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(Redox-)Biogeochemistry on the Microscale - Biofilms and Model Systems

Subject Area Mineralogy, Petrology and Geochemistry
Term from 2017 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 323566027
 
Final Report Year 2024

Final Report Abstract

The fate of nutrients and contaminants in the environment is controlled by redox reactions. In the traditional view, the presence or absence of certain biogeochemical reactions is controlled by the availability of electron donors such as S2-, Fe(II), or organic C, and acceptors such as O2, NO3-, Fe(III), or SO42-. This concept, however, cannot describe all environmental observations adequately. In the last decade, such phenomena have been described as cryptic or hidden biogeochemical cycles, and there has been an increasing awareness of the influence of spatial and temporal heterogeneity in composition and function of sediments, soils, or environmental biofilms at the interface between aquatic environments and the geosphere. Spatially resolved analytical tools such as synchrotron-based scanning transmission X- ray microscopy (STXM), confocal laser scanning microscopy (CLSM) and Raman microspectroscopy, allowed us to identify reaction mechanisms underlying the environmental processes on the micro- down to molecular scales. In this project we focused on interactions between organic matter, e.g. microbially formed extracellular polymers (EPS) with mineral phases such as Fe or Mn minerals in aquatic environments. A theoretical framework of how highly reactive, metastable phases are involved in environmental processes was developed on the sub-millimetre scale. After demonstrating the suitability of lectin staining for environmental research in redox-active environments, the spatial heterogeneity of processes resulting in local pH changes in aquatic biofilms (e.g. the hydrolysis of microbially oxidized Fe(III) to Fe oxyhydroxides) was demonstrated by a combination of CLSM with pH-sensitive probes. On the micron scale, redox-interactions between EPS of microaerophilic Fe(II)-oxidizing bacteria and dissolved, adsorbed, and precipitated Fe-species was studied, which influences Fe and C cycling in aquatic environments. Our development of in-situ electrochemistry STXM allowed us to disentangle fundamental differences in the function of different EPS structures such as twisted stalks and sheaths. This analytical approach is of high interest for entirely different fields of science such as catalyst research, thus led to various collaborative projects. In a collaborative project, for the first time the dissolution of goethite by fungal EPS - a process known from the bulk scale – was shown on individual cell-mineral interfaces. Motivated by promising preliminary experiments, we also focused on redox reactions controlling nutrient availability in soil. A collaborative study furthered our understanding of vivianite oxidation whereas other projects investigated interactions of silicic acid with soil minerals. We identified redox-processes on the Fe-mineral surfaces of paddy soil upon fertilization with silicic acid resulting in phosphate release. Surface-sensitive STXM analysis of arctic soils, revealed the control of Fe and Al mobility by Si and Ca.

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