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

Symploca PCC 8002 Kutzing is a filamentous cyanobacterium that lacks the sp ecialized cells, known as heterocysts, that protect nitrogenase from O-2 in most aerobic N-2-fixing cyanobacteria. Nevertheless, Symploca is able to c arry out N-2 fixation in the light under aerobic conditions. When cultures were grown under light/dark cycles, nitrogenase activity commenced and incr eased in the light phase and declined towards zero in the dark. Immunolocal ization of dinitrogenase reductase in sectioned Symploca trichomes showed t hat the enzyme was present only in 9 % of the cells. These cells lacked any obvious mechanical protection against atmospheric O-2 and their ultrastruc tural characteristics were similar to those of cells that did not contain a ny dinitrogenase reductase. The nitrogenase-containing cells possessed carb oxysomes 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-2 fixation and photosynthesis. The s ignificance of the localization pattern for dinitrogenase reductase is disc ussed in the context of N-2 fixation in Symploca PCC 8002.