Oxidation of Fe(II) by nitrate-reducing Fe(II)-oxidizers enriched from Fe(III)-rich microbial mats of an extreme acidic river (Río Tinto, Spain).

Gallionella dominated Fe(III)-rich microbial mats at the Río Tinto origin, but nitrate-reducing Fe(II) oxidation was only observed in acetate/NO3−/Fe2+ enrichments where Rhodoblastus dominated.

• Why it matters: It was unclear whether NRFeOx microorganisms inhabit these surface-water mats as pH shifts from circumneutral to highly acidic, and which Fe(II) oxidation pathway operates in situ.
• What they did: The team analyzed mat communities and Fe mineralogy, then ran NRFeOx enrichment cultures with either NO3−/Fe2+ aq or acetate/NO3−/Fe2+ aq and tested denitrification and Fe(II) oxidation under in situ pH.
• The result: Nitrite production in the Rhodoblastus-dominated culture suggested chemodenitrification as the main Fe(II) oxidation pathway at pH 6.2–7, while at pH 5.8 nitrate reduction occurred without Fe(II) oxidation.