Evaluation of Cryptic Sulfur Cycling in Marine Sediments
Despite the importance of marine sediments as a carbon sink and many efforts to integrate microbiological with geochemical and geological data, the understanding of microbial organic carbon mineralization processes in marine sediments remains incomplete. There is growing evidence for the existence of an unrecognized oxidative sulfur cycling (cryptic sulfur cycling) above, within and below of the main sulfate reduction zone. For example, sulfate exists in supposedly sulfate-free methanogenic marine sediments from the Aarhus Bay and Black Sea. This implies that sulfur cycling is an ongoing process in the deep biosphere. Quantification of cryptic sulfur cycling is a major challenge because it does not leave an imprint in the net sulfur budget of the surrounding environment. My goal is to quantify cryptic sulfur cycling in sediments and assess if it is of significant importance to organic carbon mineralization. Sediment samples for my project were collected from the Kattegat-Skagerrak region located in Danish, Norwegian and Swedish national waters. With these sediment samples, I conducted sediment incubation studies in airtight bags fitted with Rhizon ® samplers to allow non-destructive sampling of pore-water at different points of time. I developed experimental protocols that allow for the monitoring of cryptic sulfur cycling with the help of 34SSO4 and 18OH2O labels that were released from ampules embedded in sediment incubations, which were kept strictly anaerobic. My incubation experiments resulted in the surprising finding that once sulfate is introduced, sulfate reduction rates are extremely high, and that the rate of oxygen isotope exchange between sulfate and water mediated by sulfate reducing microbes was similar to pure culture incubations. Apparently, during the long storage of the sediments, the sulfate-reducing microbes ran out of sulfate, and were limited to the supply from sulfate from oxidative processes, fueled by oxygen that diffused into the storage bags. This relative lack of sulfate as central terminal electron acceptor led to the accumulation of organic substrates that are favorable for sulfate reducing microbes. Once sulfate was supplied, sulfate reduction immediately proceeded at astonishingly high rates. Thus, my incubation experiments cannot be considered to be representative for cryptic sulfur cycling in marine sediments. Nevertheless, I make key observations that can contribute to the body of research on cryptic sulfur cycling. The initial concentration of sulfate in the bags was approximately 37µM, a value that is similar to what has been postulated as sulfate concentrations below the sulfate methane transition. Apparently, even the high availability of organic substrates for the sulfate reducing microbes in the storage bags did not allow sulfate-reducing organisms to lower the sulfate concentration even further. This implies that there is a link between sulfate concentrations, and energetic threshold for dissimilatory sulfate reduction. Finally, inhibition of sulfate reduction with the help of molybdate reveals that sulfate consumption continues to proceed at low rates, however, with very little associated oxygen isotope exchange between sulfate and water. Potentially, inhibition with molybdate alters the sulfate reduction pathway in a manner where the process becomes less reversible. This possibility offers a new avenue to study the mechanisms of stepwise sulfate reduction by or dissimilatory sulfate bacteria. Alternatively, the decline in sulfate concentrations could be attributed to assimilatory processes. If this is the case, the slow increase in the oxygen isotope composition of sulfate would represent the elusive fingerprint of cryptic sulfur oxidation.
Mathuri, Michael Ngari, "Evaluation of Cryptic Sulfur Cycling in Marine Sediments" (2017). ETD Collection for University of Texas, El Paso. AAI10284368.