Microbial life in the underworld: Biogenicity in secondary mineral formations

One unresolved issue in geomicrobiology is the involvement of microbial activity in the formation of secondary mineral deposits, or speleothems, in caves. Although there is extensive literature demonstrating the importance of bacteria in the precipitation of calcite in noncave environments, the role that these organisms play within caves remains unclear. Evidence in support of microbial involvement in deposition of speleothems has often not been compelling. Following the "Rules for the Hunt" first proposed by Schopf and Walter to determine whether structures in rock were biogenic in origin, we propose a similar set of guidelines for evaluation of microbial association with cave features. We also illustrate methods that may help unravel the complex problem of microorganism involvement in secondary mineral deposition in caves.