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Biogeochemical Processes at Sulfide-oxidant Interfaces

Due to the steep chemical gradients, sulfide-oxidant interfaces are narrow zones of high microbial activity. The intense biochemical turnover at these interfaces can impact entire ecosystems, and control feedback mechanisms between sediment and water column biogeochemistry

Phosphorous, Phosphorites and Molybdenum

Together with colleagues from MARUM and U. Stockholm, we have used 33P-radiotracer to explore the role of microorganisms in phosphorite formation. Under oxic and anoxic conditions, large sulfide oxidizing bacteria, i.e. .Thiomargarita namibiensis, accumulated 33P-phosphate intracellularly and rapidly catalyzed the conversion of 33P-phosphate to apatite. The rates of this conversion balanced rates of phosphate release from organic matter. Normally, under anoxic conditions a net release of phosphate from the sediments to the water column is observed. However, these nitrate-reducing bacteria appear to disrupt this release by promoting phosphogenesis. The oxygen isotope composition of phosphate from pore waters at sites with large sulfide oxidizing bacteria is higher than at sites where they are absent. This indicates a fundamental difference in phosphate turnover by large sulfide oxidizing bacteria compared to the typical microbial community that inhabits marine sediments.
Panel A. Chain of approx. 40 cells of Thiomargarita namibiensis in plain light.
Panel B. Corresponding beta image (red = high beta counts).
Cell width is approximately 200 nm.
Molybdenum (Mo) cycling in upwelling regions may be linked to phosphate release and phosphorite formation by large sulfide oxidizing bacteria. Experiments with sediments from the Namibian shelf area show quantitative Mo removal at high productivity sites. The concentration of Mo in the modern oceans is only slightly higher than the threshold at which Mo centered nitrogenases are replaced by alternative, less efficient nitrogenases. A small change in the availability of Mo in the oceans could thus have a dramatic effect on the marine nitrogen cycle.

Phototrophic Sulfide Oxidation and Sulfur Disproportionation

We studied 33S and 36S isotope fractionation during sulfate reduction, sulfide oxidation and sulfur disproportionation in meromictic lakes (in collaboration with U. Maryland and U. Zagreb) and conclude that sulfide oxidation is coupled to sulfur disproportionation in these environmental settings. The isotope composition of elemental sulfur showed that the elemental S was formed by a process involving equilibrium with polysulfides. The absence of significant concentration of polysulfides in the water column of Fayetteville Green Lake indicates that the sulfide – polysulfide – sulfur equilibrium occurs within the purple sulfur bacteria.
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