I-6   Dr. Thomas Wichard, JSMC Jena

Capturing of molybdenum by bacteria: “On the way from soil to the nitrogenase”

Dr. Thomas Wichard*

*Friedrich-Schiller-Universität Jena, Institute for Inorganic and Analytical Chemistry, Jena School for Microbial Communication, Lessingstr. 8, 07743 Jena, Germany – www.jsmc.uni-jena.de ↗

The micronutrient molybdenum is a necessary component of the nitrogen-fixing enzyme nitrogenase. Molybdenum is very rare in soils and is usually present in a highly soluble form making it susceptible to leaching. Therefore, N2-fixing soil bacteria may have difficulty acquiring Mo for diazotrophic growth. The soil bacterium Azotobacter vinelandii acquires Fe by excreting small organic ligands (siderophores) that bind iron and increase its availability. To understand the capturing and acquiring of Mo at the environment-bacteria interface, (i) X-ray spectroscopy was applied to natural samples, (ii) the overall siderophore production was monitored and (iii) the uptake of Mo was determined in short term experiments. X-ray absorption near-edge (XANES) spectroscopy studies showed that Mo binds preferentially to organic matter in a competition experiment to Fe [1]. Here, Mo may be complexed by catecholate groups. These findings are intriguing as laboratory experiments have shown that A. vinelandii releases catecholate siderophores (e.g., azotochelin and protochelin), which have an exceptionally high affinity for Mo. Protochelin can hence potentially outcompete the natural organic matter for molybdate complexation and increase the bioavailability of Mo at low concentrations. Using short term uptake experiments with 98Mo, it was demonstrated that 98Mo-protochelin is available to the bacteria. Protochelin is hence a metallophore used not only for Fe but also for Mo acquisition [2]. Following up, an extensive survey of siderophore-production has revealed different patterns of metallophore-production depending on the bacterial growth phases [3]. In order to decipher novel metallophore-mediated interactions between bacteria and their environment, it is essential to identify these substances under physiological conditions in complex matrices. Therefore, we develop approaches for direct metallophore identification and other organic ligands that form stable complexes with cations or oxoanions in the natural matrix. The importance of molybdenum in the elemental cycles in terrestrial systems will be discussed in the talk.

[1] T. Wichard, B. Mishra, S.C.B. Myneni, J.P. Bellenger, A.M.L. Kraepiel, Nat. Geosci. 2009, 2, 625–629. doi:10.1038/ngeo589
[2] J.P. Bellenger, T. Wichard, A.B. Kustka, A.M.L. Kraepiel, Nat. Geosci. 2008, 1, 243–246. doi:10.1038/ngeo161
[3] M. Deicke, J.P. Bellenger, T. Wichard, J. Chromatogr. A, 2013, 1258, 50–60. doi:10.1016/j.chroma.2013.05.008

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