Iodate Reduction by Shewanella oneidensis Does Not Involve Nitrate Reductase

ABSTRACT

Microbial iodate (IO₃⁻) reduction is a major component of iodine biogeochemical cycling and is the basis of alternative strategies for remediation of iodine-contaminated environments. The molecular mechanism of microbial IO₃⁻ reduction, however, is not well understood. In several microorganisms displaying IO₃⁻ and nitrate (NO₃⁻) reduction activities, NO₃⁻ reductase is postulated to reduce IO₃⁻ as alternate electron acceptor. In the present study, whole genome analyses of 25 NO₃⁻-reducing Shewanella strains identified various combinations of genes encoding one assimilatory (cytoplasmic Nas) and three dissimilatory (membrane-associated Nar and periplasmic Napα and Napβ) NO₃⁻ reductases. Shewanella oneidensis was the only Shewanella strain whose genome encoded a single NO₃⁻ reductase (Napβ). Terminal electron acceptor competition experiments in S. oneidensis batch cultures amended with both NO₃⁻ and IO₃⁻ demonstrated that neither NO₃⁻ nor IO₃⁻ reduction activities were competitively inhibited by the presence of the competing electron acceptor. The lack of involvement of S. oneidensis Napβ in IO₃⁻ reduction was confirmed via phenotypic analysis of an in-frame gene deletion mutant lacking napβA (encoding the NO₃⁻-reducing NapβA catalytic subunit). S. oneidensis ΔnapβA was unable to reduce NO₃⁻, yet reduced IO₃⁻ at rates higher than the wild-type strain. Thus, NapβA is required for dissimilatory NO₃⁻ reduction by S. oneidensis, while neither the assimilatory (Nas) nor dissimilatory (Napα, Napβ, and Nar) NO₃⁻ reductases are required for IO₃⁻ reduction. These findings provide the first genetic evidence that IO₃⁻ reduction by S. oneidensis does not involve nitrate reductase and indicate that S. oneidensis reduces IO₃⁻ via an as yet undiscovered enzymatic mechanism.

KEYWORDS:

• Iodate
• nitrate
• reduction
• shewanella oneidensis

Additional information

Funding

Funding was provided by the US Department of Energy Office of Environmental Management and Richland Operations Office through a subcontract from the Pacific Northwest National Laboratory (PNNL). PNNL is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-AC05-76RL01830.