Diversity of chemosynthetic thiosulfate oxidizing bacteria from diffuse flow hydrothermal vents and their role in mercury detoxification

The mixing of hydrothermal fluids with seawater creates chemical, temperature, and pH gradients that support chemosynthetic primary production at deep-sea vents. These fluids are enriched with reduced sulfur compounds and their oxidation under aerobic conditions is considered the main chemosynthetic process at the vents. The main objective of my research was to gain a better understanding of the aerobic chemosynthetic thiosulfate oxidation processes at deep-sea hydrothermal vents, by studying the abundance and diversity of chemosynthetic thiosulfate oxidizing bacteria, and their role on the detoxification of heavy metals, with an emphasis on mercury detoxification. Fluids, sediments, and biomass from microbial colonization experiments were collected during several expeditions to the East Pacific Rise (EPR) at "9º50'N, 104º17'W" and to the Guaymas Basin, Gulf of California. Microbial isolations were carried out from diluted and undiluted samples. Isolates were identified by 16S rRNA gene analysis. The isolates obtained in pure cultures were related to the general Thiomicrospira, Halothiobacillus, Hydrogenovibrio, Thioclava, Thalassospira, and Pelagibaca, as well as a new isolate EPR 70, which was described as a new species, Salinisphaera hydrothermalis. The isolates were further characterized, and their functional genes encoding enzymes for carbon fixation (RubisCO) and thiosulfate oxidation (SoxB) were analyzed. The Most Probable Number (MPN) technique was carried out in order to determine the abundance of chemosynthetic thiosulfate oxidizing bacteria, and the values obtained were compared with the total number of microorganisms per sample, estimated from microscopic direct counts. Our data show that this group of microorganisms represented from 103 to 107 cells per ml of sample, which accounts for about 0.002% to 14.1% of the total cell counts per sample. The chemical composition of the fluids was analyzed, and results indicated that hydrothermal fluids were enriched in mercury with concentrations comparable to the concentrations found in contaminated surface waters. MPN counts were done with the addition of mercury in order to determine the percentage of chemosynthetic thiosulfate oxidizing bacteria that were mercury resistant. Results indicated that from 0.2 to 24.6% of the chemosynthetic bacteria were resistant, suggesting an adaptation to life in the presence of this toxic metal.