(
2005-12-29)
Wakeham, Stuart G.; McNichol, Ann P.; Kostka, Joel E.; Pease, Tamara K.
The natural abundance of radiocarbon (14C) 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 C15 and C17, 10-methyl-C16) were depleted in 14C in the contaminated sediment
(mean Δ14C value of +25 ± 19 ‰ for bacterial PLFAs) relative to PLFAs in
uncontaminated “control” sediment (Δ14C = +101 ± 12‰). We suggest that the 14C-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.
