New Mexico Geological Society Annual Spring Meeting — Abstracts

In the subsurface of Earth, myriad microorganisms make their living conducting their affairs in a world of darkness, of often extreme nutrient limitation (either organic or mineral), and frequently extreme temperatures, pressures, and highly reactive gases. Are these organisms just evolution’s “losers” who have had to retreat to the subsurface because they have been outcompeted for all the desirable real estate on the surface? Or in contrast, are they part of an emerging story of a subsurface rock-inhabiting microbial biosphere that has persisted over much of the course of Earth’s history? Evidence is slowly mounting that the latter situation may be the case.

Some clues that will help us unravel this story are emerging from studies of natural caves at depths ranging from very shallow (a few meters) to over 1-2 km in depth, and from human-delved mines some of which now reach depths in excess of 4 km. Although all of these environments are often described as “subsurface”, they represent very different environmental conditions and ecological opportunities for microbial inhabitants. Suggestions of systematic differences in microbial biodiversity, nutritional strategy, and other properties with depth are potentially distinguishable between shallow and mid-range crustal levels. Trends observed include a decreasing tendency to heterotrophy and an increasing tendency to chemoautotrophy with depth. This may be accompanied by decreasing biodiversity with depth. Sheer availability of living space may also be a major gradient between mesoporosity and macroporosity (caves) at shallower depths and very small rock fracture spaces available to microorganisms at greater depths.

Energetics of the systems can differ markedly. In general, near-surface systems (including caves) are dependent on the products of photosynthesis (organic C and/or O²) whereas at great depth, the ecosystems are anaerobic and fueled by the products of water-rock interactions that produce H² and indirectly, abiotic short chain hydrocarbons. This forms a gradient of increasing importance of abiotic energy sources with depth. Exceptions occur including geochemically enriched shallower systems that push those to supporting mineral-oxidizing chemolithotrophy.

Such comparative analyses of newly characterized subsurface geomicrobial habitats may provide insights into the early origin and evolution of life, and potentially act as biomarker proxies for long timescale geological connections, plate motions, and fluid flowpaths.