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Does aspartic acid racemization constrain the depth limit of the subsurface biosphere?

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.

journal_name

Geobiology

journal_title

Geobiology

authors

Onstott TC,Magnabosco C,Aubrey AD,Burton AS,Dworkin JP,Elsila JE,Grunsfeld S,Cao BH,Hein JE,Glavin DP,Kieft TL,Silver BJ,Phelps TJ,van Heerden E,Opperman DJ,Bada JL

doi

10.1111/gbi.12069

subject

Has Abstract

pub_date

2014-01-01 00:00:00

pages

1-19

issue

eissn

1472-4677

issn

1472-4669

journal_volume

pub_type

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Does aspartic acid racemization constrain the depth limit of the subsurface biosphere?