AME 23:293-300 (2001)  -  doi:10.3354/ame023293

ABSTRACT: The degree of host specificity of most plant root associated bacteria is poorly understood. In this study we examined the physiological diversity of oxygen utilizing, culturable diazotrophs from the rhizoplane of the high marsh perennial glasswort Salicornia virginica and compared them to diazotrophs from other salt marsh plants (tall and short Spartina alterniflora, Juncus roemerianus, and Spartina patens) from the same ecosystem. Forty-six pure culture strains were recovered from the rhizoplane of S. virginica by stab inoculating freshly collected roots into combined nitrogen-free semi-solid media, followed by streak plating of clonal outgrowths. The majority of these strains were Gram-negative obligately aerobic or microaerophilic rods, but 3 Gram-positive strains were also isolated and characterized. API 20NE test strips were used for preliminary characterization of all strains, yielding 22 physiologically different API strain groups. One representative strain was selected from each API group and tested for the presence of nifH, denoting strains capable of N₂-fixation. Seventeen strains (14 Gram-negative, 3 Gram-positive) were nifH-positive and were characterized further using BIOLOG test plates. Four well-supported strain clusters were identified by bootstrapped cluster analysis of the BIOLOG substrate utilization profiles. These clusters differed in utilization of carbohydrates, carboxylic acids, and amino acids. S. virginica diazotrophs were physiologically quite different from rhizoplane diazotrophs from the low marsh plants S. alterniflora and J. roemerianus, but much more similar to diazotrophs from another high marsh plant, S. patens. We hypothesize that the observed physiological differentiation between high marsh and low marsh diazotrophs reflects differences in selection pressures in the rhizoplane microenvironment produced by plants with differing abilities to ventilate the rhizosphere. In addition, high and low marsh branches were further resolved into host-specific strain clusters, which also implies a strong impact of other host features, such as the suite of carbon exudate compounds produced, on the distributions of specific diazotroph strains. These findings imply endemic, host-specific distributions of salt marsh diazotrophs and are consistent with the great diversity of diazotrophs that have been observed in this ecosystem to date.

KEY WORDS: Diazotrophs · Rhizoplane · Physiological specialization · Rhizosphere ventilation