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

Abstract

Diel rates of nitrogen (N2) fixation (acetylene reduction) and primary production (14CO2 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 N2 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 N2 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 N2 fixation was enhanced, indicating in situ O2 inhibition of N2 fixation. The most pronounced DCMU stimulation of daytime N2 fixation was in spring- fall. Both N2 and CO2 fixation rates were lower in winter. Winter patterns of diel N2 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 O2 suppression of N2 fixation. Seasonal changes in cyanobacterial community composition and bacterial diazotrophy may have played additional roles in determining diel rates and patterns of N: fixation and mat production. Results indicate a more important role for bacteria in the dynamics of mat N2 fixation than has been previously recognized.