OE-26:

Anaerobic ammonium oxidation coupled with extracellular electron transport by Shewanella oneidensis MR-1

Several kinds of iron reducing bacteria, such as Shewanella and Geobactor, transport electrons to various mineral surface to terminate their respiration. This process is called as extracellular electron transport (EET), which plays an important role in geochemical cycles for carbon, sulfur, phosphorus, as well as iron and manganese.¹ However, the oxidation reaction of nitrogen compounds composing nitrogen cycle associated with EET has not been widely investigated. In this study, we examined EET-coupled ammonium oxidation by a model EET microbe, Shewanella oneidensis MR-1, with highly sensitive electrochemical methods.²
We used an anaerobic electrochemical reactor with indium tin-oxide (ITO) electrode poised at +0.4 V vs. SHE, which contains 20 mM NH₄Cl. Increase of metabolic current and decrease of NH₄⁺ concentration were observed after the addition of S. oneidensis MR-1 into the reactor, although no current increase was detected without NH₄Cl. in situ mass spectroscopic analysis showed the production of nitric oxide (NO) coupled with current increase. These results suggested that S. oneidensis MR-1 oxidized ammonia coupled with EET. In accordance with the assumption that the produced NO would inhibit EET, metabolic current production ceased in 12 hours after the microbe addition even with enough NH₄⁺. Furthermore, NO scavenger (2-Phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO)) in the reactor led to a sustainable current production for more than 20 hours. These results strongly suggest that NO production by ammonium oxidation is coupled with EET.
We will discuss oxygen source for NO production in detail.

1. Nealson, KH. et al., Annu. Rev. Microbiol., 1994, 48, 311-343
2. Okamoto, A. et al., Proc. Natl. Acad. Sci. USA, 2013, 110(19), 7856_7861

keywords:Nitrogen cycle,Shewanella,Extracellular electron transfer,Electrochemistry,Ammonium