Host-specific ecotype diversity of rhizoplane diazotrophs of the perennial glasswort Salicornia virginica and selected salt marsh grasses

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 N 2 -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 Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 23: [293][294][295][296][297][298][299][300] 2001 1997, Stoffels et al. 1998, Fernandez et al. 1999, Nusslein & Tiedje 1999, suggesting that these organisms may have limited distribution patterns (i.e. endemism). Endemism would imply that microorganisms are physiologically restricted to defined niches in situ (e.g. Ferris et al. 1996, Moore et al. 1998, Bagwell & Lovell 2000a, with important implications for microbial diversity (Fenchel et al. 1997, Finlay et al....

Diazotrophic diversity in the rhizosphere of two exotic weed plants, Prosopis juliflora and Parthenium hysterophorus

“…Several uncommon diazotrophs were also reported in the rhizosphere may be due to the unique rhizosphere effect of these two exotic plants, which has to be further investigated. In present study also, it was confirmed that Prosopis and Parthenium like plants that are growing under oligotrophic conditions harbor diversified diazotrophs as reported in previous investigations in other plants (Bagwell et al 2001;Albino et al 2006;FloresMireles et al 2007;Chowdhury et al 2009). The quantitative real-time PCR is a powerful technique that is easily adapted to new primer sets and target genes to achieve specific products that amplify as many versions as possible.…”

“…To authenticate the diazotrophy, the presence of the gene nifH which codes for iron nitrogenase unit of the nitrogenase enzyme complex has become the useful marker gene in culture independent studies to provide evidence for potential nitrogen fixation (Bagwell et al 2001;Izquierdo and Nusslein 2006). The advantage of using nifH as a probe is that the protein sequence of nifH is well conserved and clustered with many known prokaryotic families having similarity to family wise clustering in 16S rRNA sequences (Postgate and Eady 1988).…”

“…Hence, it is widely accepted that the rhizosphere of any plant species is a unique niche harboring diversified bacterial and fungal communities, which serve as potential resource for bioprospecting. The rhizosphere of plant species growing profusely under stress-conditions harbors novel diazotrophs to meet their nitrogen requirement as observed in salt marsh grasses such as Spartina alterniflora, Juncus roemerianus Lovell et al 2000) and Salicornia virginica (Bagwell et al 2001), oligotrophic habitant Drosera villosa (Albino et al 2006), mangrove roots (Flores-Mireles et al 2007) and desert growing Lasiurus grass (Chowdhury et al 2009). …”

Opposite diel patterns of nitrogen fixation associated with salt marsh plant species (Spartina foliosa and Salicornia virginica) in southern California

“…More specifically, nitrogen has been found to limit the height of S. foliosa plants, which directly impacts the ability of these plants to provide adequate nesting habitat for the endangered Clapper rail (Boyer & Zedler 1999). Nitrogen fixers have been isolated from roots of S. virginica (Bagwell et al. 2001) and S. bigelovii (Rueda‐Puente et al.…”

Diversity and functional responses of nitrogen-fixing microbes to three wetland invasions

“…Nitrogen fixation in the benthos underlies the high productivity of these ecosystems by offsetting nitrogen losses to denitrification (Capone 1988). The function of nitrogen fixation is mediated by diverse microorganisms including autotrophic cyanobacteria in mats on wetland sediments (Zehr et al 1995) and plant surfaces (Currin and Paerl 1998) as well as heterotrophic sulfate reducers that intimately engage roots of cordgrasses (Brown et al 2003;Lovell 2002;Whiting et al 1986), seagrasses (Welsh 2000), mangroves (Holguin et al 2001), and other salt marsh plants (Bagwell et al 2001) by providing fixed nitrogen in exchange for organic carbon.…”