Ecologically and geologically relevant isotope signatures of C, N, and S: okenone producing purple sulfur bacteria part I
Purple sulfur bacteria (PSB) are known to couple the carbon, nitrogen, and sulfur cycling in euxinic environments. This is the first study with multiple strains and species of okenone-producing PSB to examine the carbon (C), nitrogen (N), and sulfur (S) metabolisms and isotopic signatures in controlled laboratory conditions, investigating what isotopic fractionations might be recorded in modern environments and the geologic record. PSB play an integral role in the ecology of euxinic environments and produce the unique molecular fossil okenane, derived from the diagenetic alteration of the carotenoid pigment okenone. Cultures of Marichromatium purpuratum 1591 (Mpurp1591) were observed to have carbon isotope fractionations (13_ε_biomass – CO_2), via RuBisCO, ranging from −16.1 to −23.2‰ during exponential and stationary phases of growth. Cultures of Thiocapsa marina 5653 (Tmar5653) and Mpurp1591 had a nitrogen isotope fractionation (15_ε_biomass – NH4) of −15‰, via glutamate dehydrogenase, measured and recorded for the first time in PSB. The δ^(34)S_VCDT values and amount of stored elemental sulfur for Mpurp1591 cells grown autotrophically and photoheterotrophically were dependent upon their carbon metabolic pathways. We show that PSB may contribute to the isotopic enrichments observed in modern and ancient anoxic basins. In a photoheterotrophic culture of Mpurp1591 that switched to autotrophy once the organic substrate was consumed, there were bulk biomass δ13C values that span a broader range than recorded across the Late Devonian, Permian–Triassic, Triassic–Jurassic, and OAE2 mass extinction boundaries. This finding stresses the complexities in interpreting and assigning δ13C values to bulk organic matter preserved in the geologic record.
© 2015 John Wiley & Sons Ltd. Received 8 September 2014; accepted 20 February 2015. The authors would like to thank K. Dawson, C. Bradley for constructive feedback, and four anonymous reviewers who provided input that greatly improved the manuscript. Dartmouth College and the Carnegie Institution of Washington (CIW) funded DAS. W. M. Keck Foundation (2007–6–29) to MLF and AS NASA's NASA Astrobiology Institute (NNH08ZDA002C), and CIW supported the research.
Supplemental Material - gbi12136-sup-0001-FigS1.pdf
Supplemental Material - gbi12136-sup-0002-CNSData.xlsx
Supplemental Material - gbi12136-sup-0003-FigS1Legend.docx