A NOVEL DELTA-SUBDIVISION PROTEOBACTERIAL LINEAGE FROM THE LOWER OCEAN SURFACE-LAYER

A small-subunit ribosomal RNA (16S rRNA) gene lineage (SAR324) affilia ted with the delta subdivision of the class Proteobacteria (DP) was di scovered in a 16S rRNA gene clone library prepared from a water sample collected from 250 m in the western Sargasso Sea. This clone library of nearly full-length amplicons of bacterial 16S rRNA genes has been t he subject of previous studies aimed at identifying bacteria that inha bit the lower ocean surface layer. The novel lineage was identified by randomly sequencing clones that did not hybridize to oligonucleotide probes specific for several abundant bacterioplankton groups identifie d in previous studies. Phylogenetic analysis indicated that SAR324 was most closely affiliated with the DP, although it showed no specific r elationship to any DP 16S rRNA genes in databases. Eight of the clones in the library of 148 clones were identified as members of the SAR324 lineage by hybridization to an oligonucleotide probe specific for SAR 324. Subsequent hybridizations showed that the SAR324 group is stratif ied in the lower surface layer of both the Atlantic and Pacific Oceans , with maxima between 160 and 500 m. The repeated discovery of sequenc es belonging to different gene clusters with similar distributions in this region of the water column suggests that microbial communities in the lower surface layer may be functionally specialized.