Microbial stabilization of riverine sediments by extracellular polymeric substances

Abstract

Sediment stability is a critical component for the understanding of cohesive sediment dynamics. Traditionally, physico-chemical sediment conditions have been regarded as most important drivers of sediment stability. However, over the last decade, the stabilization of sediment by biological activity, particularly the influence of highly hydrated matrices of extracellular polymeric substances (EPS) has been given increasing attention. However, most studies have focused on the sediment/water interface and, usually, of marine systems. The present study exploits current knowledge of EPS dynamics from marine systems and applies it to freshwater habitats, also considering a wide range of biological and physico-chemical variables. Natural sediments were taken from a freshwater site with high levels of heavy metal pollution (Lauffen reservoir, River Neckar, Germany). Vertical profiles from the flocculent surface layer to depth of 50 cm within the sediment were investigated, monthly, over the course of year. Tubificidae and Chironomidae larvae constituted the majority of the macrofauna. Despite the turbidity of the water column, a highly diverse and abundant microphytobenthic community of diatoms (11-82 microg g(-1) DW) was found at the sediment surface closely associated with high numbers of bacteria (10(9) cells g(-1) DW). The concentrations of all EPS moieties were remarkably high (0.1-0.5, 1.7-3.8, 0.9-5.2 mg g(-1) DW, for colloidal and bound carbohydrates and proteins, respectively) and levels were comparable to those determined in intertidal studies. The microalgal and bacterial biomass both showed strong correlations with the colloidal and bound EPS carbohydrate fractions. The data suggested that the present macrofauna as well as the metabolic activities of microalgae and bacteria interact with sedimentological factors to influence the properties of the sediment by binding fine-grained sediment, changing water content and enhancing the organic content through secretion products. The colloidal and bound EPS moieties showed strong correlation with the critical shear stress for erosion over sediment depth. It is suggested that the cohesive strength of the sediment was controlled by a high number of active adsorption sites and higher charge densities in fine grained sediments. The EPS network may significantly enhance this by embedding particles and permeating the void space but also in offering additional ionic binding sites and cross-linkages.

Biofilm origin of clay-coated sand grains

“…However, biofilms have been reported to attach dust particles to sand-grain surfaces in desert environments (Belnap and Weber, 2013). Clay coats have been reported in meander fluvial point-bar environments (Wooldridge et al, 2017), in which biofilm abundance is comparable to that in estuarine sediments (Gerbersdorf et al, 2008).…”

Dynamics and compositional changes in extracellular carbohydrates in estuarine sediments during degradation

“…Very little is known about the contribution of bacterial EPS to the EPS pool of intertidal biofilms, despite the high bacterial cell numbers in such sediments (Underwood & Paterson 1993, van Oevelen et al 2006a) and a clear spatial association between bacterial cells and EPS in artificial estuarine biofilms (Mueller et al 2006). Bacteria have been found to contribute significantly to river sediment carbohydrate content (Gerbersdorf et al 2008), and bacterial cell numbers are correlated with the total sediment carbohydrate content (Underwood & Paterson 1993). Most prior investigation of the EPS pool in intertidal sediments has concentrated on the pathways of MPB autotrophic production of carbohydrates (Underwood & Paterson 2003), and the bacterial contribution to sediment EPS dynamics is largely undescribed.…”

Influence of bioturbation on the biogeochemistry of littoral sediments of an acidic post-mining pit lake

“…Indeed, besides a higher nutrient and oxygen availability, chironomid larvae yield also labile and fresh organic matter (e.g. mucus, faeces) within sub-surface sediments that make their burrows favorable habitats for certain microbial communities (Stief and De Beer, 2002;Stief et al, 2005;Gerbersdorf et al, 2008). Despite these considerations, integrated rates of production/consumption of iron in the whole sediment column were balanced and the global turn-over remained unchanged.…”