NSF Geobiology and Low Temperature Chemistry - Racemization dating of subsurface microorganisms

Part 1 - How quickly do subsurface microbial communities metabolize and grow? Microbial communities are responsible for the transformation of organic matter that is deeply buried in marine and terrestrial sediments and of organic matter generated at depth by hydrothermal processes and rock/water interactions, such as serpentinization. This organic matter is either converted to CO2 and CH4, which often returns to the surface and enters the atmosphere, or into more microbial biomass. Heavily biodegraded petroleum is the residue of these microbial conversions.  Subsurface microbial communities also mediate the concentrations of iron, uranium, arsenic and other toxic metals and organic contaminants in groundwater. They cause diagenetic alteration of mineral phases, such as the transformation of feldspar grains into clay and change the porosity and permeability of aquifers. The rate at which all of this biogeochemical processing occurs far beneath our feet, however, is poorly constrained. This proposal seeks to develop a new method of more accurately quantifying the in situ rates of subsurface microbial biogeochemical processes by determining the average age the microorganisms themselves. If the microorganisms are growing quickly then the biogeochemical cycling must be equally fast, but if they are as old a methuselah, then the biogeochemical processes are occurring at glacial rates.


Part 2 - For the past 20 years geochemical models for carbon transformation have been used to deduce in situ metabolic rates and anabolic rates of these communities. Depending upon the assumptions made and measurements used, estimates of these rates vary by three to four orders of magnitude for any site/sample. Onstott et al.1 recently proposed that the D/L analyses of aspartic acid (Asp) provides a direct constraint on the anabolic rate of subsurface microorganisms. These results suggest that the protein turnover times for thermophilic microorganisms are less than one year and that the biogeochemical rates required to support this turnover are much greater than previously guessed from geochemical models. With further refinements in measurement protocols and testing, the Asp D/L analyses could provide a far more accurate constraint on the in situ anabolic rate than previous approaches. When this rate is combined with metaproteomic analyses, then constraints can also be placed on the rates of the most abundant metabolic pathways and thus biogeochemical transformations from analysis of a single sample. This estimate is completely independent of geochemical models or isotopic assays that have been used in the past.

To realize this goal starvation experiments will be performed over a period of 2 years on spore-forming and non-spore forming thermophiles and hyperthermophiles for which complete genomes currently exist. The rate of racemization of aspartic acid as a function of temperature will then be determined. The principal metabolic pathways will be mapped using proteomics to determine the rate of ATP production and consumption. This will then be compared to the measured rates of respiration as a function of temperature. If the racemization rate and rate of isoaspartate formation represent the major debilitating process, then estimates of the respiration rate derived from the D/L of Asp and the metabolic pathway analysis should coincide with the measured rates. These experiments will also reveal at what point the D/L of aspartic acid is so high that a microorganism truly dies, never to come back to life.