Abstract from Masters Degree thesis, from the University of Cincinnati, Department of Biological Sciences, 1999

Geomicrobiology of Sulfidic Karst Systems

Annette Summers Engel

Sulfur-oxidizing bacteria were investigated in four different sulfidic karst systems in order to compare microbial distribution in subterranean habitats, and to assess the potential microbiological impact of sulfuric acid on rock dissolution. Four caves were examined, including: Movile Cave, Romania; the Frasassi caves, Italy; Cesspool Cave, Virginia; and Lower Kane Cave, Wyoming. Microorganisms in these active caves form a variety of aqueous and sub-aerial microbial mats and biofilms, including thick filamentous streamers and microbial draperies.

There has been very little research that compares sulfur-dependent microbial communities from sulfidic karst systems, although several investigators have proposed that these bacteria may be involved in cave development due to the copious amounts of sulfuric acid that they produce. Therefore, sulfur- and thiosulfate-oxidizing bacteria were enriched for under laboratory conditions and tested for acid production using two plating methods. Various morphotypes were observed from all cave samples, including predominately rod-shaped and filamentous bacteria. Of seventy isolated strains, all were rod-shaped and assumed to be thiobacilli. Forty-six strains could generate acid. Dissolution rates were calculated for some of the isolated strains following the same basic methodology that previous researchers used to determine abiotic sulfuric acid dissolution rates for one of the caves in this study. Rates for microbially-mediated rock dissolution were significantly faster than the measured abiotic rates, suggesting that bacteria have the potential to enhance cave development.

A molecular analysis of bacteria from Cesspool Cave was conducted to identify some of the bacteria possibly responsible for sulfuric acid dissolution in the natural setting. Two strains of laboratory-isolated, sulfur- and thiosulfate-oxidizing bacteria were shown to be closely related to Thiobacillus thermosulfatus, a bacterium that has been found in acidic, subsurface environments. Clones obtained from direct PCR amplification of 16S rDNA isolated from the microbial mats were most similar to sequences of sulfur-oxidizing, filamentous bacteria, including Thiothrix and Thiovuvlum spp. These genera previously have been found in sulfidic springs, other sulfidic caves, and groundwater. One additional clone was related to Cytophaga, a heterotrophic bacterium that can degrade complex organic molecules. Based on these molecular analyses, the Cesspool Cave microbial community appears to be dominated by filamentous sulfur-oxidizers such as Thiothrix and Thiovulum. No clones related to Thiobacillus were found during the molecular analysis, indicating that the strains isolated in the laboratory may not be dominant in the caves, and the contribution of sulfuric acid from this genera may be minor in comparison to the acid generated by the filamentous bacteria.

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