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

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

Pia H. Moisander1,2,*, Michael F. Piehler1, Hans W. Paerl1

1University of North Carolina at Chapel Hill, Institute of Marine Sciences, 3431 Arendell Street, Morehead City, North Carolina 28557, USA
2Present address: Ocean Sciences Department, University of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA

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

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