonies, and precursors released by growing or senescent phytoplankton. TEP precursors can be fibrillar, forming larger colloids, or aggregations, and within hours to days after release from the cell are fully formed transparent exopolymer particles. While most exopolymeric substances range from loose slimes to tight shells surrounding cells, TEPs exist as individual particles, allowing them to aggregate and be collected by filtration. They are highly sticky, forming aggregations of solid particles known as marine snow, and are actually associated with all marine aggregations investigated thus far. TEPs have a high C:N ratio compared to the Redfield Ratio, suggesting the significance of TEPs in the promotion of carbon sequestration and particle sedimentation to the benthos, but this is complicated due to bacterial decomposition, as well as heterotrophic grazing by zooplankton such as euphausiids and protists. This also suggests that TEPs may represent a link between the oceanic microbial loop and other food webs, as well as creating short circuit food webs within the pelagic.
