Dominance of unicellular cyanobacteria in the diazotrophic community in the Atlantic Ocean

Abstract

The horizontal and vertical distribution of representatives of diazotrophic unicellular cyanobacteria wasinvestigated in the subtropical northeast Atlantic Ocean (28.87 to 42.00uN; 9.01 to 20.02uW). Samples fromstations encompassing different water conditions (from oceanic oligotrophic waters to upwelling areas and atemperature range of 13.1uC to 24.2uC) were size fractionated and analyzed fornifHby a nested polymerase chainreaction (PCR) and by tyramide signal amplification–fluorescence in situ hybridization (TSA-FISH) using probeNitro821. In samples from the surface, mixed-layer depth, and deep chlorophyll maximum waters, most (.50%)of thenifHrecovered was from the 0.2–3mm fraction and was consistent with TSA-FISH counts. The,3mmNitro821-positive cells were more abundant than the larger cells, and the proportion of single cells was larger thanthat associated with particulate matter or with larger cells. Phylogenetic analysis of representative samplesrevealed that most of the sequences belong to diazotrophic unicellular cyanobacteria Group A (UCYN-A orCandidatusAtelocyanobacterium thalassa). N2fixation in the 0.2–3mm fraction, putatively representing theactivity of UCYN-A, contributed more than 50%of the total N2fixation. There was a positive relationship of thisputative UCYN-A abundance and activity with temperature, and a negative relationship with dissolved O2. Thedominance of these putative UCYN-A organisms in nitrate-rich upwelling filament regions suggests that theactivity of this group of organisms may not be strongly controlled by the availability of fixed N.