Contemporaneous N 2 Fixation and Oxygenic Photosynthesis in the Nonheterocystous Mat-Forming Cyanobacterium Lyngbya aestuarii

Abstract

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 N 2 fixation associated with mats. The ability to contemporaneously conduct O 2 -sensitive N 2 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 (O 2 evolution) inhibitor 3(3,4-dichlorophenyl)-1,1-dimethylurea stimulated light-mediated N 2 fixation (nitrogenase activity), indicating potential inhibition of N 2 fixation by O 2 production. However, some degree of light-mediated N 2 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 14 CO 2 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) 14 CO 2 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 N 2 fixation during illumination, with N 2 fixation being confined to terminal regions. During darkness, a larger share of the filament appears capable of N 2 fixation.