Ultrastructure of Unicellular N2 Fixing Cyanobacteria From the Tropical North Atlantic and Subtropical North Pacific Oceans1

Nitrogen fixing unicellular marine cyanobacteria may have a major role in the global biogeochemistry of N; nevertheless, little is known about their phylogeny and morphology. We isolated N2 fixing unicellular cyanobacteria from the tropical North Atlantic and subtropical North Pacific Oceans and examined ultrastructural dynamics during dark:light cycles when grown in incubators. The isolate from the subtropical North Pacific was larger and showed a size variation from 3 to 7 μm but had similar morphology and cell division‐plane characteristics as the isolate from the North Atlantic (2.5 μm). Nitrogen fixation only occurred during the dark phase, and ultrastructural analysis demonstrated changes in the appearance and quantity of large carbohydrate‐like granules present in the cells. To verify the composition of these carbohydrate‐like granules, staining with periodic acid, thioacetic acid, and silver was carried out, and a positive reaction was obvious in all cells. The cells from the Atlantic seemed to empty their polysaccharide granules during the night, whereas those from the Pacific showed a decrease in the number of their granules. Our work suggests that phylogenetically related strains of unicellular N2 fixing cyanobacteria from different oceans showed similar carbohydrate‐like granules that could be used to fuel N2 fixation during darkness.