Response of sedimentary bacteria in a Louisiana salt marsh to contamination by diesel fuel

In a 28 d microcosm study, we examined the effects of diesel-contaminated sediment on the sedimentary bacterial community of a Louisiana (USA) salt marsh that has been chronically exposed to petroleum hydrocarbons for decades. Diesel contaminants in microcosms as determined from polycyclic aromatic hydrocarbon (PAH) concentration ranged from 0.55 to 55 ppm (dry weight). Bacterial metabolism (incorporation of C-14-acetate and H-3-leucine) and bacterial abundance were not affected by diesel-contaminated sediment at any concentration. Bacterial degradation of C-14- phenanthrene, however, increased in direct proportion to the amount of diesel- contaminated sediment added. Ambient sediment also exhibited significant capacity to degrade PAH. The half life of phenanthrene (based on C-14-phenanthrene-degradation experiments) ranged from 137 d in ambient sediments to 4.5 d in sediment chronically exposed to high levels of diesel-contaminated sediments for 28 d. Two- and three-ring PAH, including naphthalenes, phenanthrenes, and dibenzothiophenes, constituted the bulk of PAH composition of diesel and were rapidly metabolized. Alkylated PAH were also readily metabolized. The rapid removal of PAH suggests that even if the marsh were exposed to chronically high levels of petroleum hydrocarbons, chemical evidence of the contaminants would not be detected in sediments. Collectively, these results are consistent with the hypothesis that the bacterial community in this salt marsh has adapted to chronic exposure to petroleum hydrocarbons.