Abstract
Microbial fatty acids preserve metabolic and environmental information in their hydrogen isotope ratios (2H/1H). This ratio is influenced by parameters that include the 2H/1H of water in the microbial growth environment, and biosynthetic fractionations between water and lipid. In some microbes, this biosynthetic fractionation has been shown to vary systematically with central energy metabolism, and controls on fatty acid 2H/1H may be linked to the intracellular production of NADPH. We examined the apparent fractionation between media water and the fatty acids produced by Desulfovibrio alaskensis G20. Growth was in batch culture with malate as an electron donor for sulfate respiration, and with pyruvate and fumarate as substrates for fermentation and for sulfate respiration. A larger fractionation was observed as a consequence of respiratory or fermentative growth on pyruvate than growth on fumarate or malate. This difference correlates with opposite apparent flows of electrons through the electron bifurcating/confurcating transhydrogenase NfnAB. When grown on malate or fumarate, mutant strains of D. alaskensis G20 containing transposon disruptions in a copy of nfnAB show different fractionations than the wild type strain. This phenotype is muted during fermentative growth on pyruvate, and it is absent when pyruvate is a substrate for sulfate reduction. All strains and conditions produced similar fatty acid profiles, and the 2H/1H of individual lipids changed in concert with the mass-weighted average. Unsaturated fatty acids were generally depleted in 2H relative to their saturated homologs, and anteiso-branched fatty acids were generally depleted in 2H relative to straight-chain fatty acids. Fractionation correlated with growth rate, a pattern that has also been observed in the fractionation of sulfur isotopes during dissimilatory sulfate reduction by sulfate-reducing bacteria.
Highlights
The structures and isotopic compositions of lipids preserve information about organisms that can be archived in sediments and rocks over geological time scales
A repression in growth rate (22% of wild type) was apparent when the strains were grown as fumarate fermenters, in the absence of sulfate
The relative importance of each of these mechanisms varies by substrate, and potential differences in the δ2HNADPH produced by these different mechanisms has been invoked as a reason for differences in δ2Hlipid (Zhang et al, 2009)
Summary
The structures and isotopic compositions of lipids preserve information about organisms that can be archived in sediments and rocks over geological time scales. The ratio of hydrogen isotopes (deuterium/protium = 2H/1H) in lipids derived from environmental samples has been observed to relate to the 2H/1H of environmental water (Hayes, 2001; Sauer et al, 2001). It was shown in a range of aerobic microorganisms that the fractionation of hydrogen isotopes between media water and lipids varied with changes in growth substrate (Zhang et al, 2009). Experiments with anaerobes have shown less systematic changes in lipid 2H/1H as a function of energy metabolism, strong differences have been observed in lipid 2H/1H of organisms in pure culture versus co-cultures with another organism (Dawson et al, 2015; Osburn et al, in review)
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