Bachelor Thesis

The role of free and As-bound sulfide during arsenite oxidation by Alkalilimnicola ehrlichii, Strain MLHE-1

Linda Schneider (12/2011-08/2012)

Support: Britta Planer-Friedrich

Controversial to geochemical expectations, the concurrent presence of the reduced and oxidized arsenic species, arsenite and arsenate, has been reported for anoxic zones of highly reducing, sulfide-enriched aquatic environments. Alkalilimnicola ehrlichii (strain MLHE-1), isolated from the alkaline Mono Lake, is one organism capable of anaerobically oxidizing arsenite and sulfide by simultaneous reduction of nitrate. In this thesis, the role and transformation of free sulfide versus sulfur bond to arsenic in thioarsenates was investigated in anaerobic incubation experiments with strain MLHE-1. As expected, arsenite, sulfide and also polysulfides were stable under abiotic conditions, but oxidized completely to arsenate and non-specified sulfur species in the presence of strain MLHE-1. In experiments with monothioarsenate no change in arsenic speciation was observed, indicating that strain MLHE-1 most likely does not use this thioarsenic species.  Adding sulfide to arsenite solutions led to spontaneous formation of thioarsenates, which is most likely a pure chemical process. The higher the sulfide concentrations, the more and the higher thiolated thioarsenates formed. Subsequent transformation of thioarsenates to arsenate occurred at As:S ratios of 1:1 and 1:4. Except for a slightly shorter transformation time, no difference was observed in arsenate production between abiotic and biotic setups. At a 10fold sulfide excess arsenate production was completely inhibited and thioarsenates remained stable. Despite large differences in arsenate production, cell growth was observed in all experiments, questioning the effect of abiotic and biotic factors in thioarsenate formation and their transformation to arsenate as well as the presence of other potential growth substrates and electron donors.