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 ((CO2)-C-14 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 CO2 fixation were observed during spring through fall. During this period, an inverse temporal relationship was noted between these processes, with CO2 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, 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 CO2 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.