Sabine Ulrike Gerbersdorf

Microbial stabilization of riverine sediments by extracellular polymeric substances

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.

The stabilisation potential of individual and mixed assemblages of natural bacteria and microalgae.

It is recognized that microorganisms inhabiting natural sediments significantly mediate the erosive response of the bed (“ecosystem engineers”) through the secretion of naturally adhesive organic material (EPS: extracellular polymeric substances). However, little is known about the individual engineering capability of the main biofilm components (heterotrophic bacteria and autotrophic microalgae) in terms of their individual contribution to the EPS pool and their relative functional contribution to substratum stabilisation. This paper investigates the engineering effects on a non-cohesive test bed as the surface was colonised by natural benthic assemblages (prokaryotic, eukaryotic and mixed cultures) of bacteria and microalgae. MagPI (Magnetic Particle Induction) and CSM (Cohesive Strength Meter) respectively determined the adhesive capacity and the cohesive strength of the culture surface. Stabilisation was significantly higher for the bacterial assemblages (up to a factor of 2) than for axenic microalgal assemblages. The EPS concentration and the EPS composition (carbohydrates and proteins) were both important in determining stabilisation. The peak of engineering effect was significantly greater in the mixed assemblage as compared to the bacterial (x 1.2) and axenic diatom (x 1.7) cultures. The possibility of synergistic effects between the bacterial and algal cultures in terms of stability was examined and rejected although the concentration of EPS did show a synergistic elevation in mixed culture. The rapid development and overall stabilisation potential of the various assemblages was impressive (x 7.5 and ×9.5, for MagPI and CSM, respectively, as compared to controls). We confirmed the important role of heterotrophic bacteria in “biostabilisation” and highlighted the interactions between autotrophic and heterotrophic biofilm consortia. This information contributes to the conceptual understanding of the microbial sediment engineering that represents an important ecosystem function and service in aquatic habitats.

Biostabilization of cohesive sediments: revisiting the role of abiotic conditions, physiology and diversity of microbes, polymeric secretion, and biofilm architecture.

In aquatic habitats, micro-organisms successfully adhere to and mediate particles, thus changing the erosive response of fine sediments to hydrodynamic forcing by secreting glue-like extracellular polymeric substances (EPS). Because sediment dynamics is vital for many ecological and economic aspects of watersheds and coastal regions, biostabilization of cohesive sediments is one of the important ecosystem services provided by biofilms. Although the research on biostabilization has gained momentum over the last 20 years, we still have limited insights principally due to the complex nature of this topic, the varying spatial, temporal, and community scales examined, oversimplified ecohydraulic experiments with little natural relevance, and the often partial views of the disciplines involved. This review highlights the current state of our knowledge on biostabilization and identifies important areas for future research on: (A) the influence of abiotic conditions on initial colonization and subsequent biofilm growth, focusing on hydrodynamics, substratum, salinity, nutrition, and light climate; (B) the response of microbes in terms of physiological activity and species diversity to environmental settings as well as biotic conditions such as competition and grazing; and (C) the effects of the former on the EPS matrix, its main constituents, their composition, functional groups/substitutes, and structures/linkages. The review focuses specifically on how the numerous mutual feedback mechanisms between abiotic and biotic conditions influence microbial stabilization capacity, and thus cohesive sediment dynamics.

The engineering potential of natural benthic bacterial assemblages in terms of the erosion resistance of sediments

