MICROBIAL SULFUR TRANSFORMATIONS IN A SULFIDE-RICH CAVE IN TABASCO, MEXICO

Cueva de las Sardinas (a.k.a. Cueva de Villa Luz) is an example of active cave development and enlargement by sulfuric acid. The acid is generated in part by the microbial transformation of hydrogen sulfide that enters the cave through a series of springs. Microbial transformations lead to strong redox gradients within the cave. The Cueva de las Sardinas system produces prodigious quantities of hydrogen sulfide and other gases (H₂S to 204ppm, CO to 110ppm, oxygen as low as 9.6%).  A variety of microbial communities exist in the cave, including the stalactite-shaped, mucous-like acidic biofilms, known as “snottites”, which contain densely packed bacteria.  Molecular phylogenetic studies have documented that a Thiobacillus species is the closest relative of the dominant clone.  These bacteria likely produce the sulfuric acid (pH 0.3) that drips from the snottites.  Other genetic sequences from the snottites have closest relatives amongst the ciliates, nematodes, and flagellates.    Oxidation-reduction potential (ORP) measurements reveal strong redox gradients with values ranging from +585 to –288 mV overall.   The ORP readings ranged from –276 to –288 mV in the sulfur springs; -279 to –345 mV in the biofilm lining the springs; -82 to –298 mV in the stream waters; -257 to -386 mV in the black mud, where sulfate-reducing bacteria are active; and +500 to +585 mV in iron- and clay-rich wall coatings.  Exoenzyme studies in several of these habitats show that resident microorganisms use a variety of carbon sources, to differing degrees in different habitats.  Acridine orange staining of cells from biovermiculations lining the walls reveal a wealth of cell types, including cocci, slender and more robust rods, stalked bacteria, and more unusual shapes.  Metabolic dyes studies reveal that these communities contain many active cells.  Microbial sulfur transformations play important roles in the different communities and habitats within this cave.