The Resilience of Biofilm-Bound Sandy Systems to Cyclic Changes in Shear Stress

Sand-attached benthic biofilms drive many important biogeological processes and serve as cooperative "ecosystem engineers". In aquatic environments, biofilms undergo periodic detachment and re-colonization due to the regular changes in hydrodynamic forcing. However, legacy impacts of past microbial actions on current biofilm formation and the biostabilization of the substratum sands are yet to be fully understood. In this study, a systematic set of flume experiments were conducted to investigate the effects of different depositional histories. Changes in the erosion threshold and rate of erosion were determined from the time sequences of suspended sediment concentrations. The contents of extracellular polymeric substances (EPS) and particle morphology of the biofilm-bound sandy matrix were analyzed. Surprisingly, biostabilization is disturbance-stimulated, rather than disturbance-limited, as previously thought. Bio-sandy beds cultivated under intensive disturbance presented an EPS accumulation in each cycle, and showed a more rapid increase in bed strength and stability than when rarely disturbed. All colonies from previous cycles exhibited traces of EPS as "footprints". These stimulated and possibly accelerated the process of recolonization, thereby enhancing the erosion resistance of the bed. In contrast, a stabilized bed was better suited to mature microbial communities. A modified "Windows of Opportunity" framework was therefore put forward. Although biostabilization was not established within short quiescent periods, the system created the "opportunity" to become established in subsequent "windows" by seeding the colonization process. The stabilization, destabilization and re-stabilization of biofilm may imply a much more important role as ecosystem engineers and is relevant for a range of engineered bio-systems.