Petroleum-hydrocarbon-degrading bacteria were obtained after enrichment on crude oil (as a 'chocolate mousse') in a continuous supply of Indonesian seawater amended with nitrogen, phosphorus and iron nutrients. They were related to Alcanivorax and Marinobacter strains, which are ubiquitous petroleum-hydrocarbon-degrading bacteria in marine environments, and to Oceanobacter kriegii (96.4-96.5 % similarities in almost full-length 16S rRNA gene sequences). The Oceanobacter-related bacteria showed high n-alkane-degrading activity, comparable to that of Alcanivorax borkumensis strain SK2. On the other hand, Alcanivorax strains exhibited high activity for branched-alkane degradation and thus could be key bacteria for branched-alkane biodegradation in tropical seas. Oceanobacter-related bacteria became most dominant in microcosms that simulated a crude oil spill event with Indonesian seawater. The dominance was observed in microcosms that were unamended or amended with fertilizer, suggesting that the Oceanobacter-related strains could become dominant in the natural tropical marine environment after an accidental oil spill, and would continue to dominate in the environment after biostimulation. These results suggest that Oceanobacter-related bacteria could be major degraders of petroleum n-alkanes spilt in the tropical sea.
INTRODUCTIONA wide variety of micro-organisms are known to degrade petroleum hydrocarbons (Head et al., 2006;Prince, 2005). Most hydrocarbonoclastic bacteria metabolize either aliphatic or aromatic hydrocarbons, although some bacteria such as strains of Pseudomonas (Whyte et al., 1997) and Rhodococcus (Andreoni et al., 2000) have been shown to degrade both types of hydrocarbons. Among hydrocarbonoclastic bacteria, Alcanivorax (Hara et al., 2003;Kasai et al., 2001;Roling et al., 2004;Yakimov et al., 1998Yakimov et al., , 2005 and Cycloclasticus (Dyksterhouse et al., 1995;Kasai et al., 2002a;Maruyama et al., 2003) strains have been identified as key micro-organisms in the degradation of aliphatic and aromatic hydrocarbons, respectively, in marine environments (Harayama et al., 2004). Alcanivorax strains are distributed in natural marine environments around the world . The ability of Alcanivorax strains to use branched alkanes is high (Hara et al., 2003;McKew et al., 2007), and this could be one of the reasons why these strains predominate in crude-oil-impacted temperate marine environments (Cappello et al., 2007;Hara et al., 2003;Kasai et al., 2001;Roling et al., 2002Roling et al., , 2004Yakimov et al., 2005). Thalassolituus oleivorans has been reported to degrade aliphatic hydrocarbons (Yakimov et al., 2004), and Thalassolituus strains have recently been shown to dominate in n-alkane-containing temperate seawater microcosms (McKew et al., 2007;Yakimov et al., 2005) and in crude-oil-containing temperate estuarine seawater microcosms (Coulon et al., 2007;McKew et al., 2007).In addition to these 'professional' hydrocarbonoclastic bacteria, many 'non-professional' hydrocarbonoclastic bacteria such as Mar...