Biesgen, Danh: Soil microaggregates as habitat for microorganisms. - Bonn, 2022. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
Online-Ausgabe in bonndoc:
author = {{Danh Biesgen}},
title = {Soil microaggregates as habitat for microorganisms},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2022,
month = jul,

note = {Aggregation of soil particles is one of the most characteristic features of terrestrial soils and crucial for their functionality. Despite the high relevance of soil aggregates, only little is known about their genesis and functionality due to their complex and heterogeneous three-dimensional architecture. Furthermore, soil aggregates represent a major bacterial habitat, but so far neither the impact of aggregate structures on soil bacterial communities and their specific bacterial colonization are fully understood, nor the bacterial contribution to the formation of aggregate structures. Based on this, in this thesis I analyzed (I) whether different aggregate types have a formative influence on the soil bac-terial community and (II) whether specific aggregate size classes provide preferential habitats for specific bacteria. In addition, I analyzed (III) whether bacteria are involved in the initial formation of aggregate structures and whether these structures in turn have an influence on bacterial life. Fur-thermore, I explored (IV) the possibilities of modern light microscopic systems and image processing tools in visualizing complex aggregate structures and their bacterial colonization.
To address effects on the bacterial community structure, I analyzed aggregates varying in size, clay content and, for microaggregates, localization in the soil as free or occluded within macroaggregates (I). This was done using size-fractionated soil samples from a clay catena. The results showed that the bacterial community was influenced by both clay content and aggregate size class. Whereby with increasing clay content an increased differentiation of the bacterial community was observed, espe-cially in the smallest aggregate fraction. I used an indicator taxa analysis to test whether different aggregate size classes provide specific habitats for particular bacterial taxa (II). Here, I could demon-strate that especially in the smallest aggregate size fraction (<20 µm) differences in the general life-style of the indicator taxa were observable. Predominantly copiotrophic bacteria were identified to be enriched in the free aggregate fraction, whereas predominantly oligotrophic organisms were enriched in the occluded aggregate fraction, indicating that a further differentiation of the bacterial community occurs within this specific aggregate size fraction in dependence on the localization of the aggregates in the soil matrix. To investigate the initial aggregate genesis and the impact of these resulting aggre-gates on bacterial cells, I utilized montmorillonite and goethite as model minerals as well as different soil bacteria in the presence or absence of stress conditions in form of wetting and drying cycles (III). My analyses revealed that the presence of bacteria contributed to the formation of larger aggregate structures, whereby effects on aggregate formation were species-dependent (III). In addition, I demonstrated that the aggregate structures that were formed had a sheltering effect for the bacteria when applying wetting-drying cycles. Using a modern epifluorescence microscope, equipped with an ApoTome.2 and a color camera, I visualized a variety of microaggregates in real color along with their surficial bacterial colonization upon method establishment (IV). By applying specific staining ap-proaches and modern image processing methods I was able to demonstrate the feasibility of visualiz-ing complex soil microaggregates and observed a heterogeneous bacterial colonization on their sur-faces.
In synthesis, my results thus show that in addition to size other aggregate properties or also the spa-tial localization of aggregate structures in the soil matrix, contribute to the diversification of bacterial communities in the soil. In this context, soil bacteria are not only beneficiaries of the available struc-tures and given influencing factors, but also actively involved in the genesis of aggregate structures, with different bacterial species themselves exerting different influences. As a consequence, unique aggregate structures develop, which provide the foundation for a diverse soil bacterial community, demonstrating the fundamental importance of processes and effects at small spatial scales and their impact across larger and superordinate scales.},

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