Transparent exopolymer particles (TEP) in aquatic environments

Abstract

Since the development of methods to quantify transparent exopolymer particles (TEP) 1993, it has been shown that these gel-particles are not only ubiquitous and abundant, but also play a significant role in the biogeochemical cycling of elements and the structuring of food webs. TEP may be quantified either microscopically or colorimetrically. Although data based on measurements using one or other of these methods are not directly comparable, the results are consistent. TEP abundances in fresh and marine waters are in the same range as those of phytoplankton, with peak values occurring during phytoplankton blooms. TEP are very sticky particles that exhibit the characteristics of gels, and consist predominantly of acidic polysaccharides. In marine systems the majority of TEP are formed abiotically from dissolved precursors, which are released by phytoplankton that are either actively growing or are senescent. TEP are also generated during the sloughing of cell surface mucus and the disintegration of colonial matrices. The impact of exopolymers in the creation of microhabitats and in the cycling of trace compounds varies with the state in which the polymers occur, either as particles or as solute slimes. As particles, TEP provide surfaces for the colonization by bacteria and transfer by adsorption, trace solute substances into the particulate pool. As dissolved polymers they are mixed with the water and can neither be filtered nor aggregated. Because of their high abundances, large size and high stickiness, TEP enhance or even facilitate the aggregation of solid, non-sticky particles. They have been found to form the matrices of all marine aggregates investigated to date. By aggregating solid particles, TEP promote the sedimentation of particles, and, because their carbon content is high, their direct contribution to fluxes of carbon into deep water is significant. The direct sedimentation of TEP may represent a mechanism for the selective sequestration of carbon in deep water, because the C:N ratios of TEP lie well above the Redfield ratio. The turnover time of TEP as a result of bacterial degradation appears to range from hours to months, depending on the chemical composition and age of TEP. TEP may also be utilized not only by filter feeders (some protozoans and appendicularian) but TEP-rich microaggregates, consisting of pico- and nano-plankton are also readily grazed by euphausiids, thus permitting the uptake of particles that would otherwise be too small to be grazed directly by euphausiids. This short-circuits food chains and links the microbial food-web to the classical food-web. It is suggested that this expansion of the concept of food webs, linking the microbial loop with an aggregation web will provide a more complete description of particle dynamics. (C) 2002 Elsevier Science Ltd. All rights reserved.