Geoökologie - Master of Science

Angebotene Masterarbeit

Formation of thioarsenates during electron shuttling in coastal waters by cable bacteria

Betreuer: Britta Planer-Friedrich

Background. Many coastal waters seasonally experience oxygen depletion due to a combination of high loading with organic matter and nutrients, stratification of the water column, and elevated temperatures. Oxygen-depletion combined with reduction of sulfate to toxic sulfide should stress marine life, however, it has been found that it survives surprisingly often these anoxic periods. The reason likely are cable bacteria. Cable bacteria are filamentous, multicellular bacteria that efficiently transport electrons from one end to the other. With this long-distance electron transfer they can couple spatially separated redox half reactions over centimeter scales. For the coastal sediment, in spring, cable bacteria abound in the top centimeters of the sediment and oxidize hydrogen sulfide. The electrons from sulfide oxidation are passed to the sediment surface through internal electric ‘wires’ and donated to oxygen (blue). The hydrogen sulfide is derived from concurrent sulfate reduction but also from dissolution of a large storage of iron sulfide (black). Protons from the anodic oxidation process promote further dissolution of iron sulfide, and the dissolved ferrous iron diffuses to the surface where it reacts with oxygen to form a crust of iron hydroxide (orange). When oxygen depletion develops in late summer and sulfate reduction intensifies, the iron ‘firewall’ is ready to capture the hydrogen sulfide that piles up in the sediment (purple).

However, marine systems do not only contain Fe and S but also other potentially toxic elements. One of the most prominent ones is Arsenic. There are initial studies on arsenic mobilization during the biogeochemical processes induced by cable bacteria, but these studies have been done focusing only on the well known arsenic oxyanions arsenite and arsenate. However, under sulfidic conditions arsenic can complex with sulfur and form so-called thioarsenates. These thioarsenates are a bit tricky to determine which is why people normally don´t bother. My group has specialized on thioarsenates over the past 10 years and we could show that they are not exotic at all, but can well dominate arsenic speciation in sulfidic environments, as those coastal waters certainly are.

Tasks. During a conference meeting contact was established to Sebastiaan van de Velde who did his PhD on determination of arsenic speciation and mobility in the presence of cable bacteria. He could provide both the organisms for laboratory studies and instructions on how to cultivate them as well as potentially access to a field site (Lake Grevelingen). The idea would thus be for a master or bachelor student (or potentially also two bachelor students working jointly on that topic) to visit him at Brussels University in Belgium for 1-2 weeks to learn lab techniques and (depending on season and stratification in Lake Greveling) take natural samples for further incubation experiments in the lab. The incubation work itself including arsenic speciation analyses would then be done in our labs in Bayreuth.