Aerobic nitrogen fixation is confined to a subset of cells in the non-heterocystous cyanobacterium Symploca PCC 8002.

Symploca PCC 8002 Kützing is a filamentous cyanobacterium that lacks the specialized cells, known as heterocysts, that protect nitrogenase from O in most aerobic N -fixing cyanobacteria. Nevertheless, Symploca is able to carry out N fixation in the light under aerobic conditions. When cultures were grown under light/dark cycles, nitrogenase activity commenced and increased in the light phase and declined towards zero in the dark. Immunolocalization of dinitrogenase reductase in sectioned Symploca trichomes showed that the enzyme was present only in 9% of the cells. These cells lacked any obvious mechanical protection against atmospheric O and their ultrastructural characteristics were similar to those of cells that did not contain any dinitrogenase reductase. The nitrogenase-containing cells possessed carboxysomes that were rich in ribulose-1,5-bisphosphate carboxylase/oxygenase and phycoerythrin, a light harvesting pigment of PS II. This indicates that these cells had a capacity for both N fixation and photosynthesis. The significance of the localization pattern for dinitrogenase reductase is discussed in the context of N fixation in Symploca PCC 8002.