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-contamina ted sediment on the sedimentary bacterial community of a Louisiana (US A) salt marsh that has been chronically exposed to petroleum hydrocarb ons for decades. Diesel contaminants in microcosms as determined from polycyclic aromatic hydrocarbon (PAH) concentration ranged from 0.55 t o 55 ppm (dry weight). Bacterial metabolism (incorporation of C-14-ace tate and H-3-leucine) and bacterial abundance were not affected by die sel-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 e xhibited significant capacity to degrade PAH. The half life of phenant hrene (based on C-14-phenanthrene-degradation experiments) ranged from 137 d in ambient sediments to 4.5 d in sediment chronically exposed t o high levels of diesel-contaminated sediments for 28 d. Two- and thre e-ring PAH, including naphthalenes, phenanthrenes, and dibenzothiophen es, constituted the bulk of PAH composition of diesel and were rapidly metabolized. Alkylated PAH were also readily metabolized. The rapid r emoval of PAH suggests that even if the marsh were exposed to chronica lly high levels of petroleum hydrocarbons, chemical evidence of the co ntaminants would not be detected in sediments. Collectively, these res ults are consistent with the hypothesis that the bacterial community i n this salt marsh has adapted to chronic exposure to petroleum hydroca rbons.