Aqueous geochemistry of a sulfurous freshwater spring: implications for sulfur cycling and resident microbial communities
Blount Springs offers a sulfide-rich environment inhabited by a diverse microbial community in which to study the sulfur redox reactions and the microbially mediated processes via the analysis of sulfur isotopes of sulfide and sulfate. The average δ34SH2S at the wellheads is +31.1 ±0.3 / (n=9) and the average δ34SSO4 in the biofilm and downstream locations is highly variable with a mean value of +16.9 ±7.5 / (n=9). Sulfur isotope fractionations from H2S to SO4 range from 7.1 to 13.9 /. Utilizing the sulfur isotope fractionations it is concluded that (i) the sulfide is most likely derived from Thermochemical Sulfate Reduction (TSR) in the subsurface; (ii) the sulfide is subsequently oxidized to sulfate via the microbially related process of chemosynthetic sulfide oxidation; and (iii) the sulfide is also likely consumed by the abiotic processes of sulfide oxidation and outgassing of H2S. The isotopic fractionations from H2S to SO4 corroborate the isotopic fractionations observed in the laboratory during chemosynthetic sulfide oxidation. The carbon isotopic composition of DIC and the concentration of DIC support the hypothesis of microbial consumption of organic matter. Visualization of the biofilm via macroscopic and microscopic imaging revealed a morphologically diverse community. Biofilm of white, pink, and orange color were observed over the course of the study. Microscopic images revealed rod-shaped, coccoid, and filamentous cells. PCR amplification confirmed the presence of bacterial DNA. Aerobic lithotrophs, such as Thioplaca and Beggiatoa are possible groups of bacteria responsible for the chemosynthetic oxidation of sulfide at Blount Springs.