Evidence for tropical endemicity in the Deltaproteobacteria Marine Group B/SAR324 bacterioplankton clade

The fine-scale phylogeny of the deeply branching Deltaproteobacteria Marine Group B/SAR324 (MGB/SAR324) bacterioplankton clade was analyzed using partial 16S rRNA gene sequence and internal transcribed spacer (ITS) sequences. Both sets of analyses revealed considerable sequence variation, which, along with bootstrap calculations, strongly support the discrimination of 3 'species' level clusters (i.e. displaying < 98% within-cluster and > 97% between-cluster 16S rRNA gene sequence identity) within this clade, which are here designated as MGB/SAR324 clade groups I and II and the MGB/SAR276 clade. The biogeographical ranges of these 3 clades were analyzed using newly acquired 16S and ITS sequences from Stn ALOHA (22.45 degrees N, 158 degrees W), along with sequences available in the public domain. MGB/SAR324 clade group I and the MBG/SAR276 clade display a clearly restricted distribution, occurring only in tropical and subtropical waters at depths < 250 m. These groups represent the first reported species- and genus-level phylotypes displaying a 'tropical-only' global distribution. MGB/SAR324 clade group II, which has been previously described as ubiquitous, occurs across the latitudinal gradient, but is generally restricted to deep, colder waters in the lower latitudes. Enzyme sequences associated at least with MGB/SAR324 clade group II appear to indicate a role for this group in dissolved organic phosphate cycling.