Detection of stratified microbial populations related to Chlorobium and Fibrobacter species in the Atlantic and Pacific oceans

A gene lineage (SAR406) related to Chlorobium and Fibrobacter species was found in 16S rRNA gene clone libraries prepared from samples from two oceans. The clone libraries were constructed from total picoplankton genomic DNA to assess bacterial diversity in the lower surface layer. The samples were collected by filtration from a depth of 80 m at a site in the western Sargasso Sea and from a depth of 120 m at a site in the Pacific Ocean, approximately 70 km from the Oregon coast. The PCR and primers which amplified nearly full-length 16S rRNA genes were used to prepare the clone libraries. Among the diverse gene clones in these libraries were two related clones (SAR406 and OCS307) which could not be assigned to any of the major bacterial phyla. Phylogenetic analyses demonstrated that these genes were distant relatives of the genus Fibrobacter and the green sulfur bacterial phylum, which includes the genus Chlorobium. The inclusion of SAR406 in phylogenetic trees inferred by several methods resulted in support from bootstrap replicates for the conclusion that Fibrobacter and Chlorobium species and SAR406 are a monophyletic group. An oligonucleotide probe that selectively hybridized to clone SAR406 was used to examine the distribution of this gene lineage in vertical profiles from the Atlantic and Pacific Oceans and in monthly time series at 0 and 200 m in the Atlantic Ocean. During stratified periods, the genes were most abundant slightly below the deep chlorophyll layer. Seasonal changes in the surface abundance of SAR406 rDNA were highly correlated with chlorophyll a levels (r = 0.75).