GEOMICROBIOLOGICAL INTERACTIONS IN THE SUBSURFACE ENVIRONMENT OF LECHUGUILLA CAVE, NEW MEXICO

Lechuguilla Cave, an ancient, deep, oligotrophic subterranean environment, contains an abundance of "corrosion residues" (CR), deposits rich in secondary Fe-, Mn, and Al-oxide minerals and clays. CR cover large expanses of walls and ceilings in Lechuguilla and Spider caves, and are now being reported from other caves. Levels of iron, and to a lesser extent manganese, in oxides in corrosion residues, are high (25-80 and 21 wt% respectively) and demonstrate a high degree of enrichment in the corrosion residues over what is present in the bedrock, negating the hypothesis that these materials are simply the detrital products of corrosive air. Underlying the CR is a layer of soft altered carbonate host rock (termed "punk rock") that exceeds 10 cm. Studies of bacterial metabolic activity within CR and punk rock demonstrated the presence of a moderately large, actively respiring, morphologically diverse microbial community. To assess the possibility that CR is formed by microbial processes, we carried out a culture-independent, small subunit ribosomal RNA (SSU rRNA) sequence-based study of the microbial communities. DNA was extracted from CR samples from two sites approximately 300 m below the surface, and from cultures of iron- and manganese-oxidizing bacteria; rRNA genes were amplified by PCR, cloned, and sequenced. An analysis of the resultant clones revealed that the dominant clone-types from the two sites grouped with low-temperature archaea in both the crenarchaeota and euryarchaeota kingdoms, or with members of the lactobacilli. In many cases, similarity values to database sequences were extremely low indicating a lack of known close relatives. Other clone sequences were most closely related to those of manganese-oxidizing bacteria, providing support for our hypothesis that microorganisms may assist in corrosion of the host rock, liberating Fe and Mn for oxidation as an energy source, and deposition of secondary Fe and Mn minerals in CR.