“…This activity could also be visualized through fluorescence microscopy of native subsurface samples by the co‐occurrence of ferric iron, the oxidation product, and the presence of nitrate‐reducing microorganisms, Acidovorax and Tessaracoccus , clearly identified by CARD‐FISH (Figure S8 ). The genomic analysis of these microorganisms showed the lack of recognizable genes associated with this metabolic activity (Carlson et al, 2013 ; Leandro et al, 2017 ; Martínez et al, 2020 ; Mateos et al, 2022 ; Puente‐Sánchez, Pieper, & Arce‐Rodríguez, 2016 ), so we have to conclude that the observed oxidation of iron, at least for the tested microorganisms, is due to the chemical oxidation of Fe promoted by the high oxidizing capacity of nitrite and nitric oxide generated by the use of nitrate as electron acceptor. These results strongly support the notion that some nitrate reducers can efficiently generate reactive nitrogen species able to oxidize iron under strict anaerobic conditions (Bryce et al, 2018 ; Carlson et al, 2013 ; Straub et al, 2004 ).…”