Does aspartic acid racemization constrain the depth limit of the subsurface biosphere?
Publication Year
2014
Type
Journal Article
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. © 2013 John Wiley & Sons Ltd.
Keywords
abundance,
amino acid,
aspartic acid,
biosphere,
maintenance,
protein,
racemization,
survival,
turnover,
South Africa,
Bacteria (microorganisms),
aspartic acid,
bacterial DNA,
Article,
bacterium,
cell division,
chemistry,
cytology,
DNA sequence,
genetics,
metabolism,
microbial viability,
microbiology,
molecular genetics,
nucleotide sequence,
sediment,
South Africa,
Temperature,
time,
aspartic acid,
Bacteria,
Cell Division,
DNA,
Bacterial,
Geologic Sediments,
microbial viability,
Molecular Sequence Data,
sequence analysis,
DNA,
Soil Microbiology,
South Africa,
Temperature,
Time Factors
Journal
Geobiology
Volume
12
Pages
1-19