Microbial assemblages as ecosystem engineers of sediment stability

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. 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). 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: x8.5 and x10.8, CSM: x2.5 and x5.7, respectively). The data also suggested synergistic effects between proteins and carbohydrates, which enhanced adhesion and stability. 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. 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 strategies.