The secretion of extracellular polymeric substances (EPS) by bacteria has been recognized as important across a wide range of scientific disciplines, but in natural sediments, EPS production by microalgae as a mechanism of sediment stabilization has received much more attention than bacterial products. In the present study, the stabilization potential of a natural benthic bacterial assemblage was tested in cultures growing on noncohesive glass beads. The surface erosion resistance as determined by a cohesive strength meter was significantly enhanced over time compared with controls. Nutrient enrichment of the bacterial assemblages by a general broth (bacteria+) resulted in enhanced stabilization (x 3.6) compared with nutrient-depleted (bacteria) assemblages (x 1.8). This correlated with higher bacterial biomass and EPS concentrations in enriched cultures. Substratum stability was closely related to bacterial cell numbers (R2=0.75/0.78) and EPS protein concentrations (R2=0.96/0.53) (for bacteria/bacteria+ treatments, respectively), but not to EPS carbohydrates. This study implies a greater significance of extracellular proteins in substratum cohesion within the EPS complex than recognized previously. The data show both the importance of bacterial assemblages for microbial sediment stabilization and that a change in abiotic conditions can significantly affect sediment stabilization.

Impairment of the bacterial biofilm stability by triclosan

The accumulation of the widely-used antibacterial and antifungal compound triclosan (TCS) in freshwaters raises concerns about the impact of this harmful chemical on the biofilms that are the dominant life style of microorganisms in aquatic systems. However, investigations to-date rarely go beyond effects at the cellular, physiological or morphological level. The present paper focuses on bacterial biofilms addressing the possible chemical impairment of their functionality, while also examining their substratum stabilization potential as one example of an important ecosystem service. The development of a bacterial assemblage of natural composition – isolated from sediments of the Eden Estuary (Scotland, UK) – on non-cohesive glass beads (<63 µm) and exposed to a range of triclosan concentrations (control, 2 – 100 µg L−1) was monitored over time by Magnetic Particle Induction (MagPI). In parallel, bacterial cell numbers, division rate, community composition (DGGE) and EPS (extracellular polymeric substances: carbohydrates and proteins) secretion were determined. While the triclosan exposure did not prevent bacterial settlement, biofilm development was increasingly inhibited by increasing TCS levels. The surface binding capacity (MagPI) of the assemblages was positively correlated to the microbial secreted EPS matrix. The EPS concentrations and composition (quantity and quality) were closely linked to bacterial growth, which was affected by enhanced TCS exposure. Furthermore, TCS induced significant changes in bacterial community composition as well as a significant decrease in bacterial diversity. The impairment of the stabilization potential of bacterial biofilm under even low, environmentally relevant TCS levels is of concern since the resistance of sediments to erosive forces has large implications for the dynamics of sediments and associated pollutant dispersal. In addition, the surface adhesive capacity of the biofilm acts as a sensitive measure of ecosystem effects.

Anthropogenic Trace Compounds (ATCs) in aquatic habitats - research needs on sources, fate, detection and toxicity to ensure timely elimination strategies and risk management.

Anthropogenic Trace Compounds (ATCs) that continuously grow in numbers and concentrations are an emerging issue for water quality in both natural and technical environments. The complex web of exposure pathways as well as the variety in the chemical structure and potency of ATCs represents immense challenges for future research and policy initiatives. This review summarizes current trends and identifies knowledge gaps in innovative, effective monitoring and management strategies while addressing the research questions concerning ATC occurrence, fate, detection and toxicity. We highlight the progressing sensitivity of chemical analytics and the challenges in harmonization of sampling protocols and methods, as well as the need for ATC indicator substances to enable cross-national valid monitoring routine. Secondly, the status quo in ecotoxicology is described to advocate for a better implementation of long-term tests, to address toxicity on community and environmental as well as on human-health levels, and to adapt various test levels and endpoints. Moreover, we discuss potential sources of ATCs and the current removal efficiency of wastewater treatment plants (WWTPs) to indicate the most effective places and elimination strategies. Knowledge gaps in transport and/or detainment of ATCs through their passage in surface waters and groundwaters are further emphasized in relation to their physico-chemical properties, abiotic conditions and biological interactions in order to highlight fundamental research needs. Finally, we demonstrate the importance and remaining challenges of an appropriate ATC risk assessment since this will greatly assist in identifying the most urgent calls for action, in selecting the most promising measures, and in evaluating the success of implemented management strategies.

