PENDULOUS MICROBIAL/MINERAL FORMATIONS IN AN ACTIVE SULFIDE-DOMINATED CAVE AND POSSIBLE LITHIFIED SPELEOTHEM ANALOGS IN AN ANCIENT, INACTIVE CAVE

Puzzling speleothems in Lechuguilla Cave, NM, known as "u-loops" have caused speculation about a potential biological origin of these structures. This cave was formed by a process known as "hypogenesis," dissolution by sulfuric acid at the subsurface interface of rising hydrogen sulfide-laden water with oxygenated ground water. However, the cave is millions of years old and long since left its chemically aggressive origins behind. The physical resemblance of u-loops to living structures in an active hydrogen sulfide-dominated cave, Cueva de Villa Luz, Tabasco, Mexico has fueled further speculation that these may represent preserved and living examples of the same microbial/mineral/mucopolysaccharide structures. Preliminary analyses of lithified u-loops reveals no preserved organic material or microfossils. However, techniques employed in our recent work on putative biospeleothems in another cave may be applicable to this problem.

The active Villa Luz system produces prodigious quantities of hydrogen sulfide and other gases (H2S to 204ppm, CO to 110ppm, oxygen as low as 9.6%, and unquantified SO2, CO2, COS, and an unknown aldehyde). This cave demonstrates the tremendous rate at which a sulfuric acid cave can slough off material and create a truly immense void. It also serves as an example of very rapid mineral precipitation (sulfur, gypsum, selenite, pyrite, and others). The stalactite-shaped (i.e. pendulous), mucous-like bacterial and mineral acidic biofilms, known as "snottites", contain densely packed bacteria adapted to living in the presence of sulfuric acid with pH values as low as 0.3. Thiobacillus is the closest relative of the dominant clone type via molecular phylogenetic techniques. EDS analysis reveals numerous gypsum crystals and elemental sulfur apparently forming in situ. Similar mineral formation is also occurring in other microbial mats and stringy structures lining the cave springs, under rocks, and on other surfaces. Sulfur isotopic data show values consistent with microbial metabolism of the various sulfur compounds within the cave. Strategies for verifying whether u-loops are lithified snottites will be detailed.