Biology Department Seminar

Our conceptual understanding of microbe-arsenic (As) interactions has changed considerably over the past decade, due in large part to the surge in interest as to why microorganisms engage in As redox transformations, and how these transformations influence As movement and bioavailability in the environment. Contemporary research has focused largely on arsenite and arsenate reduction, reporting on studies concerning the biochemistry and genetics involved in As(III) oxidation and As(V) reduction. A review of a number of environmental studies reveals that As(V) and As(III) are the major arsenicals found in soils and natural waters. However, methylated forms of As are also frequently found in very high concentrations such as in marine and geothermal environments. The importance of these methylated forms in terms of ecological function and environmental fate are completely unknown. The foundational hypothesis underpinning our current efforts is that As(III) biomethylation represents an important branch point in the global arsenic cycle. Although presumed to be a detoxification strategy for microorganisms, we suggest that As biomethylation has far broader environmental implications; namely, interspecies C transfer that functionally intertwines global As and C cycling in important ways not yet described. A mechanism of arsenite resistance through methylation and subsequent volatization is described. Heterologous expression of arsM from Rhodopseudomonas palustris was shown to confer As(III) resistance to an arsenic sensitive strain of E. coli. ArsM catalyzes the formation of a number of methylated intermediates from arsenite, with trimethylarsine as the end product. The net result is loss of arsenic, both from the medium and from the cells. Since ArsM homologues are widespread in nature, this microbial-mediated transformation is proposed to have an important impact on the global arsenic cycle. In addition, new findings on microbial arsenite oxidation will be presented.