A new approach to investigate the interactions between sediment transport and ecotoxicological processes during flood events

Extreme hydrodynamic events such as flood events or dredging activities bear the risk of eroding sediments in rivers, reservoirs, harbour basins or estuaries. One of the key concerns associated with these erosion processes is the re-mobilisation of sediment-bound pollutants in highly contaminated sediments. To date, much research has been conducted to characterise flow and sediment processes associated with hydrological events such as floods. Furthermore, there is a large body of literature describing the interaction of contaminants associated with particulate matter to aquatic biota. However, there is little knowledge regarding interactions between hydro-sedimentological and ecotoxicological processes. Understanding of the ecotoxicological consequences and associated risks to aquatic wildlife associated with hydraulic events can provide critical information to regulatory bodies or managing authorities. Specifically, it will aid in assessing risks associated with current management practices and will aid in developing more sustainable future management practices for waterways or harbours. Therefore, a combined experimental methodology between hydraulic engineers and ecotoxicologists was developed to investigate the ecological and toxicological relevance of sediment re-suspension and transport during erosion. An overview of this methodology is given in the present paper.

Vertical migration of phytoplankton in coastal waters with different UVR transparency

BackgroundThe vertical migration of phytoplankton was investigated in natural waters using in situ fluorescence profiling, chlorophyll a concentrations and life counts at two study sites differing in coloured dissolved organic matter (cDOM) concentrations. The data from the corresponding water depths (50-cm intervals down to 10 m) and times (hourly, before dawn to sunset, several days) were related to the highly resolved (2 nm) underwater ultraviolet radiation (UVR)/photosynthetic active radiation (PAR) transparency (290 to 700 nm).ResultsChlorophyll a maxima of mainly motile dinoflagellates were observed in situ at all days and at both study sites (open marine, brackish waters), independent on prevailing weather conditions or cDOM concentrations. Phytoplankton migration was triggered solely by irradiance in the 400- to 700-nm wavelength range (PAR) at the particular water depth, irrespective of PAR/UVR ratios and surface UVR (290 to 400 nm), after an illumination period of about 40 min. Interestingly, the PAR tolerance levels of the phytoplankton, which have been lower in cDOM-rich waters, matched their light acclimation values determined by parallel PAM measurements.ConclusionsThe response of the phytoplankton to PAR is not a sufficient protection strategy versus increasing UVR levels, which might have wide ecological implications beyond the level of primary producers to impact important ecosystem functions such as the delicate trophic interactions.

A multi-disciplinarily designed mesocosm to address the complex flow-sediment-ecology tripartite relationship on the microscale

BackgroundThe stabilization of fine sediments via biofilms (‘biostabilization’) has various economic and ecological implications but is presently unaddressed within lotic waters. To investigate natural biofilm growth and functionality in freshwater sediments under controlled boundary conditions, a unique mesocosm was constructed that combines established know-how from engineering and natural sciences and consists of six straight flumes. To test the comparability of biofilm growth within one flume and between the flumes, extracellular polymeric substances (EPSs), microbial biomass and microbial community composition were closely monitored over time and space as well as in relation to biofilm adhesiveness (proxy for biostabilization).ResultsMost importantly, biofilm development and biostabilization capacity revealed no significant differences within flume regions or between the flumes and the biofilms significantly stabilized the substratum as compared to abiotic controls. However, interesting temporal successions in biofilm growth phases became visible in shifting abundance and diversity of bacteria and microalgae resulting in varying EPS secretion and biostabilization.ConclusionsThese findings demonstrated the importance of biostabilization for fine sediment dynamics in freshwaters. Secondly, this unique setup allows comparable biofilm growth under controlled environmental conditions, an important requisite for future research on the ecological significance and impact of biostabilization for ecosystem functioning at varying environmental scenarios.

The effect of light intensity and shear stress on microbial biostabilization and the community composition of natural biofilms

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