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 disciplines, but in natural sediments the microalgae have been the most investigated EPS producers and sediment stabilizers. In the present paper, the stabilization potential of a natural benthic bacterial assemblage was tested in cultures growing on non-cohesive glass beads over 5 weeks. The substrate erosion resistance as determined by CSM (Cohesive Strength Meter) and by MFPC (Magnetic Force Particle Capture) was significantly enhanced over time compared to controls. Nutrient enrichment of the bacterial assemblages (bacteria+) resulted in enhanced stabilization (x 3.6) as compared with nutrient-depleted (bacteria) assemblages (x 1.8). This correlated with higher bacterial biomass and EPS concentrations. Substrate stability was thus closely related to bacterial cell numbers (R2 = 0.75 / 0.78) and EPS protein concentrations (R2 = 0.96 / 0.53) (for bacteria / nutrient treatment, respectively), but not to EPS carbohydrates. This study implies more significance for the proteins in substratum cohesion within the EPS complex than previously recognized. The data show the importance of bacterial assemblages for microbial sediment stabilization and secondly, that a change in abiotic conditions can affect their stabilization potential significantly

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 mu g 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.</p

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.Publisher PDFPeer reviewe

Floating photovoltaic plants: ecological impacts versus hydropower operation flexibility

Floating photovoltaic power plants are a quickly growing technology in which the solar modules float on water bodies instead of being mounted on the ground. This provides an advantage, especially in regions with limited space. Floating modules have other benefits when compared to conventional solar power plants, such as reducing the evaporation losses of the water body and operating at a higher efficiency because the water reduces the temperature (of the modules). So far, the literature has focused on these aspects as well as the optimal design of such solar power plants. This study contributes to the body of knowledge by i) assessing the impact of floating solar photovoltaic modules on the water quality of a hydropower reservoir, more specifically on the development of algal blooms, and by ii) studying the impact that these modules have on the hydropower production. For the first part, a three-dimensional numerical-hydrodynamic water-quality model is used. The current case (without solar modules) is compared to scenarios in which the solar modules increasingly cover the lake, thus reducing the incident sunlight from 0% to finally 100%. The focus is on microalgal growth by monitoring total chlorophyll-a as a proxy for biomass. For the second part, as the massive installation of solar modules on a reservoir may constrain the minimum water level (to avoid the stranding of the structures), the impact on hydropower revenues is examined. Here, a tool for optimal hydropower scheduling is employed, considering both different water and power price scenarios. The Rapel reservoir in central Chile serves as a case study. The response of the system strongly depends on the percentage that the modules cover the lake: for fractions below 40%, the modules have little or no effect on both microalgal growth and hydropower revenue. For moderate covers (40-60%), algal blooms are avoided because of the reduction of light in the reservoir (which controls algal growth), without major economic hydropower losses. Finally, a large solar module cover can eradicate algal blooms entirely (which might have other impacts on the ecosystem health) and results in severe economic hydropower losses. Altogether, an optimum range of solar module covers is identified, presenting a convenient trade-off between ecology health and costs. However, a massive deployment of these floating modules may affect the development of touristic activities in the reservoir, which should be examined more closely. In general, the findings herein are relevant for decision-makers from both the energy sector and water management.Deutscher Akademischer Austausch Dienst (DAAD). German Research Foundation (DFG): DFG-NO 805/11-1. Chilean Council of Scientific and Technological Research: CONICYT/FONDAP/15110019