Glycollate Excretion by N2-fixing Cyanobacteria Treated With Photorespiratory Inhibitors

We have examined the effects of various inhibitors of glycollate metabolism on glycollate excretion, photosynthetic CO₂ assimilation and N₂ fixation by cyanobacteria. α-HPMS (α-hydroxy-α(2-pyridyl)-methane sulphonate) increased the rate of glycollate excretion by Anabaena cylindrica considerably, but also inhibited net CO₂ photoassimilation and N₂ fixation. α-HPMS inhibted glycollate dehydrogenase in A. cylindrica extracts but did not affect ribulose bisphosphate carboxylase or phosphoglycollate phosphatase activities. Diamox (5 acetamido-1,3,5-thiadizaole-S-sulphonamide) also stimulated glycollate excretion, inhibited CO₂ fixation and caused a low inhibition of N₂ fixation. INH (isonicotinic acid hydrazide) enhanced glycollate excretion by A. cylindrica, suggesting the metabolism of glycollate via the glycineserine pathway, but again inhibited CO₂ fixation and N₂ fixation. Glyoxylate also stimulated glycollate excretion by A. cylindrica, Anabaena variabilis and Nostoc muscorum. The data are discussed in terms of the specificity of glycollate metabolism inhibitors in cyanobacteria and the regulation of glycollate metabolism in these organisms.