SAR11 dominance among metabolically active low nucleic acid bacterioplankton in surface waters along an Atlantic Meridional Transect

Low nucleic acid (LNA) bacterioplankton (sorted by flow cytometry) were characterised in surface water samples along a meridional transect from 48°N to 40° S across the Atlantic Ocean. The LNA bacterioplankton abundance and metabolic activity, assessed by their 35S-methionine uptake rate, were similar along the transect, representing 36 ± 6 and 36 ± 11% of total bacterioplankton, respectively. Fluorescence in situ hybridisation analysis of the flow-sorted cells revealed that the LNA bacterioplankton population was dominated (59 ± 4%) by and contained virtually all the identifiable SAR11 clade cells throughout the Atlantic Ocean. Therefore, the present study provides ecological characterisation of this flow-sorted group and suggests both phylogenetic and functional constancy of the LNA bacterioplankton at the basin-scale.