Microbial stabilization of lotic fine sediments

Abstract

The microbial stabilization of fine sediments constitutes an essential ecosystem function with great ecological and economic implications e.g. in the context of reservoir and waterway management. Although this process is well researched in intertidal mudflats, there is still a major lack of knowledge for lotic systems. To perform fundamental research in this field and to account for the associated very high level of complexity, expertise of natural and engineering science was combined in an interdisciplinary approach. A highly sophisticated mesocosm setup was designed and constructed to guarantee fully controllable and reproducible natural-like boundary conditions during biofilm formation. The overall aim of the performed studies in this doctoral thesis was a comprehensive investigation of all relevant parameters of the cultivated biofilms, such as the microbial biomass, the produced extracellular polymeric substances (EPS), and the composition of the microbial community as well as the stability of the biofilm. This extensive approach should allow the identification of functional key parameters of the biofilm as well as essential interactions and their impact on the overall biofilm ecosystem and resulting biostabilization. In a series of long-term experiments, different influencing factors on biofilm development and corresponding biostabilization were assessed. The first potential impact factor that was analyzed was the experimental setup itself. Furthermore, the influence of the seasonal changes of the microbial community in the utilized river water and the effects of different levels of bed shear stress and illumination intensity were assessed. The results of these different experiments provided essential new insights into the process of biostabilization of lotic fine sediments. Firstly, the reliability of the used experimental setup could be proven, as no significant differences could be detected in biofilm formation and biostabilization comparing different mesocosm sections. The fact that very similar biofilms were developing when the boundary conditions were identical was a crucial prerequisite for any further investigations. In addition, the relevance of biostabilization in lotic systems, which was doubted for a long time, could be proven. However, freshwater and brackish habitat can be very different (e.g. in terms of nutrient availability). This was exemplarily indicated by significantly lower microbial biomass in the analyzed freshwater biofilms compared to biofilms from well-studied intertidal mudflats. Moreover, the very complex interplays between bacteria and diatoms in the biofilm matrix were underlined which led to a focus on this subject during further subsequent studies via an extensive genetic and microscopic profiling. Secondly, the important role of EPS during biostabilization could be demonstrated, whereby the significance of extracellular proteins, such as adhesives produced by sessile diatoms, was suggested. This observation may extend the current EPS research which focusses on extracellular carbohydrates due to their high quantitative fraction in the EPS matrix. Furthermore, the interactions between the microbes, the extracellular matrix and the overall stability of the biofilm system appeared to be much more complex than formerly assumed. Thirdly, the importance of the microbial community in the biofilm system could be elucidated. Even though a high correlation between mere microbial biomass and biostabilization could be detected, especially the seasonality experiments emphasized the impact of the life style of key players among the diatoms. These insights could be extended during the experiments analyzing the different levels of abiotic boundary conditions, where differently stable biofilms were clearly dominated by different assemblages of dominant bacteria. These observations constitute very important new insights into microbial biostabilization as a direct correlation between microbial ecology and the overall, actually measurable ecosystem function of the biofilm could be shown for the first time. Concluding, the insights into the fundamental principles of biostabilization gathered during this thesis can be seen as important steps for further fundamental research. The construction of a reliable unique setup is complete, the reproducible biofilm cultivation in this setup is verified and first investigations of different driving factors during biostabilization were performed. These analyses paved the way for further studies to analyze currently hardly assessed boundary conditions and deeper assessments in order to generate a sound database for future modelling approaches of the dynamics of microbially stabilized lotic fine sediments.