A coastal marine sulfide-oxidizing autotrophic bacterium produces hydrophilic filamentous sulfur as a novel metabolic end product. Phylogenetic analysis placed the organism in the genus Arcobacter in the epsilon subdivision of the Proteobacteria. This motile vibrioid organism can be considered difficult to grow, preferring to grow under microaerophilic conditions in flowing systems in which a sulfide-oxygen gradient has been established. Purified cell cultures were maintained by using this approach. Essentially all 4,6-diamidino-2-phenylindole dihydrochloride-stained cells in a flowing reactor system hybridized with Arcobacter-specific probes as well as with a probe specific for the sequence obtained from reactor-grown cells. The proposed provisional name for the coastal isolate is "Candidatus Arcobacter sulfidicus." For cells cultured in a flowing reactor system, the sulfide optimum was higher than and the CO 2 fixation activity was as high as or higher than those reported for other sulfur oxidizers, such as Thiomicrospira spp. Cells associated with filamentous sulfur material demonstrated nitrogen fixation capability. No ribulose 1,5-bisphosphate carboxylase/oxygenase could be detected on the basis of radioisotopic activity or by Western blotting techniques, suggesting an alternative pathway of CO 2 fixation. The process of microbial filamentous sulfur formation has been documented in a number of marine environments where both sulfide and oxygen are available. Filamentous sulfur formation by "Candidatus Arcobacter sulfidicus" or similar strains may be an ecologically important process, contributing significantly to primary production in such environments.In the marine environment, hydrogen sulfide is a ubiquitous end product of anaerobic processes of organic matter remineralization (5,23,29,30). At ridge crest sites on the ocean floor, it is produced from the geothermal transformation of sulfate and elemental sulfur leaching via seawater-basaltic rock interaction (26,40,62). When brought into contact with the aerobic biosphere, hydrogen sulfide becomes an energy-yielding substrate for chemosynthetic colorless sulfur-oxidizing bacteria. Members of this group include free-living rods or ovoids of the genera Thiobacillus, Thiomonas, Acidiphilium, Thiomicrospira, and Thiovulum (31,32,34,42) as well as the morphologically conspicuous gliding and nongliding filamentous forms of the genera Beggiatoa, Thioploca, and Thiothrix (19,47,48,69). These organisms are characterized by their ability to catalyze the oxidation of sulfide and its chemically and biologically mediated partial oxidation products (polysulfides, S n 2Ϫ ; elemental sulfur, S 0 ; sulfane monosulfonic acids, HSS n O 3 2Ϫ ]; and polythionates, S n O 6 2Ϫ ) (14, 33, 66) coupled to the fixation of carbon dioxide to organic carbon by utilizing the same CalvinBassham-Benson cycle enzymes as those used by oxygenic phototrophs.Because of the differential rates of oxidation of hydrogen sulfide and derived oxidation products, intermediates may substantially accu...

Silicon (Si) is the second most abundant element in soil and effectively counteracts the effects of various abiotic stresses, such as drought, heavy metal toxicity and salinity, on plants. In the present study the ameliorating effects of Si nutrition supplied as 2 mmol L )1 sodium silicate were investigated on hydroponically grown canola (Brassica napus L.) plants under salinity stress (i.e. 150 mmol L )1 sodium chloride). Salinity decreased plant growth parameters such as tissue fresh and dry weights. These decreases were accompanied by increased lignin contents, Na + ion accumulation, increased lipid peroxidation and decreased chlorophyll contents in plants. Silicon nutrition, however, enhanced plant growth parameters and led to the prevention of lignin and the Na + accumulation in shoots, reduced levels of lipid peroxidation in the roots and higher levels of chlorophyll. As a result of salinity, catalase activity in the whole plant and both soluble and cell wall peroxidase activities in the shoots decreased. Silicon nutrition, however, increased the reactive oxygen species scavenging capacity of saltstressed plants through increased catalase and cell wall peroxidase activities. Thus, silicon nutrition ameliorated the deleterious effects of salinity on the growth of canola plants through lower tissue Na + contents, maintaining the membrane integrity of root cells as evidenced by reduced lipid peroxidation, increased reactive oxygen species scavenging capacity and reduced lignification.

Damping off of soybean and corn, caused by Pythium spp., is favored by cool temperatures and wet soil conditions and is primarily managed using fungicide seed treatments. The goal of this research was to determine the effect of temperature on aggressiveness and fungicide sensitivity of Pythium spp. recovered from soybean and corn in Iowa. A total of 21 isolates of four of the most prevalent Pythium spp. in Iowa were screened. Seed and seedling assays were used to quantify the aggressiveness of P. lutarium, P. oopapillum, P. sylvaticum, and P. torulosum on soybean and corn at 13, 18, and 23°C. Isolates recovered from soybean or corn were equally pathogenic on both hosts. P. torulosum was more aggressive at 13°C compared with 18 and 23°C. Conversely, P. sylvaticum was more aggressive at 18 and 23°C than at 13°C. A plate assay was used to assess fungicide sensitivity to seven fungicides that are commonly used as seed treatments, and EC50 values at each of the three temperatures were determined and compared. EC50 values for P. torulosum were higher for all fungicides tested at 13°C, compared with 18 or 23°C, whereas EC50 values for P. sylvaticum were higher for all fungicides at 18 and 23°C compared with 13°C. These data contribute to our understanding of the effect of soil temperature on the risk of soybean and corn damping off, which may aid in the development of more effective management practices.

Methanotroph abundance was analyzed in control and long-term nitrogen-amended pine and hardwood soils using rRNA-targeted quantitative hybridization. Family-specific 16S rRNA and pmoA/amoA genes were analyzed via PCR-directed assays to elucidate methanotrophic bacteria inhabiting soils undergoing atmospheric methane consumption. Quantitative hybridizations suggested methanotrophs related to the family Methylocystaceae were one order of magnitude more abundant than Methyloccocaceae and more sensitive to nitrogen-addition in pine soils. 16S rRNA gene phylotypes related to known Methylocystaceae and acidophilic methanotrophs and pmoA/amoA gene sequences, including three related to the upland soil cluster Alphaproteobacteria (USCalpha) group, were detected across different treatments and soil depths. Our results suggest that methanotrophic members of the Methylocystaceae and Beijerinckiaceae may be the candidates for soil atmospheric methane consumption.