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Does Aspartic Acid Racemization Constrain the Depth Limit of the Subsurface Biosphere?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 approximately 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.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Onstott, T C.
(Princeton Univ. NJ, United States)
Magnabosco, C.
(Princeton Univ. NJ, United States)
Aubrey, A. D.
(Jet Propulsion Lab., California Inst. of Tech. Pasadena, CA, United States)
Burton, A. S.
(NASA Johnson Space Center Houston, TX, United States)
Dworkin, J. P.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Elsila, J. E.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Grunsfeld, S.
(River Hill High School Clarksville, MD, United States)
Cao, B. H.
(California Univ. Merced, CA, United States)
Hein, J. E.
(California Univ. Merced, CA, United States)
Glavin, D. P.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Kieft, T. L.
(New Mexico Inst. of Mining and Technology Socorro, NM, United States)
Silver, B. J.
(Arcadis US, Inc. Tampa, FL, United States)
Phelps, T. J.
(Oak Ridge National Lab. TN, United States)
Heerden, E. Van
(Orange Free State Univ. Bloemfontein, South Africa)
Opperman, D. J.
(Orange Free State Univ. Bloemfontein, South Africa)
Bada, J. L.
(Scripps Institution of Oceanography San Diego, CA, United States)
Date Acquired
August 5, 2016
Publication Date
December 11, 2013
Publication Information
Publication: Geobiology
Volume: 12
Issue: 1
Subject Category
Life Sciences (General)
Report/Patent Number
Funding Number(s)
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