Metal Reduction and Protein Secretion Genes Required for Iodate Reduction by Shewanella oneidensis.

The metal-reducing gammaproteobacterium Shewanella oneidensis reduces iodate (IO3-) as an anaerobic terminal electron acceptor. Microbial IO3- electron transport pathways are postulated to terminate with nitrate (NO3-) reductase, which reduces IO3- as an alternative electron acceptor. Recent studies with S. oneidensis, however, have demonstrated that NO3- reductase is not involved in IO3- reduction. The main objective of the present study was to determine the metal reduction and protein secretion genes required for IO3- reduction by Shewanella oneidensis with lactate, formate, or H2 as the electron donor. With all electron donors, the type I and type V protein secretion mutants retained wild-type IO3- reduction activity, while the type II protein secretion mutant lacking the outer membrane secretin GspD was impaired in IO3- reduction. Deletion mutants lacking the cyclic AMP receptor protein (CRP), cytochrome maturation permease CcmB, and inner membrane-tethered c-type cytochrome CymA were impaired in IO3- reduction with all electron donors, while deletion mutants lacking c-type cytochrome MtrA and outer membrane ß-barrel protein MtrB of the outer membrane MtrAB module were impaired in IO3- reduction with only lactate as an electron donor. With all electron donors, mutants lacking the c-type cytochromes OmcA and MtrC of the metal-reducing extracellular electron conduit MtrCAB retained wild-type IO3- reduction activity. These findings indicate that IO3- reduction by S. oneidensis involves electron donor-dependent metal reduction and protein secretion pathway components, including the outer membrane MtrAB module and type II protein secretion of an unidentified IO3- reductase to the S. oneidensis outer membrane.IMPORTANCE Microbial iodate (IO3-) reduction is a major component in the biogeochemical cycling of iodine and the bioremediation of iodine-contaminated environments; however, the molecular mechanism of microbial IO3- reduction is poorly understood. Results of the present study indicate that outer membrane (type II) protein secretion and metal reduction genes encoding the outer membrane MtrAB module of the extracellular electron conduit MtrCAB are required for IO3- reduction by S. oneidensis On the other hand, the metal-reducing c-type cytochrome MtrC of the extracellular electron conduit is not required for IO3- reduction by S. oneidensis These findings indicate that the IO3- electron transport pathway terminates with an as yet unidentified IO3- reductase that associates with the outer membrane MtrAB module to deliver electrons extracellularly to IO3.

Iodate and nitrate transformation by Agrobacterium/Rhizobium related strain DVZ35 isolated from contaminated Hanford groundwater.

Nitrate and radioiodine (129I) contamination is widespread in groundwater underneath the Central Plateau of the Hanford Site. 129I, a byproduct of nuclear fission, is of concern due to a 15.7 million year half-life, and toxicity. The Hanford 200 West Area contains plumes covering 4.3 km2 with average 129I concentrations of 3.5 pCi/L. Iodate accounts for 70.6% of the iodine present and organo-iodine and iodide make up 25.8% and 3.6%, respectively. Nitrate plumes encompassing the 129I plumes have a surface area of 16 km2 averaging 130 mg/L. A nitrate and iodate reducing bacterium closely related to Agrobacterium, strain DVZ35, was isolated from sediment incubated in a 129I plume. Iodate removal efficiency was 36.3% in transition cultures, and 47.8% in anaerobic cultures. Nitrate (10 mM) was also reduced in the microcosm. When nitrate was spiked into the microcosms, iodate removal efficiency was 84.0% and 69.2% in transition and anaerobic cultures, respectively. Iodate reduction was lacking when nitrate was absent from the growth medium. These data indicate there is simultaneous reduction of nitrate and iodate by DVZ35, and iodate is reduced to iodide. Results provide the scientific basis for combined nitrogen and iodine cycling throughout the Hanford Site.

Field evidence for co-metabolism of trichloroethene stimulated by addition of electron donor to groundwater.

For more than 10 years, electron donor has been injected into the Snake River aquifer beneath the Test Area North site of the Idaho National Laboratory for the purpose of stimulating microbial reductive dechlorination of trichloroethene (TCE) in groundwater. This has resulted in significant TCE removal from the source area of the contaminant plume and elevated dissolved CH(4) in the groundwater extending 250 m from the injection well. The delta(13)C of the CH(4) increases from -56 per thousand in the source area to -13 per thousand with distance from the injection well, whereas the delta(13)C of dissolved inorganic carbon decreases from 8 per thousand to -13 per thousand, indicating a shift from methanogenesis to methane oxidation. This change in microbial activity along the plume axis is confirmed by PhyloChip microarray analyses of 16S rRNA genes obtained from groundwater microbial communities, which indicate decreasing abundances of reductive dechlorinating microorganisms (e.g., Dehalococcoides ethenogenes) and increasing CH(4)-oxidizing microorganisms capable of aerobic co-metabolism of TCE (e.g., Methylosinus trichosporium). Incubation experiments with (13)C-labeled TCE introduced into microcosms containing basalt and groundwater from the aquifer confirm that TCE co-metabolism is possible. The results of these studies indicate that electron donor amendment designed to stimulate reductive dechlorination of TCE may also stimulate co-metabolism of TCE.

Activity-dependent labeling of oxygenase enzymes in a trichloroethene-contaminated groundwater site.

A variety of naturally occurring bacteria produce enzymes that cometabolically degrade trichloroethene (TCE), including organisms with aerobic oxygenases. Groundwater contaminated with TCE was collected from the aerobic region of the Test Area North site of the Idaho National Laboratory. Samples were evaluated with enzyme activity probes, and resulted in measurable detection of toluene oxygenase activity (6-79% of the total microbial cells). Wells from both inside and outside contaminated plume showed activity. Toluene oxygenase-specific PCR primers determined that toluene-degrading genes were present in all groundwater samples evaluated. In addition, bacterial isolates were obtained and possessed toluene oxygenase enzymes, demonstrated activity, and were dominated by the phylotype Pseudomonas. This study demonstrated, through the use of enzymatic probes and oxygenase gene identification, that indigenous microorganisms at a contaminated site were cometabolically active. Documentation such as this can be used to substantiate observations of natural attenuation of TCE-contaminated groundwater plumes.