May 11 – PhD defense by Engy Ahmed

Microbe-mineral interactions in soil
Investigation of biogenic chelators, microenvironments and weathering processes

by Engy Ahmed

Date and time: Monday, May 11 at 13.00
Place: Nordenskiöldsalen, Geohuset (link to the house plan)

Supervisor: Dr. Sara Holmström
Assistant Supervisor: Dr. Volker Brüchert, Dr. Anders Andersson, Prof. Nils Holm

Opponent: Prof. Roger Finlay, Department of Forest Mycology and Plant, Swedish University of Agricultural Sciences, Sweden

Examining committee:
Prof. Erland Bååth, Department of Biology, Lund University, Sweden
Prof. Kristín Vala Ragnarsdóttir, School of Engineering and Natural Sciences, University of Iceland, Iceland
Prof. Stefan Karlsson, School of Science and Technology, Örebro University, Sweden

The interplay between geology and biology has shaped the Earth during billions of years. Microbe-mineral interactions are prime examples of this interplay and underscore the importance of microorganisms in making Earth a suitable environment for all forms of life. The present thesis takes an interdisciplinary approach to obtain an integrated understanding of microbe-mineral interactions. More specifically it addresses how the composition and distribution of biogenic chelators (siderophores) differ with regard to soil horizon and mineral type in situ, what siderophore type soil microorganisms produce under laboratory conditions, what role microbial surface attachment plays in mineral weathering reactions and what central roles and applications siderophores have in the environment.

Podzol, the third most abundant soil in Europe and the most abundant soil in Scandinavia, was chosen for a field experiment, where three mineral types (apatite, biotite and oligoclase) were inserted in the organic, eluvial and upper illuvial soil horizons. The study started with an investigation of the siderophore composition in the bulk soil profile and on the mineral surfaces (Paper I), which was followed by a study of the siderophore producing capabilities of microorganisms isolated from the soil profile under laboratory conditions (Paper II). Subsequently, a study was done on the impact of microbial surface attachment on biotite dissolution (Paper III). Finally, the roles of siderophores in nature and their potential applications were reviewed (Paper IV).

The major findings were that the concentration of hydroxamate siderophores in the soil attached to the mineral surfaces was greater than those in the surrounding bulk soil, indicating that the minerals stimulate the microbial communities attached to their surfaces to produce more siderophores than the microorganisms in the bulk soil. Each mineral had a unique assemblage of hydroxamate siderophores that makes the mineral type one of the main factors affecting siderophore composition in the natural environment. Siderophore production varied between the microbial species originating from different soil horizons, suggesting that the metabolic properties of microbes in deep soil horizons function differently from those at upper soil horizons. Microbial surface attachment enhanced the biotite dissolution, showing that attached microbes have a greater influence than free living populations on weathering reactions in soil. In conclusion, our findings reflected that the complicated relationship between microorganisms and mineral surfaces reinforces the central theme of biogeochemistry that the mineral controls the biological activity in natural environments. However, the importance of these relationships to the overall biogeochemical systems requires further investigation.

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