Direct and indirect formation of organohalogens by microorganisms

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Organohalogens influence atmospheric chemistry and thereby earth climate. Thus, it is necessary to identify the sources of organohalogens and to obtain a detailed understanding of the different mechanisms that lead to their formation. From recent studies it became evident that both biotic (enzymatically catalysed), but also abiotic redox reactions involving humic substances and iron minerals can lead to the formation of organohalogens. However, the mechanisms of these processes, the identity of the reactive organic and iron species, the global distribution and geochemical parameters influencing the enzymatic and abiotic reactions, and the relative contribution of abiotic vs. biotic processes to the global organohalogen budget are currently unclear. The main goals of this research project are: i) to determine whether microbial redox reactions of iron minerals and humic substances can produce reactive iron species and humic compounds that lead to abiotic formation of volatile organohalogens (VOX) and ii) to quantify the diversity, abundance, and activity of halogenating enzymes in the environment. For this purpose soil samples from organohalogenreleasing field sites will be collected. Laboratory microcosms will be setup with these samples to determine the role of humic substances, iron minerals, and iron-/humics-metabolizing bacteria for VOX formation and to evaluate the effect of changing geochemical conditions, including redox status, hydration (water) status, and temperature on VOX formation. Further we propose to analyze and quantify the genetic diversity and in situ gene expression of halogenating enzyme and the geochemical parameters controlling enyzme activity in the microcosms and at the Southern African and Russian field sites. Comprehensive knowledge on the direct and indirect formation of organohalogens by microorganisms will not only further our understanding of natural biological halogenations processes, but also facilitate scientific efforts to quantify the sources, sinks, and global burdens of organic halogen compounds.