Microbial Mats

(paper presented at the 15th International Symposium on Environmental Biogeochemistry meeting, 200. Wroclaw, Poland)

Biogeochemical Diversity of Microbial Mats from Lower Kane Cave, Wyoming, USA

Annette Summers Engel, Philip C. Bennett, Libby A. Stern

Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712  email: pbennett@mail.utexa.edu, lstern@mail.utexas.edu

Key words: karst geomicrobiology, microbial geochemistry, sulfide oxidation

Introduction

Microorganisms occupy a variety of subsurface habitats, often altering their geologic surroundings and the physicochemical conditions of their environment. Carbonate caves associated with sulfide-rich groundwater represent an energy-rich subsurface habitat where an assortment of microorganisms can successfully compete, with the potential for developing complex microbial communities. We are studying several sulfidic caves that have developed in the Madison Limestone of the Bighorn Basin approximately 130 km west of Yellowstone National Park and 120 km north of Thermopolis, WY, USA. The Bighorn Basin contains extensive oil fields and thermal and non-thermal springs that discharge along the flanks of the basin, and has several areas of known karstification. Most of the caves are epigenic, but several formed or were modified by sulfuric acid speleogenesis (Egemeier 1981).

The focus of this study is Lower Kane Cave, near Lovell Wyoming, USA, where a sulfidic spring and cave stream support a rich community of sulfur-oxidizing acidophilic and neutrophilic bacteria. The neoformed cave minerals are dominated by gypsum, suggesting the cave is formed almost exclusively by sulfuric acid dissolution of the carbonate rocks. In addition to the chemotrophic sulfur-oxidizing bacteria, however, we find evidence of a more diverse ecosystem, including methanogens, sulfate-reducers, iron-oxidizers, iron-reducers, nitrogen bacteria, undifferentiated heterotrophs, as well as higher organisms. We report here preliminary data on the community structure of microbial mats dominated by sulfur-oxidizers, but with close associations of other aerobic and anaerobic populations.

Materials and Methods

The cave was initially examined during spring 1999, and again during summer 2000. Samples of microbial mats, stream waters, and mineral deposits were collected and preserved for laboratory characterization. Water samples were analyzed for major ion chemistry, organic carbon, dissolved gases, and stable isotopes.  Samples of microbial mats were prepared for examination by scanning electron microscopy SEM), and major element analysis of the ashed residue using ICP-OES.  Microbial community structure was characterized by standard culturing methods, examination of gross morphology by SEM, and PCR amplification of small-subunit rDNA using Eubacteria and Archaeabacteria specific probes.

Results and Discussion

There are four principle springs discharging into LKC, with an average temperature of 22.6^oC and pH of 7.3.  H₂S concentrations varied from 0.05 to 0.96 mg/L during the summer 2000 sampling; compared to 66 mg/L measured in the spring of 1999, suggesting that the geochemical system is quite dynamic.

Three distinct microbial mats were present in the sub-aqueous passage of the cave: 1) thin gelatinous black mats and coatings; 2) thick red and orange mats; and 3) white filamentous mats. Black mats only occur immediately adjacent to the spring orifice, with the red mats several meters down stream of the spring. White mats were found in the water column at several locations, but always isolated from the other mats, most often in fast-moving water. The microbial community in the black mats is still uncharacterized, but lacks culturable sulfur-oxidizing bacteria, yet SEM examination suggests the presence of Fe-S framboid structures. Iron-oxidizing bacteria, with minor acid-producing sulfur-oxidizing bacteria, dominate the red mats.  These red mats have abundant iron oxide bacterial sheaths, but the source of iron is still unknown.  White mats are dominated by neutrophilic sulfur-oxidizing bacteria but with abundant methanogens identified by PCR methods. We also found small bubbles of methane in the white mats.

A subaerial microbial habitat was found on the cave walls associated with soft gypsum paste and sulfur deposits. Mucous-like biofilms with a pH of 0 to 1 formed droplets of sulfuric acid suspended from the cave-walls and ceilings.  Acid-producing, sulfur-oxidizing bacteria dominate the subaerial community.

Conclusions

Carbonate rock is dissolving due to sulfuric acid dissolution, releasing other elements that cycle through the biogeochemical system. These elements can be beneficial, as nutrient and energy sources, or detrimental to the microbes. Consequently, colonization of different areas of the cave may be controlled by elemental substrate concentration and abundance, and the elemental composition of the mats reflects the dissolving bedrock, more so than the incoming water. Mat geochemistry may also reflect different metabolic groups that may be concentrating specific elements in the mats.

References

Egemeier, S. J. (1981): Cavern development by thermal waters. NSS Bulletin, 43, 31-51.