High concentrations of uranium, inorganic mercury [Hg(II)], and methylmercury (MeHg) have been detected in streams located in the Department of Energy reservation in Oak Ridge, TN. To determine the potential effects of the surface water contamination on the microbial community composition, surface stream sediments were collected 7 times during the year, from 5 contaminated locations and 1 control stream. Fifty-nine samples were analyzed for bacterial community composition and geochemistry. Community characterization was based on GS 454 FLX pyrosequencing with 235 Mb of 16S rRNA gene sequence targeting the V4 region. Sorting and filtering of the raw reads resulted in 588,699 high-quality sequences with lengths of >200 bp. The bacterial community consisted of 23 phyla, including Proteobacteria (ranging from 22.9 to 58.5% per sample), Cyanobacteria (0.2 to 32.0%), Acidobacteria (1.6 to 30.6%), Verrucomicrobia (3.4 to 31.0%), and unclassified bacteria. Redundancy analysis indicated no significant differences in the bacterial community structure between midchannel and near-bank samples. Significant correlations were found between the bacterial community and seasonal as well as geochemical factors. Furthermore, several community members within the Proteobacteria group that includes sulfate-reducing bacteria and within the Verrucomicrobia group appeared to be associated positively with Hg and MeHg. This study is the first to indicate an influence of MeHg on the in situ microbial community and suggests possible roles of these bacteria in the Hg/MeHg cycle.

No abstract

Shewanella oneidensis MR-1 is a mesophilic bacterium with a maximum growth temperature of Ϸ35°C but the ability to grow over a wide range of temperatures, including temperatures near zero. At room temperature (Ϸ22°C) MR-1 grows with a doubling time of about 40 min, but when moved from 22°C to 3°C, MR-1 cells display a very long lag phase of more than 100 h followed by very slow growth, with a doubling time of Ϸ67 h. In comparison to cells grown at 22°C, the cold-grown cells formed long, motile filaments, showed many spheroplast-like structures, produced an array of proteins not seen at higher temperature, and synthesized a different pattern of cellular lipids. Frequent pilus-like structures were observed during the transition from 3 to 22°C.Shewanella oneidensis MR-1 was first isolated from sediments of Lake Oneida in New York State (20). The cells are gram-negative straight rods capable of moving by means of a single polar flagellum. S. oneidensis MR-1 is a mesophilic, facultative anaerobe having a respiratory type of metabolism, with an optimal growth temperature of Ϸ30°C. Lake Oneida is a shallow freshwater system that freezes over completely during the winter (from early to mid-December to mid-or late January, with complete ice-out in March or April), with ice thickness reaching up to 70 cm (19). During May, water temperatures begin to rise, reaching a maximum of 25°C in midsummer (19).Because of their ability to metabolically reduce metals such as U(VI), Tc(VII), and Cr(VI), organisms such as Shewanella are considered candidates for bioremediation of subsurface metal-contaminated areas. However, subsurface temperatures are often low, and predictions of the utility of Shewanella in such environments require an understanding of the effects of low temperatures on their metabolism and general ecology. Recently, it was shown that S. oneidensis MR-1, while capable of growth at low temperature, exhibited a growth transition at about 10°C, and below these temperatures showed a differential increase in rRNA synthesis compared to DNA synthesis (4). Here we report on other properties of MR-1 grown at low temperature (3°C), including a dramatically different phenotype with changes in morphology, growth rate, ultrastructure, and protein and lipid composition. MATERIALS AND METHODSCells of Shewanella oneidensis strain MR-1 (ATCC 700550) were grown overnight aerobically in batch cultures at 22°C in 150-ml flasks containing 50 ml of Luria-Bertani (LB) broth Miller (Difco) at 130 rpm. For growth measurements, new cultures were started by transferring 1.0 ml of the original culture to a 250-ml Erlenmeyer flask containing 100 ml of LB and incubated at 130 rpm at two temperatures (3 and 22°C). All incubations were done in triplicate. Cells analyzed for proteins and phospholipid fatty acids were harvested by centrifugation and washed with sterile phosphate-buffered saline buffer, wet weight was determined, and the cells were frozen at Ϫ70°C until analysis.The turbidity of the cultures was measured at 600 nm. For measuring cell ...

No abstract

No abstract