Microbiology and geochemistry in a hydrogen-sulphide-rich karst environment

Cueva de Villa Luz, a hypogenic cave in Tabasco, Mexico, offers a remarkabl e opportunity to observe chemotrophic microbial interactions within a karst environment. The cave water and atmosphere are both rich in hydrogen sulphide. Measured H2S levels in the cave atmosphere reach 210 ppm, and SO2 commonly exceeds 3 5 ppm. These gases, plus oxygen from the cave air, are absorbed by freshwat er that accumulates on cave walls from infiltration and condensation. Oxida tion of sulphur and hydrogen sulphide forms concentrated sulphuric acid. Dr ip waters contain mean pH values of 1.4, with minimum values as low as 0.i. The cave is fed by at least 26 groundwater inlets with a combined flow of 200-300 l/s, Inlet waters fall into two categories: those with high H2S con tent (300-500 mg/l), mean P-CO2 = 0.03-0.1 atm, and no measurable O-2; and those with less than 0.1 mg/l H2S, mean P-CO2= 0.02 atm, and modest O-2 con tent (up to 4.3 mg/l). Both water types have a similar source, as shown by their dissolved solid content. However, the oxygenated water has been expos ed to aerated conditions upstream from the inlets so that original H2S has been largely lost due to outgassing and oxidation to sulphate, increasing t he sulphate concentration by about 4%. Chemical modelling of the water show s that it can be produced by the dissolution of common sulphate, carbonate, and chloride minerals. Redox reactions in the cave appear to be microbially mediated. Sequence ana lysis of small subunit (16S) ribosomal RNA genes of 19 bacterial clones fro m microbial colonies associated with water drips revealed that 18 were most similar to three Thiobacilli sop., a genus that often obtains its energy f rom the oxidation of sulphur compounds. The other clone was most similar to Acidimicrobium ferrooxidans, a moderately thermophilic, mineral-sulphide-o xidizing bacterium. Oxidation of hydrogen sulphide to sulphuric acid, and h ence the cave enlargement, is probably enhanced by these bacteria. Two cave-enlarging processes were identified. (1) Sulphuric acid derived fr om oxidation of the hydrogen sulphide converts subaerial limestone surfaces to gypsum. The gypsum falls into the cave stream and is dissolved. (2) Str ongly acidic droplets form on the gypsum and on microbial filaments, dissol ving limestone where they drip onto the cave floors. The source of the H2S in the spring waters has not been positively identifi ed. The Villahermosa petroleum basin within 50 bm to the northwest, or the El Chichon volcano similar to 50 km to the west, may serve as source areas for the rising water. Depletion of S-34 values (-11.7 parts per thousand fo r sulphur stabilized from H2S in the cave atmosphere), along with the hydro chemistry of the spring waters, favour a basinal source. (C) 2000 Elsevier Science B.V. All rights reserved.