Focus of this thesis

To summarize, up to now the development of different biofilms was investigated in three very specialized research approaches: the first field of research is the assessment of fundamental principles during the formation and maturation of bacterial biofilms focusing on molecular biology e.g. genomics, transcriptomics and/or proteomics. The second target of investigation is the process of microbial biostabilization of fine sediments concentrating on intertidal mudflats and the role of diatoms, their produced EPS and their adaption to different environmental conditions. The third field of study analyses the correlations between the maturing microbial community, flow of substances and structural elements as a result of different boundary conditions in lotic biofilms. While each of these individual disciplines were

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able to unravel important pieces of information, there is still a lack of a comprehensive understanding concerning microbial biostabilization in riverine systems.

This current lack of knowledge constitutes the starting point of this thesis. The most important aim of this thesis was to elucidate the relevance of microbial stabilization of fine sediments in lotic systems. As stated above, the lack of high ion concentrations in the running water may lead to very low stabilization compared to intertidal habitats. However, the microbial community of the riverine benthos may traverse a very different adaption process than intertidal assemblages. As a result, the stability of the biofilm matrix and its corresponding stabilization capacity of fine sediments may be influenced in up to now unknown ways. Especially the microbial ecology appears to be a decisive influencing factor for the temporal and structural development of the biofilm system. Thus, this thesis aims to assess the community composition of the biofilm in high detail and to relate the state of the microbial community to the overall biofilm stability. In this context, the community composition of both, the apparently structural very significant diatoms as well as the metabolic very versatile bacteria was analyzed. This approach allows conclusions about potential interaction between these two different taxa to impact the biofilm habitat.

Furthermore, by identifying dominant species among bacteria as well as diatoms, the relevance of possible functional key players can be contrasted with potential functional redundancies. This investigation constitutes unique fundamental research in the principles of riverine microbial biostabilization with a special focus on microbial ecology. The overall aim is to do the first steps in order to understand the importance of specific microbial species in this very complex process. Regarding a potential application of the gathered knowledge in future sediment management concepts, parameters are required that are more precise and easier to interpret than complex ecological characteristics. This is the reason why this thesis additionally aims to evaluate the significance of parameters for microbial biostabilization that are accessible via straight forward analytic approaches and with high cost and time-effectiveness. A detailed assessment of bacterial as well as algal biomass and the major two EPS compounds (carbohydrates and proteins) was performed in order to elucidate their suitability as proxies for biofilm stability and corresponding microbial biostabilization.

Furthermore, it may be assumed that parallel to the observations in intertidal mudflats, different boundary conditions might exhibit an important impact on the structural and functional development of the biofilms in a riverine system. As described in section 1.3.6,

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some knowledge could be gained regarding lotic systems. However, studies on the development of riverine biofilms on fine sediments are currently relatively rare and there is still a significant lack of investigations that include microbial biostabilization in this habitat.

Thus, this thesis aims to perform a comprehensive assessment of the impact of three major boundary conditions on the stabilization capacity of the biofilm system. Concerning abiotic environmental parameters, especially two abiotic boundary conditions appear to be of high importance. Firstly, light intensity shapes the autotrophic microbial community and influences its metabolic productivity with direct consequences for EPS production. Hence, light intensity possesses an essential relevance for the collocation and spatial organization of autotrophic microbes and the biochemical characteristics of the biofilm extracellular matrix. As a result, the physio-chemical features of the EPS and their potential gluing effect on the surrounding sediment grains may be found to be significantly influenced by the intensity of light available for autotrophic primary production. Secondly, different levels of bed shear stress can determine the degree and mode of microbial attachment to the underling substrate. During the formation of a biofilm, this physical forcing may constitute a major driving factor that shapes the spatial and structural arrangement of biofilm compartments as well as their three-dimensional appearance which in turn may be able to have a significant impact on the functionality and stability of the biofilm system and the underling fine sediment.

However, it is important to note that especially a microbial community in a riverine biofilm is not only subjected to direct abiotic conditions such as light intensity or flow velocity. In lotic systems, the benthic-pelagic feedback loop can be very pronounced e.g. due to the continuous water movement in combination with comparably low water depths in littoral zones. In this context, benthic biofilms constantly undergo a process of detachment of biofilm compartments as well as settling down and attachment of suspended particles and microbes. In addition, as the whole river can be seen as one great continuously changing ecosystem described in the river continuum concept by Barmuta and Lake (1982), these suspended particles can be transported to downstream river section to be deposited there and to influence local benthic communities. Furthermore, the whole biocoenosis of a river ecosystem is influenced significantly by seasonal changes (e.g. shading due to littoral vegetation, water temperature or entrainment of leafs). As a result, the microbial community in the running wave as well as benthic biofilms are subjected to a seasonal succession process. Although this factor may have a huge impact on the development of biofilms, its relevance in regards of microbial biostabilization of riverine fine sediments is up to now not

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addressed. Thus, this thesis aims to elucidate the significance of the seasonal succession process in the microbial community for the corresponding stabilization capacity of the biofilm. This investigation is closely related to the aforementioned investigation of potential functional microbial key players.