AME 39:271-279 (2005)  -  doi:10.3354/ame039271

ABSTRACT: Microbial communities growing on the surfaces of standing dead stems of salt marsh grasses contribute to fixation of atmospheric nitrogen (N₂) in these N-limited intertidal systems. Salt marshes in North Carolina, USA, have been shownto exhibit N₂ fixation throughout the growth season, but the composition of these diazotrophic communities is poorly known. This study investigated the diversity of the epiphytic N₂-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 α-, γ-, and δ-proteobacteria, was present. Half of the recovered unique nifH sequences fell into the cluster with anaerobes. N₂-fixation rates under natural irradiance ranged from 0.001to 2.58 nmol C₂H₄ µg chl a^–1 h^–1 (0.07 to 206 nmol C₂H₄ cm^–2 h^–1) and decreased from spring tofall. Nitrogen-fixation rates had a positive relationship with chlorophyll a, suggesting phototrophs contributed to N₂ 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.

KEY WORDS: NifH · Nitrogen fixation · Epiphytes · Salt marsh · Spartina alterniflora · Cyanobacteria · Anaerobes · Diversity