CONTEMPORANEOUS N2 FIXATION AND OXYGENIC PHOTOSYNTHESIS IN THE NONHETEROCYSTOUS MAT-FORMING CYANOBACTERIUM LYNGBYA-AESTUARII

The nonheterocystous filamentous cyanobacterial genus Lyngbya is a widespread and frequently dominant component of marine microbial mats. It is suspected of contributing to relatively high rates of N2 fixation associated with mats. The ability to contemporaneously conduct O2-sensitive N2 fixation and oxygenic photosynthesis was investigated in Lyngbya aestuarii isolates from a North Carolina intertidal mat. Short-term (< 4-h) additions of the photosystem II (O2 evolution) inhibitor 3(3,4-dichlorophenyl)-1,1-dimethylurea stimulated light-mediated N2 fixation (nitrogenase activity), indicating potential inhibition of N2 fixation by O2 production. However, some degree of light-mediated N2 fixation in the absence of 3(3,4-dichlorophenyl)-1,1-dimethylurea was observed. Electron microscopic immunocytochemical localization of nitrogenase, coupled to microautoradiographic studies of (CO2)-C-14 fixation and cellular deposition of the tetrazolium salt 2,4,5-triphenyltetrazolium chloride, revealed that (i) nitrogenase was widely distributed throughout individual filaments during illuminated and dark periods, (ii) (CO2)-C-14 fixation was most active in intercalary regions, and (iii) daylight 2,4,5-triphenyltetrazolium chloride reduction (formazan deposition) was most intense in terminal regions. Results suggest lateral partitioning of photosynthesis and N2 fixation during illumination, with N2 fixation being confined to terminal regions. During darkness, a larger share of the filament appears capable of N2 fixation.