Microdiversity of culturable diazotrophs from the rhizoplanes of the salt marsh grasses Spartina alterniflora and Juncus roemerianus

Salt marshes dominated by Spartina alterniflora (smooth cordgrass) are among the most productive ecosystems known, despite nitrogen limitation. Rhizoplane/rhizosphere diazotrophy (nitrogen fixation) serves as a significant source of combined nitrogen in these systems. Several recent studies have demonstrated remarkable physiological and phylogenetic macro- and microdiversity within this important functional group of organisms. However, the ecological significance of this diversity is presently unknown. The physiological characteristics of the culturable, oxygen-utilizing fraction of the rhizoplane diazotroph assemblages from Spartina alterniflora and from another salt marsh grass, the black needle rush Juncus roemerianus, were examined in combination with an assessment of the phylogenetic relatedness by whole genome DNA-DNA hybridization. Analysis of substrate utilization data permitted quantitative evaluation of fully cross-hybridizing strain groups and physiological clusters. Phylogenetically related strains, defined by DNA homology greater than or equal to 90% relative to the positive control, displayed extensive physiological diversity. Seven bootstrap-supported physiological clusters, composed largely of phylogenetically dissimilar strains, showed similar utilization patterns for at least one class of ecologically relevant substrates (carbohydrates, carboxylic acids, or amino acids). These diazotrophs appear to be physiologically adapted for utilization of specific substrates or classes of substrates, lending support to diazotrophic functional redundancy. Micro-environmental heterogeneity is credited for promoting this diversity by selecting for physiologically specialized diazotroph populations to occupy defined niches in situ. One outcome of this physiological diversity is maintenance of a crucial environmental function (nitrogen fixation) over a broad range of environmental conditions.