PNNL

Abstract

Microbial iodate (IO3-) reduction is a major component of the iodine biogeochemical reaction network 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 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 Nap and Nap) NO3- reductases. S. oneidensis was the only Shewanella strain whose genome encoded a single NO3- reductase (Nap). 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 Nap in IO3- reduction was confirmed via phenotypic analysis of an in-frame gene deletion mutant lacking napA (encoding the NO3--reducing NapA catalytic subunit). S. oneidensis ?napA was unable to reduce NO3-, yet reduced IO3- at rates higher than the wild-type strain. Thus, NapA is required for dissimilatory NO3- reduction by S. oneidensis, while neither the assimilatory (Nas) nor dissimilatory (Nap, Nap, and Nar) NO3- reductases are required for IO3- reduction. These findings oppose the traditional view that NO3- reductase reduces IO3- as alternate electron acceptor and indicate that S. oneidensis reduces IO3- via an as yet undiscovered enzymatic mechanism.