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Obligately microaerophilic, nitrogen-fixing bacteria were found associated with roots of Spartina afternflora Loisel and in root-associated sediments from salt marshes in Nova Scotia, Canada, and Georgia. These bacteria differ from previously described species and thus represent a new species. The cells of all strains which we studied are small, rigid, curved, motile, and rod shaped and have single polar flagella. Metabolism is respiratory, and the strains utilize organic and amino acids, but not carbohydrates, as sole carbon sources. Poly-p-hydroxybutyrate is not produced. These traits and the guanine-plus-cytosine contents of the deoxyribonucleic acids of these strains (28.3 ? 0.1 mol%) indicate that they are members of the genus Campyfobacter Sebald and Veron 1963. However, these strains can be distinguished from the previously described species of Campylobacter by the presence of nitrogenase, by their tolerance of and apparent requirement for NaC1, by the production of pigment from tryptophan, by a combination of other biochemical traits, and by their association with plant roots. Therefore, we propose that these strains represent a new species, Campylobacter nitrojigifis, and we designate strain CI (= ATCC 33309) as the type strain.Campyfobacter species are commonly occurring pathogens and commensal organisms that are associated with a diverse range of animal hosts (23, 24,31). Of particular interest in this genus is the Campyfobacter jejuni-Campylobacter cofi group (Campyfobacter fetus subsp. jejuni), which increasingly is being recognized as a cause of enteritis in humans (3, 10). In addition to the human-and animal-associated campylobacters, free-living strains of Campyfobacter have been isolated in culture (11, 15), but the taxonomic status of these organisms has not been established yet. One of the free-living strains, strain CIT (type strain), was found associated with the roots of Spartina afternflora Loisel growing in a salt marsh near Halifax, Nova Scotia, Canada; this strain was found to be capable of fixing nitrogen (15). In this paper we describe the isolation in culture of several additional nitrogen-fixing Campyfobacter strains from S. afternflora roots and root-associated sediments from a salt marsh on Sapelo Island, Georgia. We propose that these nitrogen-fixing, Spartina-associated strains represent a new species, Campyfobacter nitrojigifis. MATERIALS AND METHODSIsolation. Roots of S. altevnifora Loisel were taken from a salt marsh on Sapelo Island, Georgia, and were washed free of sediment in adjacent creek water. Root pieces (length, 2 cm) were placed in tubes of semisolid diazotroph medium (15). Other tubes were seeded with root-associated sediment samples. After 72 h at room temperature, the cultures were transferred to fresh tubes of semisolid malate-salts (MS) medium (15), and nitrogenase activity was determined by acetylene reduction assays (4). After 24 h, tubes of MS medium were capped with serum stoppers, and acetylene was injected to give a final partial pressure of C2Hz of 2 kPa...

High rates of acetylene reduction were observed in systems containing excised rhizomes of the Caribbean marine angiosperms Thalassia testudinum, Syringodium fili/orme and Di. planthera wrightii, and the temperate marine angiosperm Zostera marina. For 4 plant and plant-sediment systems the ratio of acetylene reduced/N 2 fixed varied from 2.6 to 4.6. For T. testudinum the estimated rates of nitrogen fixation are in agreement with estimated requirements of the plant for nitrogen. For a typical T. testudinum stand, N 2 fixation is estimated to be 100 to 500 kg N/hectare per year. Numbers of Ne-fixing bacteria in the rhizosphere sediments were roughly 50 to 300 times more abundant than those in the nonrhizosphere sediments, and in both types of sediments were of the same orders as the estimated numbers of heterotrophic aerobes.

Roots of field-grown tropical maize, Panicum maximum Jacq. and Digitaria decumbens Stent., and of sorghum and wheat grown in monoxenic culture with the diazotroph Spirillum lipoferum (syn. Azospirillum spp.) were examined for tetrazolium-reducing bacteria following incubation of roots in a malate-phosphate buffer-2,3,5-triphenyltetrazolium chloride medium. Bacteria were observed between and in cells of the cortex, in intercellular spaces between the cortex and endodermis, in xylem cells, and in and between pith cells. In maize, colonization of the inner cortex and stele appears to occur in the absence of significant bacterial colonization or collapse of outerlying tissues. Bacteria in the stele remained viable after a 6-h treatment of roots with chloramine-t, indicating that the endodermis was intact. Infection of the inner cortex and stele appears to occur initially in branches, and then to spread longitudinally into main roots. Inter- and intra-cellular infections of the cortex were observed in monoxenic systems. Tetrazolium reduction and prominent crystal formation was not specific for diazotrophic bacteria, but S. lipoferum was isolated from surface-sterilized roots, and S. lipoferum-like organisms were observed in the endorhizosphere. A correlation of inner cortex and stele infections with the presence of branches appears to explain previous observations that excised roots of grasses exhibiting high nitrogenase activity are characteristically branched roots with an intact cortex.

How are high rates of production by Thalassia testudinum KS~rm maintained in notably nutrient-poor tropical waters ? Yield-nutrient supply correlations indicate that a significant proportion of the phosphorus, and virtually all nitrogen for leaf growth are taken-up from the sediments, and that growth is generally limited by availability of nitrogen. Considerations of supply and demand suggest that the sediments could not be a primary source of phosphate, but the sediments may act as a "storage bank" for phosphate taken up from the sea water by T. testudinum. It is believed that inorganic nitrogen in the root layer is derived from fixation of gaseous nitrogen by anaerobic bacteria. Maintenance of anaerobic conditions in the root layer appears to be essential for good growth of T. testudinum, and it is suggested that such conditions facilitate nitrogen fixation.

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