Affiliation:
1. Exobiology Branch, NASA Ames Research Center, Moffett Field, California
Abstract
ABSTRACT
Biogeochemical transformations occurring in the anoxic zones of stratified sedimentary microbial communities can profoundly influence the isotopic and organic signatures preserved in the fossil record. Accordingly, we have determined carbon isotope discrimination that is associated with both heterotrophic and lithotrophic growth of pure cultures of sulfate-reducing bacteria (SRB). For heterotrophic-growth experiments, substrate consumption was monitored to completion. Sealed vessels containing SRB cultures were harvested at different time intervals, and δ
13
C values were determined for gaseous CO
2
, organic substrates, and products such as biomass. For three of the four SRB, carbon isotope effects between the substrates, acetate or lactate and CO
2
, and the cell biomass were small, ranging from 0 to 2‰. However, for
Desulfotomaculum acetoxidans
, the carbon incorporated into biomass was isotopically heavier than the available substrates by 8 to 9‰. SRB grown lithoautotrophically consumed less than 3% of the available CO
2
and exhibited substantial discrimination (calculated as isotope fractionation factors [α]), as follows: for
Desulfobacterium autotrophicum
, α values ranged from 1.0100 to 1.0123; for
Desulfobacter hydrogenophilus
, the α value was 0.0138, and for
Desulfotomaculum acetoxidans
, the α value was 1.0310. Mixotrophic growth of
Desulfovibrio desulfuricans
on acetate and CO
2
resulted in biomass with a δ
13
C composition intermediate to that of the substrates. The extent of fractionation depended on which enzymatic pathways were used, the direction in which the pathways operated, and the growth rate, but fractionation was not dependent on the growth phase. To the extent that environmental conditions affect the availability of organic substrates (e.g., acetate) and reducing power (e.g., H
2
), ecological forces can also influence carbon isotope discrimination by SRB.
Publisher
American Society for Microbiology
Subject
Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology
Cited by
118 articles.
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