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 primar y production ((CO2)-C-14 fixation) were examined seasonally on a North Carolina Atlantic coastal, intertidal, benthic microbial mat communit y 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 perio d, 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 dichloropheny l)-1,1 dimethylurea (DCMU), daytime N, fixation was enhanced, indicati ng in situ O-2 inhibition of N-2 fixation. The most pronounced DCMU st imulation 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 fixa tion were the reverse of those in summer, with maximum rates at midday . The reversal was related to seasonal changes in daily and hourly pho tosynthetic rates, leading to differential O-2 suppression of N-2 fixa tion. Seasonal changes in cyanobacterial community composition and bac terial diazotrophy may have played additional roles in determining die l rates and patterns of N-2 fixation and mat production. Results indic ate a more important role for bacteria in the dynamics of mat N-2 fixa tion than has been previously recognized.