This dissertation investigated the role of biology in several biogeochemical cycles in acid sulfate chloride (ASC) geothermal springs in Yellowstone National Park (YNP). Elemental sulfur (S°) is associated with many geothermal springs, yet little is known about the organisms involved in its cycling. The aqueous and solid phase geochemistry near the source of Dragon Spring, an ASC spring in the Norris Geyser Basin (NGB) of YNP, was used to guide the enrichment and isolation of two novel S°-reducing Crenarchaeota affiliated with the order Desulfurococcales. Both isolates are chemoorganotrophs, dependent on complex peptide-containing carbon sources, S°, and anaerobic conditions for respiration-dependent growth. Physiological characterization suggests the isolates are adapted to the physicochemical conditions of Dragon Spring which is supported by quantitative PCR analysis which indicates that the isolates represent a significant fraction of the microbial community associated with S° precipitates in several ASC geothermal springs in the NGB in YNP. Both isolates are capable of utilizing naturally-occurring, complex forms of carbon as a carbon and energy source, and naturally-formed S° as terminal electron acceptor for respiration-dependent growth, suggesting a role for these microbes in the biological cycling of carbon and sulfur in these environments.

Our understanding of the flow of carbon, energy, and other materials between microbial producer species and heterotrophic consumers is limited, in particular in geothermal systems. Novel invertebrate larvae related to Odontomyia sp. (Stratiomyidiae: Diptera) were observed grazing microbial mat biomass in several ASC geothermal springs in the NGB. DNA-based methods were used to demonstrate that stratiomyid larvae graze interstitial algal populations within the mat biomass. Results also indicate that the biomass grazed by larvae contained elevated levels of monomethylated Hg (MeHg) and total mercury (THg). As a consequence of grazing the mat biomass, larvae biomagnified MeHg in their tissues at 2.7- to 5.5-times the concentrations measured in mat biomass. The results of this analysis indicate the Killdeer contained MeHg at a concentration 4.64-fold greater than larval tissue. Collectively, this data suggests that larval grazing behavior represents a key pathway for the transfer of MeHg to species within a geothermal mat-based food web.