Diversity and activity of epiphytic nitrogen-fixers on standing dead stems of the salt marsh grass Spartina alterniflora

Microbial communities growing on the surfaces of standing dead stems of salt marsh grasses contribute to fixation of atmospheric nitrogen (N-2) in these N-limited intertidal systems. Salt marshes in North Carolina, USA, have been shown to exhibit N-2 fixation throughout the growth season, but the composition of these diazotrophic communities is poorly known. This study investigated the diversity of the epiphytic N-2-fixing microbial community on the surfaces of dead stems (shoots) of the cord grass Spartina alterniflora in a salt marsh in North Carolina. Sequencing of the nifH gene and microscopy showed that a diverse diazotrophic community consisting of at least 8 diazotrophic cyanobacterial taxa, as well as an assemblage of alpha-, gamma-, and delta-proteobacteria, was present. Half of the recovered unique nifH sequences fell into the cluster with anaerobes. N-2-fixation rates under natural irradiance ranged from 0.001 to 2.58 nmol C2H4 mu g chl a(-1) h(-1) (0.07 to 206 nmol C2H4 cm(-2) h(-1)) and decreased from spring to fall. Nitrogen-fixation rates had a positive relationship with chlorophyll a, suggesting phototrophs contributed to N-2 fixation. The dominant diazotrophs in this study differed from epiphytic communities described elsewhere in the southeast United States, suggesting regional differences are present in the diversity of these communities