Unicellular nitrogen fixing cyanobacteria from the tropical North Atlantic and subtropical North Pacific Oceans

Nitrogen fixation is important in adding “new” nitrogen ( sensu Dugdale) to nutrient limited oceanic waters. Until recently, a group of large, filamentous cyanobacteria, Trichodesmium spp., were believed to be the major N₂-fixers in the tropical and subtropical oceans. The work presented here shows that a group of unicellular cyanobacteria of the Order Chroococcales, found both in the tropical North Atlantic and Subtropical North Pacific oceans, can also be major N₂-fixers. Rates of N₂ fixation by unicellular cyanobacteria in the tropical North Atlantic were equivalent to those of Trichodesmium spp. in the summer of 2001 and about 20% in the spring of 2002. Rates in the subtropical N Pacific were only 2.6% of those of Trichodesmium spp. in the fall of 2002. Cell abundances of unicellular cyanobacteria were one order of magnitude higher in the Atlantic (2.5 μm) than in the Pacific. In the Pacific Ocean, small cells (3 μm) were 10-fold greater in abundance than large cells (7 μm). Unicellular cyanobacteria from both oceanic basins clustered together based on 16S rDNA sequences but grouped separately based on nifH sequences, thus suggesting a common ancestry but spatial genetic divergence. Isolates were obtained from both oceans and cells had abundant polysaccharide granules which could serve as energy storage structures for nighttime N ₂ fixation energetic requirements. Storage granules increased both in size and number during the daytime with C fixation and decreased during the nighttime. After carrying out continuous culture experiments with isolates from both oceans under phosphorous (P) limiting conditions, we observed that isolates kept their size differences when grown under 1 μM P (2.5 and 3 μm) and 4 μM P (7 μm). Small cells had lower half saturation constants (Km) for P than large cells. Size differences between oceans could be a strategy to cope with differences in limiting P conditions between the two basins. Isolates were highly stenothermal, and optimal growth temperatures for isolates from both oceans ranged between 26 to 30°C. Ratios of N:P for the three isolates approached Redfield at maximum growth rates.