MICROBIALLY-ENHANCED WEATHERING IN SUBSURFACE HABITATS: SULFUR-OXIDIZING BACTERIA AND THE CAVE ENVIRONMENT

Annette Summers Engel (aengel@geol.lsu.edu)

Department of Geological Sciences, C1140, The University of Texas at Austin, Austin, Texas 78712

While most caves and karst features form from carbonic acid dissolution of carbonate rocks, caves can also form by sulfuric acid dissolution (Egemeier 1981). Sulfuric acid is derived from hydrogen sulfide-rich waters that rise up into carbonate strata and become oxidized 1) through abiotic reactions from mixing of waters having different concentrations of hydrogen sulfide or mixing reduced and oxygen-rich waters (Palmer 1995), as well as 2) from biological oxidation reactions (Altas and Bartha 1998).

One of the major problems interpreting caves formed by sulfuric acid speleogenesis is recognizing the mechanisms of sulfuric acid formation. However, in modern caves containing sulfidic waters, there is a wide range of microorganisms in these systems. A vast majority of them are sulfur-oxidizing bacteria that live within cave springs and streams, as well as growing on cave-wall surfaces (Table 1). Some of these bacteria are reportedly acidophilic due to the production of sulfuric acid (Hose and Piscarowicz 1999; Vlasceanu et al., in press). Therefore, sulfur-oxidizing bacterial populations can be examined as a means of understanding sulfuric acid speleogenesis. Additionally, studying the microbial communities in sulfidic caves can provide a better appreciation of other sulfidic habitats, relative microbial abundance and viability in the subsurface, and potential biogeochemical interactions pertinent to global sulfur cycling. Unfortunately, although sulfur-oxidizing microorganisms have been identified, few studies have addressed the possible geomicrobiological impacts that the microbial communities may have on the cave environment. For these reasons, this work examined sulfur-oxidizing bacterial populations from four sulfidic cave systems and tested whether their metabolic activity can enhance cave formation and modification.

The caves used in this study include: Movile Cave, Romania; Grotta Grande del Vento-Grotta del Fiume (Frasassi caves), Italy; Cesspool Cave, Virginia; and Lower Kane Cave, Wyoming. Sub-aqueous and sub-aerial microbial habitats are ubiquitous in these caves. Movile Cave contains the only floating mats, but also has wall biofilms and anaerobic, submerged mats. The Frasassi caves contain thick, filamentous submerged mats in very turbulent water, as well as extensive wall biofilms associated with elemental sulfur and gypsum mineralization. Several delicate filamentous and web-like mats form in Cesspool Cave, predominately limited to the sediment interface in shallow, slow-moving pools. Some mats can also be found in rifle zones and forming thin wall mats near the water. Biofilms and mats in Lower Kane Cave are diverse, including thick filamentous mats in fast-moving water coming from spring discharge areas, red and black mats in pools, and as biofilms on gypsum and elemental sulfur cave-wall surfaces.

Chemoautotrophic, elemental sulfur- and thiosulfate-based media were used to grow sulfur-oxidizing bacteria from the caves at pH values of 4 and 6. Thiosulfate-based media were the most successful for achieving enrichment diversity, although growth was observed on all media and pH combinations used. The greatest numbers of bacteria were obtained from filamentous mats from Lower Kane Cave and the cave-wall biofilms from the Frasassi caves. The least number of cultured bacteria were from the Movile Cave wall mats and mats from Lower Kane Cave pools.

A summary of successful laboratory enrichment data shows that several morphotypes were observed, although rod-shaped microorganisms dominated the cave samples and pH enrichments (Figure 1). Short rods were, on average, less than 1 m m in length, and long rods were greater than 1 m m. All the rods observed were Gram negative. At pH 6, filamentous bacteria were observed from Movile Cave floating mat, Movile Cave submerged mats, Cesspool Cave wall mat, Lower Kane Cave black mats, and Lower Kane Cave stream mats. Minor occurrences of Gram positive cocci and fungi were also observed in enrichment cultures. At pH 4, no filamentous bacteria were enriched for, although there were minor amounts of vibrio/spiril morphotypes, as well as cocci and fungi. Isolation of bacterial strains resulted in only short or long rods. The majority of the isolated colonies were small and transparent, however some were pigmented (brown, black, white, yellowish-orange). Most of the pigmented colonies were strains isolated from Lower Kane Cave black and red mats, and from Frasassi cave-wall biofilms.

Molecular phylogenetic analysis of some of the laboratory strains suggested that most are closely related to several species of the genus Thiobacillus (from the Frasassi caves and Cesspool Cave). Clones obtained from direct PCR amplification of 16S rDNA gene sequences were most similar to sequences of sulfur-oxidizing, filamentous bacteria, including Thiothrix and Thiovulum spp. (Engel 1999).

Individual strains were evaluated for sulfuric acid generation using two methods to detect for acid and to quantify acidification rates. These methods involved incorporating either a pH detector (bromocresol green) or calcium carbonate into solid media. Of 70 strains isolated in the laboratory from the four caves, 47 generated acid on the bromocresol green plates. However, only 30% of all the clones were capable of dissolving supplemental calcium carbonate in the growth media. The Frasassi cave-wall biofilms had the most strains that generated acid and dissolved calcium carbonate (Vlasceanu et al., in press). Based on comparisons with in situ, abiotic dissolution rates reported by Galdenzi et al. (1997) and Sarbu (1996) for the Frasassi caves and Movile Cave respectively, preliminary rates of microbially-mediated, sulfuric acid rock dissolution were significantly faster than previously measured abiotic dissolution rates for the cave systems.

Results of this study indicate that there is considerable diversity in populations of sulfur-oxidizing bacteria living in karst systems and that their metabolic activity can have a significant impact on carbonate rock weathering. If active sulfidic karst is characterized by sulfuric acid reactions that are derived from both abiotic and biotic processes, it can be proposed that ancient systems, such as Carlsbad Cavern and Lechuguilla Cave, had the same processes occurring during times in their formation histories. Therefore, any future interpretation of ancient karst development should include a geomicrobiological component.

TABLE 1. Composite list of known active sulfidic caves and documented sulfur-oxidizing microorganisms.

*(M) - microscopic identification and culturing; (G) - genetic identification

Figure 1. Summary of averaged morphological data from the four cave systems, organized by enrichment media (thiosulfate or elemental sulfur) and pH 6 (upper graph) or pH 4 (lower graph). Ramo: Ramo Sulfureo site in the Frasassi caves; Grotta: Grotta Sulfurea site in the Frasassi caves. Short rods were less than 1 micron in cell length, whereas long rods were more than 1 micron in length, on average.

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