Natural-abundance radiocarbon as a tracer of assimilation of petroleum carbon by bacteria in salt marsh sediments

The natural abundance of radiocarbon (C-14) provides unique insight into the source and cycling of sedimentary organic matter. Radiocarbon analysis of bacterial phospholipid lipid fatty acids (PLFAs) in salt-marsh sediments of southeast Georgia (USA)-one heavily contaminated by petroleum residues-was used to assess the fate of petroleum-derived carbon in sediments and incorporation of fossil carbon into microbial biomass. PLFAs that are common components of eubacterial cell membranes (e.g., branched C-15 and C-17, 10-methyl-C-16) were depleted in C-14 in the contaminated sediment (mean Delta C-14 value of +25 +/- 19 parts per thousand for bacterial PLFAs) relative to PLFAs in uncontaminated control sediment (Delta C-14 = +101 +/- 12 parts per thousand). We suggest that the C-14-depletion in bacterial PLFAs at the contaminated site results from microbial metabolism of petroleum and subsequent incorporation of petroleum-derived carbon into bacterial membrane lipids. A mass balance calculation indicates that 6-10% of the carbon in bacterial PLFAs at the oiled site could derive from petroleum residues. These results demonstrate that even weathered petroleum may contain components of sufficient lability to be a carbon source for biomass production by marsh sediment microorganisms. Furthermore, a small but significant fraction of fossil carbon is assimilated even in the presence of a much larger pool of presumably more-labile and faster-cycling carbon substrates. (c) 2006 Elsevier Inc. All rights reserved.