EXTRACELLULAR POLYMERIC SUBSTANCES (EPS) AND EXTREME ENVIRONMENTS

It has been realized for a long time that microbial organisms may exist as free-living individuals as well as in groups, attached to surfaces or to each other, and that under many conditions cells often prefer to attach to surfaces. Under the fluctuating, often less-predictable conditions of natural systems, such attachment may confer advantages and a certain degree of stability to cells. It is now realized that most bacteria and other microorganisms (e.g., cyanobacteria, archaea, diatoms) exist as attached biofilms. The biofilm consists of microbial cells that have attached to a surface (or aggregated), and have surrounded themselves within a gelatinous matrix of EPS, and may exhibit varying degrees of community structure (Decho, 1990). Biofilm formation appears to be a common microbial process that occurs under a wide range of conditions and environments, and whose influences span aquatic, terrestrial, the epi- and endo-biont communities of plants and animals, and as well as disease and health processes (Stoodley et al., 2002).

Cited works frame the historical and conceptual development of EPS research: early emphasis on exopolysaccharides and their biosynthesis (Sutherland, 1982), roles of microbial exopolymer secretions in marine systems (Decho, 1990), biofilms as complex communities (Stoodley et al., 2002), EPS in sediment stability and diatom extracellular carbohydrates (Underwood and Paterson, 2003), EPS-mediated mineral precipitation in microbial mats (Dupraz et al., 2009), photosynthesis-induced biofilm calcification and ancient oceans (Arp et al., 2001), and the origin and role of transparent exopolymer particles in particulate matter sedimentation (Passow et al., 2001).

- EPS are secreted upon surface attachment and biofilm formation, forming a matrix of molecules with a wide range of sizes and conformations. Physical forms span tight, dense gels to looser, dispersed slimes and include water channels, leading to varying diffusivities and the capacity to bind/sequester ions, nutrients, and other molecules, creating steep biogeochemical gradients at micrometer scales. - EPS composition is broader than previously thought: beyond polysaccharides, EPS in natural biofilms contains proteins (structural and enzymatic), lipids, and abundant extracellular DNA (some actively secreted), as well as refractory amyloid proteins that may act as a structural rebar resisting degradation. A significant fraction of EPS in natural systems does not fit classical protein/carbohydrate/lipid categories. - The EPS matrix provides three-dimensional architecture for cellular interactions, including capsule formation immediately surrounding cells, enabling organized macrostructures in biofilms across diverse environments. - The physical structure of natural EPS remains poorly understood; advanced tools such as AFM, NMR spectroscopy, and CSLM are being used to image and characterize EPS in situ. A CSLM image shows EPS surrounding cyanobacteria and heterotrophic bacteria within stromatolitic mats (scale bar 10 µm). - In geobiology, EPS can both inhibit and promote mineral precipitation: freshly secreted EPS sequesters Ca2+ via carboxyl groups (bidentate bridges), inhibiting CaCO3 precipitation; partial degradation or steric changes can allow local Ca2+ saturation and EPS functional groups to nucleate aragonite (CaCO3) via unidentate binding, initiating carbonate precipitation relevant to stromatolite/microbial mat processes and global carbon flux. - EPS enhances sediment cohesion and stability against resuspension; differing EPS physical forms exert quantitatively different stabilizing effects. EPS also influences the optical properties of sediments and seawater (reflectance, scattering, absorbance). - In open waters, EPS mediates aggregation into TEP and marine snow, especially during the later stages of phytoplankton blooms and during excessive mucilage events, facilitating sedimentation of cells, EPS, and sorbed ions/trace elements to the seafloor and contributing substantially to vertical carbon and element fluxes in oceans. - EPS production and biofilms are pervasive across aquatic, terrestrial, and host-associated systems, impacting disease and health processes.

The work synthesizes how EPS underpin biofilm structure and function, explaining microbial advantages of surface attachment under fluctuating natural conditions. By detailing EPS physicochemical diversity and architecture, it links microscale processes (ion binding, diffusion modulation, enzymatic activity) to macroscale outcomes (sediment stability, optical properties, mineral precipitation, and carbon export via marine snow). The dual role of EPS in carbonate precipitation provides a mechanistic bridge between microbial ecology and geobiology, informing interpretations of stromatolites, microbial mats, and ancient carbon cycles. Recognition that EPS includes proteins, DNA, and amyloids, not only polysaccharides, reframes the biochemical landscape of biofilms and suggests new avenues for understanding resilience, degradation resistance, and horizontal gene dynamics in communities. Advanced imaging/spectroscopic approaches are highlighted as crucial for in situ characterization, addressing gaps in structural understanding.

EPS are extracellular polymeric substances produced and secreted by microorganisms that vary in physical state, chemical properties, and functional roles, often associated with biofilms. EPS production influences processes of broad biological and geobiological importance: altering sediment physical stability and optical signatures, participating in biogeomineral precipitation (e.g., CaCO3), and mediating global fluxes of carbon and other elements in oceans through aggregation and sedimentation. Biofilms and their EPS are prominent across extreme and diverse environments, from Antarctic epipontic communities to hypersaline ponds and hydrothermal vents.

The physical structure of natural EPS is poorly understood. Historical emphasis on polysaccharides reflects culture-based artifacts (carbon-rich conditions) and investigator focus, potentially biasing compositional interpretations. A considerable fraction of natural EPS does not fit classical biochemical categories, complicating comprehensive chemical characterization.