'Rare biosphere' bacteria as key phenanthrene degraders in coastal seawaters

By coupling DNA-SIP and pyrosequencing approaches, we identified Cycloclasticu s sp. as a keystone degrader of polycyclic aromatic hydrocarbons (PAH) despite being a member of the ‘rare biosphere’ in NW Mediterranean seawaters. We discovered novel PAH-degrading bacteria ( Oceanibaculum sp., Sneathiella sp. ) and we identified other groups already known to possess this function ( Alteromonas sp ., Paracoccus sp. ). Together with Cycloclasticus sp., these groups contributed to potential in situ phenanthrene degradation at a rate >0.5 mg l −1 day −1 , sufficient to account for a considerable part of PAH degradation. Further, we characterized the PAH-tolerant bacterial communities, which were much more diverse in the polluted site by comparison to unpolluted marine references. PAH-tolerant bacteria were also members of the rare biosphere, such as Glaciecola sp. Collectively, these data show the complex interactions between PAH-degraders and PAH-tolerant bacteria and provide new insights for the understanding of the functional ecology of marine bacteria in polluted waters.