FATE OF ELEMENTAL SULFUR IN A SULFIDIC CAVE AQUIFER MIXING ZONE

Open Access
Author:
Clark, Christian Evan
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 09, 2017
Committee Members:
  • Jennifer Macalady, Thesis Advisor
  • Katherine Freeman, Committee Member
  • Christopher House, Committee Member
Keywords:
  • Caves
  • sulfur cycling
  • biogeochemistry
  • microbiology
  • Iron cycling
  • Frasassi Caves
  • disproportionation
  • Desulfocapsa
  • dark biosphere
  • subsurface
  • sediment geochemistry
  • high-throughput
Abstract:
The Frasassi Cave system (Italy) is intersected by a perennially sulfidic deep aquifer that mixes with oxygenated meteoric water in the upper 1-10 meters. It therefore presents an ideal setting to study dark sulfur cycling under oxygen-limiting conditions. Conspicuous white biofilms are ubiquitous in the cave streams and pools within the oxic-anoxic mixing zone of the aquifer. Elemental sulfur concentrations in biofilms are 20 to 25% S(0) by mass, while underlying anoxic sediments contain less than 5% S(0). I analyzed paired biofilm-sediment samples using Illumina high-throughput sequencing with universal (bacterial and archaeal) 16S rRNA primers and found that the most common biofilm type is dominated by Gamma-, Beta-, Epsilon-, and Deltaproteobacteria. Underlying sediment microbial communities are nearly identical except for the lower abundance of Gammaproteobacteria related to freshwater Beggiatoa strains. My community composition data are consistent with Beggiatoa-dominated microbial mats in which cells orient along the oxic-anoxic interface at the sediment surface, as measured in microsensor profiles. Microsensor profiles indicate net acid production in the sediments but not in the biofilms, suggesting that sulfide is primarily oxidized to S(0) rather than sulfuric acid in the biofilms. Deltaproteobacteria populations in both biofilms and sediments are dominated by diverse relatives of Desulfocapsa thiozymogenes, an autotroph that carries out sulfur disproportionation in culture. Although the biochemistry and ecology of sulfur disproportionation are not yet well understood, my data suggest that S(0) disproportionation is an important sink for the large quantity of S(0) generated in the system.