Transparent Exopolymer Particles (TEP): an overlooked factor in the process of BIOFILM FORMATION in aquatic environments

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Abstract

We hypothesize that transparent exopolymer particles (TEP), present in high concentrations in most sea and freshwaters, are critical agents for biofilm initiation and development in many natural and anthropogenic aquatic environments. These gel-like particles appear in many forms, amorphous blobs, clouds, sheets, filaments or clumps ranging in size from ~2 to ~200 µm. TEP are mostly polysaccharide, negatively charged, very sticky and are frequently colonized by bacteria. TEP may be considered a 'planktonic' subgroup of exopolymeric substances (EPS), widely studied in biofilm research. Recognition of TEP involvement in biofilm formation has important implications for a comprehensive understanding of the complexities of this process in aquatic environments and may also contribute to the considerable efforts being made in the global water industry to mitigate the harmful effects of biofouling in water treatment and desalination plants.

In this paper we propose the concept that transparent exopolymer particles (TEP), ubiquitous in large numbers in both marine and freshwaters (Passow (2002), are an important but hitherto overlooked factor for the development of biofilms, both in natural and anthropogenic environments. The properties of these small (~2 to 200 µm), negatively charged, mostly polysaccharide, gel-like particles strongly suggest their potential to play a key role in the initiation and growth of biofilm (Berman and Holenberg 2005).

What is TEP?

In 1993 Alldredge et al. reported a high abundance of previously undetected, transparent micro-particles in seawater that were visualized by staining with Alcian Blue, a dye specific for acid mucopolysaccharides. These were dubbed “Transparent Exopolymer Particles” or TEP. It quickly became evident that TEP are ubiquitous and numerous in most freshwater and marine environments and play important roles in these ecosystems. TEP range in size from ~2 μ,m to 100-200 μ,m and appear in many forms, amorphous blobs, clouds, sheets, filaments or clumps. Per definition, TEP are deformable, gel-like particles suspended in the water mass. This contrasts to the polysaccharide-containing biofilm matrix that adheres to substrate surfaces, however, TEP may be considered a 'planktonic' subgroup of EPS [exopolymeric substances] (Decho 1990, Hoagland et al. 1993, Wotton 2004).

The presence of highly surface active polysaccharides (Mopper et al. 1991) in TEP explains the strong tendency of these particles to form metal ion bridges and hydrogen bonds. As a result, TEP are usually extremely sticky with a high probability of attachment upon collision, about 2 to 4 orders of magnitude more sticky than phytoplankton or mineral particles (Passow 2002, Engel et al. 2004, Mari and Dam 2004). TEP are essential for the aggregation of particles in the open water, and for coating natural surfaces (Verdugo et al. 2004). In some aquatic environments, TEP appear to form abiotically from dissolved organic exudation products by processes of coagulation and gelation (Chin et al. 1998, Mari 1999, Passow 2000) or by bubble adsorption (Mopper et al. 1991 , Zhou et al. 1998). Considerable amounts of TEP are also produced from the gelatinous envelopes surrounding diatoms and other algae (Passow & Alldredge 1994) and from bacterial mucous (Stoderegger & Herndl 1999). TEP may also be formed at senescence by algae and cyanobacteria (Grossart et al. 1997, Berman & Viner-Mozzini 2001, Berman-Frank et al 2007) TEP have been termed “macrogels”, and constitute a significant portion of the gel phase that forms an intermediate stage in the dissolved organic matter (DOM) to particulate organic matter (POM) continuum in seawater and freshwaters (Verdugo et al 2004).

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