The ability of rhizosphere diazotrophs to remain competitive during increased nitrogen availability in situ was tested in a salt marsh grass stand. Nitrogen (16.3 g m(-2)) or nitrogen (16.3 g m(-2)) and phosphorus (18.0 g m(-2)) were added to plots of short form Spartina alterniflora for either 2 weeks or 8 weeks. The diazotroph assemblage composition was monitored via the polymerase chain reaction using nifH specific primers followed by denaturing gradient gel electrophoresis (DGGE) analysis. DGGE profiles from the short-term experiments (2 and 8 weeks) were compared to profiles from control (no additions) and from long-term (>10 y) nutrient addition plots. Nitrogen fixation activity was assayed in each short-term treatment and control plot using an acetylene reduction technique. The control and nutrient addition DGGE profiles were very similar throughout the short-term experiments. One DGGE band that was prominent in the control plots was not found in the long-term nutrient addition plots. Diazotrophy may provide a competitive advantage for some species in this system, as indicated by results from the long-term nutrient amended plots. However, the rhizosphere environment in situ appears to limit the immediate impacts of increased nutrient availability on the diazotroph assemblage composition. Results from the short-term nutrient amended plots showed no measurable effect on the diazotroph assemblage. These results indicate substantial short-term stability of the diazotroph assemblage composition, but the potential for change in the face of long-term changes in nutrient availability.
Sixty-two partial formyltetrahydrofolate synthetase (FTHFS) structural gene sequences were recovered from roots of salt marsh plants, including Spartina alterniflora, Salicornia virginica, and Juncus roemerianus. Only S. alterniflora roots yielded sequences grouping with FTHFS sequences from known acetogens. Most other FTHFS or FTHFS-like sequences grouped with those from sulfate-reducing bacteria. Several sequences that grouped with Sphingomonas paucimobilis ligH were also recovered.The acetogens are anaerobic bacteria that utilize the acetylcoenzyme A (CoA) pathway for synthesis of acetyl-CoA from C 1 compounds (6, 33). The acetogens are diverse (6, 29, 31), physiologically versatile (3, 7, 33), and apparently ubiquitous in anoxic and suboxic environments (7). It is not unlikely that acetogens are ecologically significant producers of acetate, a key intermediate in terminal carbon metabolism, in many environments.Salt marshes are among the most productive ecosystems known. Much of salt marsh terminal carbon metabolism occurs in saturated, anoxic sediments and is due to sulfate-reducing bacteria or, in deeper, sulfate-limited sediments, to methanogenic Archaea. The root zones of salt marsh plants are foci of high microbial biomass and intensive activity by bacteria (reviewed in reference 20). These bacteria occur in sediments impacted by the roots (the rhizosphere), on the root surfaces (the rhizoplane), and within the root cortex (the endorhizosphere) (5,20). Despite intermittent oxygen availability in the rhizospheres of salt marsh plants due to gas transport processes (13, 32), root-associated anaerobes, including sulfate reducers, methanogens, and acetogens, have been previously documented (9,12,(19)(20)(21)27). Recently, Küsel et al. (17) demonstrated the presence of acetogenic bacteria in the rhizoplane and endorhizosphere of the seagrass Halodule wrightii. With the exception of recovery of a few partial sequences of the structural gene encoding the acetyl-CoA pathway enzyme, formyltetrahydrofolate synthetase (FTHFS), from roots of the dominant cordgrass Spartina alterniflora (19), there has been no investigation of salt-marsh-plant-associated acetogens or acetogenesis. Consequently, we have little knowledge of what types of plants might harbor these organisms or what types of acetogens might be associated with salt marsh plants.In this study, we have recovered and phylogenetically analyzed partial FTHFS sequences to examine the diversity of acetogenic and other FTHFS-containing bacteria associated with roots of dominant salt marsh plants. We found a clear demarcation between sequences recovered from the low-marsh dominant S. alterniflora (smooth cordgrass; designated Spartina hereafter) and those from two common high-marsh plants, Salicornia virginica (common pickleweed; designated Salicornia hereafter) and Juncus roemerianus (black needlerush; designated Juncus hereafter).Live roots were collected from Salicornia, Juncus, and the short and tall growth forms of Spartina from the Crab Haul Creek basin ...
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