HR: 0800h
AN: B31B-0995    [Abstracts]
TI: Hydrogen-Isotopic Ratios of Lipids From Hydrogen-Consuming Bacteria
AU: * Campbell, B J
EM: bjcampbell@umail.ucsb.edu
AF: Department of Earth Sciences, University of California Bldg. 526, Santa Barbara, CA 93106
AU: Fox, D N
EM: fox@umail.ucsb.edu
AF: Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara, CA 93106
AU: Sessions, A L
EM: als@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology, MC 170-25, 1200 E. California Blvd., Pasadena, CA 91125
AU: Valentine, D L
EM: valentine@geol.ucsb.edu
AF: Department of Earth Sciences, University of California Bldg. 526, Santa Barbara, CA 93106
AB: Molecular hydrogen (H₂) plays several key roles in aquatic sediments. Metabolized by a wide variety of prokaryotes, it both serves a vehicle for interspecies electron transfer and exerts thermodynamic control over microbial metabolic processes. H₂ is typically depleted in deuterium (D) by up to 600‰ relative to water, providing a potential isotopic marker for H₂-consuming organisms. Preservation of that isotopic signal in sediments requires that molecular H₂ can be an indirect source of carbon-bound hydrogen in microbial lipids, a question that is the target of our ongoing laboratory-based investigations. Here we report compound-specific H-isotopic analyses of lipids extracted from H₂-consuming bacteria grown under defined isotopic conditions (δD-H₂ and δD-H₂O). Cupriavidus necator, an aerobe, is a facultatively lithoautotrophic "knallgas" bacterium. C. necator was grown on H₂ + O₂ + CO₂ in liquid media with differing δD-H₂O values. The results of compound-specific isotopic analysis show a strong correlation between the H-isotopic ratio of lipids and that of H₂O in the growth medium. Cultures grown in media of δD-H₂O = -24‰ produced lipids in which δD values were from -258‰ to -197‰. In media of δD-H₂O = +527‰, δD-lipid values were from +117‰ to +228‰. In media of δD-H₂O = +1115‰, δD-lipid values were from +514‰ to +675‰. Linear regression was performed on the data from each lipid compound (R² > 0.9994 in all cases). Regression lines intercepted the δD-lipid axis between -966‰ ([D]/[H] = 5.23×10^-6) and -827‰ ([D]/[H] = 1.60×10^-5); slopes were between and 0.670 and 0.764. These results indicate that the isotopic composition of lipids is entirely controlled by that of water, i.e. that the isotopic depletion of H₂ is not recorded in the lipids - in sharp contrast to previous results from another hydrogenotroph, Sporomusa sp., where part of lipid H is derived indirectly from H₂. The results are consistent with two possibilities: 1) no hydrogen from H₂ is incorporated, even indirectly, into lipid molecules in C. necator, or 2) isotopic equilibration of H₂ with an internal H pool occurs prior to any incorporation in lipids. Without varying δD-H₂ in the experiment, it is not possible to distinguish between these explanations. Finally, three strains were grown in this experiment: the wild type and two different hydrogenase mutants. Systematic variation in the slopes of regression lines suggested that H from water was fractionated more strongly by the cytoplasmic hydrogenase of C. necator than by the membrane-bound hydrogenase. Desulfobacterium autotrophicum, an anaerobe, is a facultatively lithoautotrophic sulfate-reducing bacterium. D. autotrophicum was cultivated on H₂ + CO₂ in liquid media with differing δD-H₂O values. In order to characterize fully the H isotopic systematics, D. autotrophicum was also grown on formate + CO₂ under otherwise identical conditions. Isotopic analyses of this organism are ongoing and will be presented.
DE: 0400 BIOGEOSCIENCES
SC: Biogeosciences [B]
MN: Fall Meeting 2005