Ecosystem Engineering: Sediment entrainment and flocculation mediated by microbial produced extracellular polymeric substances (EPS)

• Applicant: Dr. Sabine Ulrike Gerbersdorf
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management ; Microbial Ecology and Applied Microbiology
• Term: from 2011 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 189216230

The current project and its follow-up period concerns microbial biostabilization of fine sediments, a truly important function of biofilms, but rarely addressed in freshwaters. For the current project, key questions were raised that concerned (a) the significance of biostabilization in different niches of freshwaters to unravel (b) fundamental feedback mechanisms between selected environmental parameters (light intensity, hydrodynamic regime, nutrients) and the settlement and growth of potential microbial engineers (bacteria, cyanobacteria, microalgae), as well as (c) address the influence of microbial biomass and species diversity on the quantity and quality of their secreted sediment glue, the extracellular polymeric substances (EPS) to (d) link them to the changed behaviour of fine sediments (stabilization, adhesiveness, flocculation). Up to now, we provided the first experimental evidence of the impressive sediment stabilization capacity of naturally growing biofilm in lotic waters (up to 11 times as compared to controls) and the impact on floc forms, sizes and settling velocity to influence further sediment transport. While these insights into the functionality of young growing biofilms are truly unique, we believe it highly valuable to complement the picture with investigations into matured biofilm and the effects of abruptly changed hydrodynamics as they quite commonly occur in fluvial systems (e.g. by reservoir flushing, floods). Severe shifts in the microbial community were discovered during biofilm growth; this implied strong interactions between bacterial and microalgal species and gave indications of their competition and mutual dependence. While we saw a clear influence of biodiversity on biofilm functions (here biostabilization), we could not yet unravel the role of each species or taxa in the complex natural biofilm. In order to learn more about the key-players in sediment engineering, we need to setup cultures which would also help to reveal the distinctive role of (species-specific) secreted EPS in sediment binding. In addressing these questions on key players and key glue for biostabilization, we enter uncharted territory but this would also greatly assist in the interpretation of our current data. Last but not least, all data on biostabilization depend strongly on the method of determining this functionality. We thus made significant further developments to a relatively young method, the Magnetic Particle Induction MagPI (e.g. revealing the functioning principle, building stronger electromagnets, avoiding remanence). Still, the scientific recognition of our MagPI data on biofilm adhesion depends upon a deeper understanding of their relation to the critical bed shear stress/sediment stability (determined by our straight erosion flume SETEG). Moreover, the project would greatly profit from pursuing a better objectivity and comparability of MagPI data by developing standardized particles and a full-automated system.

• DFG Programme: Research Grants