MICROBIAL BIOFABRICS IN CAVES: REFLECTIONS INFORM THE FUTURE

Long before it was possible to employ ‘-omics’ technologies to environmental samples, a microbial community composition study was conducted on two types of speleothems in a geothermal mine adit—a hot cave (~50C), near Glenwood Springs, CO (Spear, et al, 2007). One speleothem consisted of 2- to 3- cm long, 3- to 4-mm wide, leather-like, hollow, soda-straw stalactites. A combination of light and electron microscopy revealed that the stalactites are composed of mineralized biofabrics with several cell morphologies in a laminated form, with gypsum and sulfur as dominant mineral components. A clone and Sanger sequence approach on the 16S rRNA gene presence yielded a diverse gradient of organisms, from the underlying biofabric that covers the cave walls, to the soda-straw base, to the soda-straw tip. Thermus spp., Chloroflexi and Gammaproteobacteriawere present in varying amounts, depending on location, but the tip of the stalactite was primarily composed of ammonia-oxidizing crenarchaeota (AOA). The second type of speleothem was a dumpling-like raft floating in pools on the floor of the adit. These dumplings were composed of a complex multimineral / microbial biomass assemblage dominated by amorphous, gypsum-based minerals and a diverse suite of microbiota. Both of these speleothems reveal that biofabrics can be complex and important for secondary mineralization, weathering and deposition processes in karst environments. Now, eight years later, a new examination must be done on this, and many other cave systems with the tools that are available; high-throughput DNA sequencing, stable-isotope geochemistry, clumped-isotope geochemistry, advanced mineralogic characterization, biosignature evaluation and petrographic thin-sectioning. Such geobiological investigation would better inform on cave and karst formation, the preservation and biosignature potential of cave microbiota, subsurface microbiological processes and the climatic record of the Earth—at a minimum. A foundation has been lain in cave microbiology that must now be taken to the next level to better inform on the bounds of life, on Earth and elsewhere.

Spear, J.R., H.A. Barton, C.E. Robertson, C.A. Francis and N.R. Pace. 2007. “Microbial Community Biofabrics in a Geothermal Mine Adit.” Applied and Environmental Microbiology, 73: 6172 – 6180.