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SP6: The importance of soil microaggregates as habitat for microorganisms and their metabolic activity, and the role of microorganisms in aggregate formation and stabilization

Wie aim at an in-depth understanding of the process of soil microaggregate formation and microaggregate stability in relation to its chemical composition and microarchitecture, and the role of microaggregates for soil functioning, including the habitat function for microorganisms. Microaggregates and clay particles represent a major habitat for bacteria in soil. The arrangement and aggregation of soil components to a three dimensional structure is thus among the controlling factors of biological processes. Microorganisms contribute to the formation, stability and destabilization of soil microaggregates. The work proposed within this subproject covers both aspects, the habitat function of microaggregates for bacteria and the role of bacteria in microaggregate formation and stability.

We will prove the hypothesis that different types of microaggregates, obtained from a soil toposequence of variable clay content, host different types of bacterial communities and that this selection is influenced by the physicochemical characteristics of the microaggregates. To this end, bacterial community composition and diversity will be analysed in fractionated microaggregates using molecular methods. The obtained data that describe the microbial communities will be analysed in combination with those that describe the physicochemical parameters of the microaggregates, which will be provided by different partners of this research unit, to identify those factors that exert an effect on bacterial colonization.

A second major aim of this subproject is the study of the localization of metabolically active cells, whereby different bacterial taxa are expected to be active in spatially restricted specific microsites of microaggregates. The characteristics of these microsites are analysed at micrometre scale and thus single-cell-level, based on fluorescent microscopy in combination with mass spectrometric methods such as LA-IRMS, LA-ICP-MS and nanoSIMS. The habitats of active and inactive cells are compared to identify possible controls of microbial activity in and on microaggregates. To assess the role of bacteria for aggregate formation, we focus on selected model organisms in this first phase of the project and perform aggregation experiments to identify to what extent different bacteria contribute to aggregate formation and stabilization. Microbial destabilization of microaggregates will be analysed by studying the degradation of labelled extracellular polymeric substances, which serve as stabilizing organic material in a central microcosm experiment. With these different approaches, we will obtain data that allow to obtain a deeper understanding regarding the question how microorganisms affect microaggregate formation and their disaggregation on the one hand and the regulatory mechanisms of microaggregates on microbial presence and activity on the other hand.