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.

Purpose Sediment erosion and transport is a governing factor in the ecological and commercial health of aquatic ecosystems from the watershed to the sea. There is now a general consensus that biogenic mediation of submersed sediments contributes significantly to the resistance of the bed to physical forcing. This important ecosystem function has mainly been linked to microalgae ("ecosystem engineers") and their associated extracellular polymeric substances (EPS), yet little is known about the impact of bacterial assemblages and how their varying interactions with microalgae affect the overall biostabilization potential of the combined community. Materials and methods Natural assemblages of bacteria and diatoms-originating from sediment and water samples from the Eden Estuary (Scotland, UK)-were growing on noncohesive glass beads over 5 weeks. The adhesion and the stability of the biofilm was determined by magnetic particle induction (MagPI) and by Cohesive Strength Meter (CSM), respectively, and related to EPS (spectrophotometric determination of carbohydrates and proteins), bacterial cell numbers (flow cytometry), bacterial community (fluorescence in situ hybridization (FISH)), diatom biomass (spectrophotometric determination of chlorophyll a), and diatom assemblage composition (microscopy). Results and discussion The adhesive properties and stability of the biofilm were significantly enhanced over time as compared to controls. The diatoms profited from additional nutrients, while bacteria dominated in nutrient-limited cultures. Subsequent shifts in the microbial population at a species level resulted in varying patterns of EPS production which moderated the biostabilization capacity: Cultures with strong diatom development were less stable than cultures dominated by bacteria (MagPI: ×8.5 and ×10.8, CSM: ×2.5 and ×5.7, respectively). The data also suggested synergistic effects between proteins and carbohydrates, which enhanced adhesion and stability. Conclusions Bacteria populations under these conditions can be regarded as "ecosystem engineers" since their role in sediment stabilization is more important than previously recognized. Abiotic factors such as nutrients altered the interactions between bacteria and microalgae to influence the overall microbial stabilization potential ("engineering web") by affecting the quantity and quality of EPS. Data from MagPI and CSM correlated well (R 2 = 0.82, P < 0.0001), and the new technique, MagPI, is to be recommended for studies on growing biofilms since it determines subtle changes in sediment/biofilm properties with high sensitivity.Recommendations and perspectives Further studies should examine the highly species-specific interactions between microalgae and bacteria and their effects on EPS secretion to impact stability as well as postentrainment of sediments under varying abiotic scenarios. Our growing understanding of the ecosystem functionality of "bioengineering" will have wider implications for water framework directive and sediment/pollutant management strategi...

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.