Symploca PCC 8002 Kützing is a filamentous cyanobacterium
that lacks the specialized cells, known as
heterocysts, that protect nitrogenase from O2 in most aerobic
N2-fixing cyanobacteria. Nevertheless, Symploca is
able to carry out N2 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 O2 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 N2 fixation and photosynthesis. The significance
of the localization pattern for dinitrogenase
reductase is discussed in the context of N2 fixation in Symploca
PCC 8002.