Many, though not all, non‐heterocystous cyanobacteria can fix N2. However, very few strains can fix N2 aerobically. Nevertheless, these organisms may make a substantial contribution to the global nitrogen cycle. In this general review, N2 fixation by laboratory cultures and natural populations of non‐heterocystous cyanobacteria is considered. The properties and subcellular location of nitrogenase in these organisms is described, as is the response of N2 fixation to environmental factors such as fixed nitrogen, O2 and the pattern of illumination. The integration of N2 fixation with other aspects of cell metabolism (in particular photosynthesis) is also discussed. Similarities and differences between different individual strains of non‐heterocystous cyanobacteria are highlighted.

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The qualitative distribution and quantitative estimates of nitrogenase (EC 1.7.99.2), glutamine synthetase (EC 6.3.1.2), phycoerythrin and ribulose 1,5‐bisphosphate carboxylase/oxygenase (EC 4.1.1.39) were studied in the cyanobacterium Nostoc residing in internal cephalodia of the tripartite lichen Nephroma arcticum L. Polyclonal antisera, raised in rabbit against the proteins, and goat anti‐rabbit IgG conjugated to 10 nm gold were used as probes to detect the antigens by transmission electron microscopy. Western blot analyses demonstrated the monospecificity of the antisera. Nitrogenase was localized in heterocysts, with vegetative cells showing a label intensity comparable to the background. Distribution of the antigen within the heterocysts was uniform. Glutamine synthetase labelling was very low, but appeared to be distributed in both cell types. An intense phycoerythrin labelling was associated with the thylakoid region of the vegetative cells, whereas a much lower labelling was observed in the heterocyst. No significant differences were found between cyanobionts in younger and older cephalodia except for the nitrogenase labelling, which was higher in heterocysts of the cyanobiont in younger cephalodia. Most of the ribulose 1,5‐bisphosphate carboxylase/oxygenase (Rubisco) label was present in vegetative cells. The Rubisco label was pronounced in the carboxysomes, whereas the label in the cytoplasm, on a unit area basis, was much lower. Heterocysts showed a label intensity similar to that of the vegetative cell cytoplasm. In Nostoc of the bipartite lichen Peltigera canina L., the Rubisco protein showed a comparable distribution pattern, but the average number of carboxysomes per vegetative cell was about 4 times higher.

Methylammonium uptake was studied in a glutamine auxotroph of the cyanobacterium Anabaena cycadeae lacking glutamine synthetase activity. The uptake pattern was found to be biphasic, consisting of an initial fast phase lasting up to 60 s followed by a slower second phase. When cells were preincubated with L-methionine-DL-sulphoximine, an irreversible inhibitor of glutamine synthetase activity, the second uptake phase was abolished although the first phase was unaffected. Since the glutamine auxotroph did not have any glutamine synthetase activity the inhibition of the second phase by L-methionine-DL-sulphoximine could not have been due to the inhibition of glutamine synthetase activity. Thus, it is suggested that the two uptake phases may represent two different ammonium transport systems, the second one being sensitive to L-methionine-DL-sulphoximine.Ammonium transport Anabaenacycadeae C.yanobacteria

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