Abstract: 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.