AbstractsEarth & Environmental Science

Sulfur metabolisms leave behind a record of their activity in the sulfur they utilize. This thesis seeks to advance our understanding of some of these processes with a particular focus on dissimilatory sulfate reduction. The multiple sulfur isotope composition of two porewater sulfate profiles in the anoxic marine sapropel of Mangrove Lake, Bermuda was investigated. The porewater sulfate profiles exhibit the distinct isotopic signatures of microbial sulfate reduction and sulfur reoxidation which simple diagenetic models can reproduce. The reoxidative cycle includes sulfide oxidation to elemental sulfur followed by the disproportionation of the elemental sulfur to sulfate and sulfide, and this process turns over from 50 to 80% of the sulfide produced by sulfate reduction. We suggest that the reoxidative S cycle in any environment can best be identified within two regions of the multiple sulfur isotope fractionation spectrum. Paper 1 is titled Mass-dependent sulfur isotopefractionation during reoxidative sulfur cycling: A case study from Mangrove Lake, Bermuda. The process of evolutionary adaptation has largely been assumed inconsequential on the sulfur isotopic fractionation produced during dissimilatory sulfate reduction and recorded in the isotope rock record. Yet, the diversity of sulfur isotope phenotypes displayed by species of sulfate reducing microorganisms isolated from modern environments amounts to strong evidence that evolutionary adaptation does matter. If this is the case, important information about the evolutionary history of DSR may be preserved in the rock record. However, the relationship between evolutionary adaptation and isotope phenotype is unexplored. To begin addressing this gap in knowledge, the impact of evolutionary adaptation on the fitness and sulfur isotopic phenotype of the dissimilatory sulfate reducer Desulfovibrio vulgaris Hildenborough (DvH) was investigated. The increases in fitness that were observed did not result in a change of the isotopephenotype. At least in the conditions of the experiment this result indicates that the isotopephenotype is not very sensitive to evolutionary adaptation on the hundreds of generations timescale. This suggests that lengthier timescales are necessary for evolutionary-driven divergence of the isotope phenotype. Paper 2 is titled Evolutionary adaptation of a sulfate reducing bacterium and its sulfur isotope phenotype. To address the issues raised in paper 2, pure cultures of Desulfomicrobium baculatm were evolved in batch culture for 300 generations. A greater than two fold increase in growth rate over the course of the experiment was measured as well as a change in isotope phenotype (??) from 15 to 12 %. The response of ?? to evolutionary adaptation resembles in some ways the isotopic response of physiological adaptations to changing environmental conditions. While in the narrow context of the environment where the evolutionary adaptation took place, the change in isotope phenotype is incontestable, it remains to be seen if this difference in…