RESUMO
The role of rhizosphere bacteria in facilitating Se and Hg accumulation in two wetland plants, saltmarsh bulrush (Scirpus robustus Pursh) and rabbitfoot grass (Polypogon monspeliensis (L.) Desf.), was studied. Ampicillin-amended plants (i.e., with inhibited rhizosphere bacteria) supplied with Na(2)SeO(4) or HgCl(2) had significantly lower concentrations of Se and Hg, respectively, in roots than plants without ampicillin. These results were confirmed by inoculating axenic saltmarsh bulrush plants with bacteria isolated from the rhizosphere of plants collected from the field; these plants accumulated significantly more Se and Hg compared to axenic controls. Therefore, rhizosphere bacteria can increase the efficiency of Se and Hg phytoremediation by promoting the accumulation of Se and Hg in tissues of wetland plants.
RESUMO
Indian mustard (Brassica juncea L.) accumulates high tissue Se concentrations and volatilizes Se in relatively nontoxic forms, such as dimethylselenide. This study showed that the presence of bacteria in the rhizosphere of Indian mustard was necessary to achieve the best rates of plant Se accumulation and volatilization of selenate. Experiments with the antibiotic ampicillin showed that bacteria facilitated 35% of plant Se volatilization and 70% of plant tissue accumulation. These results were confirmed by inoculating axenic plants with rhizosphere bacteria. Compared with axenic controls, plants inoculated with rhizosphere bacteria had 5-fold higher Se concentrations in roots (the site of volatilization) and 4-fold higher rates of Se volatilization. Plants with bacteria contained a heat-labile compound in their root exudate; when this compound was added to the rhizosphere of axenic plants, Se accumulation in plant tissues increased. Plants with bacteria had an increased root surface area compared with axenic plants; the increased area was unlikely to have caused their increased tissue Se accumulation because they did not accumulate more Se when supplied with selenite or selenomethionine. Rhizosphere bacteria also possibly increased plant Se volatilization because they enabled plants to overcome a rate-limiting step in the Se volatilization pathway, i.e. Se accumulation in plant tissues.
RESUMO
Se can be accumulated by plants and volatilized to dimethylselenide, providing an attractive technology for Se phytoremediation. To determine the rate-limiting steps in Se volatilization from selenate and selenite, time- and concentration-dependent kinetics of Se accumulation and volatilization were studied in Indian mustard (Brassica juncea). Time-dependent kinetic studies showed that selenate was taken up 2-fold faster than selenite. Selenate was rapidly translocated to the shoot, away from the root, the site of volatilization, whereas only approximately 10% of the selenite was translocated. For both selenate- and selenite-supplied plants, Se accumulation and volatilization increased linearly with external Se concentration up to 20 &mgr;M; volatilization rates were also linearly correlated with root Se concentrations. Se-volatilization rates were 2- to 3-fold higher from plants supplied with selenite compared with selenate. Se speciation by x-ray absorption spectroscopy revealed that selenite-supplied plants accumulated organic Se, most likely selenomethionine, whereas selenate-supplied plants accumulated selenate. Our data suggest that Se volatilization from selenate is limited by the rate of selenate reduction, as well as by the availability of Se in roots, as influenced by uptake and translocation. Se volatilization from selenite may be limited by selenite uptake and by the conversion of selenomethionine to dimethylselenide.
RESUMO
In anoxic Spartina alterniflora-dominated sediments along a naturally occuring salinity gradient (the Cooper River estuary, South Carolina, U.S.A.), dimethylsulfoniopropionate (DMSP) was metabolized to dimethyl sulfide (DMS) and acrylate by sediment microbes. The rate of DMSP degradation and acrylate mineralization by sediment microbes was similar at all sites along this 25-km transect. However, sediments amended with acrylate (or DMSP) showed significantly higher rates of N2 fixation (measured as acetylene reduction activity) (ARA) in the saline sediments downstream than brackish sediments. These results are consistent with the fact that acrylate stimulated the rates of both denitrification and CO2 production in the saline sediments at the mouth of the river more than tenfold over rates in brackish sediments. Enrichment experiments indicate that microbes capable of using DMSP or acrylate were not present in upstream sediments despite the fact that microbial biomass, percent organic matter, and both glucose-stimulated ARA and denitrification were highest upstream. It appears that acrylate utilizing, N2 fixing, and denitrifying populations are insignificant in the lower salinity sediments of the estuary. These results may reflect the availability of DMSP, which averaged 10.3 nmol g wet wt-1 of saline sediments and levels less than our detection limit (1 &mgr;M) in brackish sediments.
