Seasonal nitrogen fixation dynamics in a marine microbial mat: Potential roles of cyanobacteria and microheterotrophs

Diel rates of nitrogen (N 2 ) fixation (acetylene reduction) and primary production ( 14 CO 2 fixation) were examined seasonally on a North Carolina Atlantic coastal, intertidal, benthic microbial mat community dominated by the filamentous, nonheterocystous cyanobacterial genera Microcoleus and Lyngbya. Highest hourly and daily rates of N 2 and CO 2 fixation were observed during spring through fall. During this period, an inverse temporal relationship was noted between these processes, with CO 2 fixation closely tracking irradiance and N 2 fixation rates remaining low during daylight and becoming maximal at night. Under the influence of the photosynthetic (PS 2) inhibitor 3‐(3,4 dichlorophenyl)‐1,1 dimethylurea (DCMU), daytime N 2 fixation was enhanced, indicating in situ O 2 inhibition of N 2 fixation. The most pronounced DCMU stimulation of daytime N 2 fixation was in spring‐fall. Both N 2 and CO 2 fixation rates were lower in winter. Winter patterns of diel N 2 fixation were the reverse of those in summer, with maximum rates at midday. The reversal was related to seasonal changes in daily and hourly photosynthetic rates, leading to differential O 2 suppression of N 2 fixation. Seasonal changes in cyanobacterial community composition and bacterial diazotrophy may have played additional roles in determining diel rates and patterns of N 2 fixation and mat production. Results indicate a more important role for bacteria in the dynamics of mat N 2 fixation than has been previously recognized.