Finding the Biotic Fringe

Abstract:

The deepest extent of inhabited rock, known as the biotic fringe, is determined by the interplay of geochemical and biochemical reaction rates. As a consequence it is unlikely that a single parameter, such as temperature, will be generally diagnostic. More probably, shifting combinations of compositional factors, together with temperature and pressure changes, will determine the position and dynamic movements of the biotic fringe. As an example, during serpentinization the biotic fringe may be determined by the depletion of carbon through carbonate precipitation or abiotic organic synthesis at mineral surfaces at temperatures and pressures that are readily inhabited elsewhere. During other weathering, diagenetic and alteration processes, the transition from sterile to populated may be determined by supply rates of reductants, oxidants, nutrients, or their diverse combinations. Where geochemical composition and mineral catalysts permit rapid abiotic organic synthesis and redox equilibration, microbes are unlikely to be able to compete. This is especially true if such conditions inhibit the function of enzymatic catalysts. As abiotic rates slow, opportunities for catalysis emerge as can the biotic fringe where enzymes can function effectively. There are many ways to turn this conceptual model into one that makes quantitative predictions. Rates of many abiotic redox and organic synthesis reactions are amenable to experimental study, and parallel biotic rate experiments yield particularly useful results. Likewise, determining how enzymes are inactivated at the limits of their function will add explicit biochemical constraints. Exploring for the biotic fringe in the subsurface involves using compositional data to evaluate apparent temperatures of equilibration that reveal anomalous approaches to redox equilibrium at conditions where abiotic rates are insufficient. Meanwhile, sampling the inhabitants of the biotic fringe will reveal surprising extents of enzyme behavior.