The role of sulfur in a primordial arsenic biochemical pathway - Mono Lake

DAAD PPP 54367250

Life as we know it today requires phosphorus. Arsenic, which lies directly below phosphorus in the periodic table, shows sufficient chemical similarity that many organisms try to substitute arsenate for phosphate, but it is a poison to almost all organisms. However, microorganisms have been described that can utilize arsenate as electron acceptor. They have been isolated e.g. from Mono Lake, an arsenic- and sulfide-rich, oxygen-depleted, hypersaline, closed basin soda lake, which is thought to reflect primordial conditions as predominated at the early stages of Earth development or may still exist in extraterrestrial environments. Is As-metabolizing life in Mono Lake an indication that arsenic could have had (and, in specific niches, potentially still has) an important biological role in  the early development and evolution of life prior to the evolution of modern photosynthesis 2.7 billion years ago? This question is currently pursued intensively e.g. by the groups of P. Davies, J. Hollibaugh, R. Oremland, J. Stolz, E. Lebrun, B. Schoepp-Coethenet.

We postulate that such an early arsenic cycle would have been significantly affected by the presence of sulfide. Instead of a direct oxidation of arsenite to arsenate, we propose that arsenite in sulfidic environments reacts much more readily with polysulfides (formed from sulfide and elemental sulfur) to yield thioarsenates which in turn can be transformed to arsenate abiotically or microbially catalyzed. Thioarsenates have just recently been discovered in more and more sulfidic environments and are already known to be key arsenic species in Mono Lake.

The long-term goal of the envisaged project is to identify reaction pathways, reaction rates and products of thioarsenate formation and degradation with arsenate, nitrate or oxygen as electron acceptors and to identify different groups of microbes or certain gene expressions responsible for these reactions to prove the role of sulfur in a primordial arsenic biochemical pathway.

Two organisms currently investigated in greater detail are the anaerobe chemoautotrophic arsenate-respiring MLMS-1 and the facultative, chemoautotrophic arsenite-oxidizing MLHE-1.