Does aspartic acid racemization constrain the depth limit of the subsurface biosphere? |
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| Authors: | T C Onstott C Magnabosco A D Aubrey A S Burton J P Dworkin J E Elsila S Grunsfeld B H Cao J E Hein D P Glavin T L Kieft B J Silver T J Phelps E van Heerden D J Opperman J L Bada |
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| Institution: | 1. Department of Geosciences, Princeton University, , Princeton, NJ, USA;2. Indiana Princeton Tennessee Astrobiology Initiative (IPTAI), NASA Astrobiology Institute, Indiana University, , Bloomington, IN, USA;3. Jet Propulsion Laboratory, California Institute of Technology, , Pasadena, CA, USA;4. Astromaterials Research and Exploration Science Directorate, NASA Johnson Space Center, , Houston, TX, USA;5. Division of Solar System Exploration, NASA Goddard Space Flight Center, , Greenbelt, MD, USA;6. River Hill High School, , Clarksville, MD, USA;7. Department of Chemistry and Chemical Biology, School of Natural Sciences, University of California‐Merced, , Merced, CA, USA;8. Department of Biology, New Mexico Institute of Mining and Technology, , Socorro, NM, USA;9. ARCADIS US, Inc., , Cranbury, NJ, USA;10. Division of Biosciences, Oak Ridge National Laboratory, , Oak Ridge, TN, USA;11. Department of Microbial, Biochemical and Food Biotechnology, University of the Free State, , Bloemfontein, South Africa;12. Division of Geosciences Research, Scripps Institution of Oceanography, University of California, , San Diego, CA, USA |
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| Abstract: | Previous studies of the subsurface biosphere have deduced average cellular doubling times of hundreds to thousands of years based upon geochemical models. We have directly constrained the in situ average cellular protein turnover or doubling times for metabolically active micro‐organisms based on cellular amino acid abundances, D/L values of cellular aspartic acid, and the in vivo aspartic acid racemization rate. Application of this method to planktonic microbial communities collected from deep fractures in South Africa yielded maximum cellular amino acid turnover times of ~89 years for 1 km depth and 27 °C and 1–2 years for 3 km depth and 54 °C. The latter turnover times are much shorter than previously estimated cellular turnover times based upon geochemical arguments. The aspartic acid racemization rate at higher temperatures yields cellular protein doubling times that are consistent with the survival times of hyperthermophilic strains and predicts that at temperatures of 85 °C, cells must replace proteins every couple of days to maintain enzymatic activity. Such a high maintenance requirement may be the principal limit on the abundance of living micro‐organisms in the deep, hot subsurface biosphere, as well as a potential limit on their activity. The measurement of the D/L of aspartic acid in biological samples is a potentially powerful tool for deep, fractured continental and oceanic crustal settings where geochemical models of carbon turnover times are poorly constrained. Experimental observations on the racemization rates of aspartic acid in living thermophiles and hyperthermophiles could test this hypothesis. The development of corrections for cell wall peptides and spores will be required, however, to improve the accuracy of these estimates for environmental samples. |
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