Bacterial diversity in marine hydrocarbon seep sediments

Marine seeps introduce significant amounts of hydrocarbons into oceans and create unusual habitats for microfauna and -flora. In the vicinity of chronic seeps, microbes likely exert control on carbon quality entering the marine food chain and, in turn, hydrocarbons could influence microbial community composition and diversity. To determine the effects of seep oil on marine sediment bacterial communities, we collected sediment piston cores within an active marine hydrocarbon seep zone in the Coal Oil Point Seep Field, at a depth of 22 m in the Santa Barbara Channel, California. Cores were taken adjacent to an active seep vent in a hydrocarbon volcano, on the edge of the volcano, and at the periphery of the area of active seepage. Bacterial community profiles were determined by terminal restriction fragment length polymorphisms (TRFLPs) of 16S ribosomal genes that were polymerase chain reaction (PCR)-amplified with eubacterial primers. Sediment carbon content and C/N ratio increased with oil content. Terminal restriction fragment length polymorphisms suggested that bacterial communities varied both with depth into sediments and with oil concentration. Whereas the apparent abundance of several peaks correlated positively with hydrocarbon content, overall bacterial diversity and richness decreased with increasing sediment hydrocarbon content. Sequence analysis of a clone library generated from sediments collected at the periphery of the seep suggested that oil-sensitive species belong to the gamma Proteobacteria and Holophaga groups. These sequences were closely related to sequences previously recovered from uncontaminated marine sediments. Our results suggest that seep hydrocarbons exert a strong selective pressure on bacterial communities in marine sediments. This selective pressure could, in turn, control the effects of oil on other biota in the vicinity of marine hydrocarbon seeps.