Measuring carbon and N 2 fixation in field populations of colonial and free‐living unicellular cyanobacteria using nanometer‐scale secondary ion mass spectrometry 1

Unicellular cyanobacteria are now recognized as important to the marine N and C cycles in open ocean gyres, yet there are few direct in situ measurements of their activities. Using a high‐resolution nanometer scale secondary ion mass spectrometer (nano SIMS ), single cell N 2 and C fixation rates were estimated for unicellular cyanobacteria resembling N 2 fixer Crocosphaera watsonii . Crocosphaera watsonii ‐like cells were observed in the subtropical North Pacific gyre (22°45′ N, 158°0′ W) as 2 different phenotypes: colonial and free‐living. Colonies containing 3–242 cells per colony were observed and cell density in colonies increased with incubation time. Estimated C fixation rates were similarly high in both phenotypes and unexpectedly for unicellular cyanobacteria 85% of the colonial cells incubated during midday were also enriched in 15 N above natural abundance. Highest 15 N enrichment and N 2 fixation rates were found in cells incubated overnight where up to 64% of the total daily fixed N in the upper surface waters was attributed to both phenotypes. The colonial cells retained newly fixed C in a sulfur‐rich matrix surrounding the cells and often cells of both phenotypes possessed areas (<1 nm) of enriched 15 N and 13 C resembling storage granules. The nano SIMS imaging of the colonial cells also showed evidence for a division of N 2 and C fixation activity across the colony where few individual cells (<34%) in a given colony were enriched in both 15 N and 13 C above the colony average. Our results provide new insights into the ecophysiology of unicellular cyanobacteria.