Abiotic oxidation of Fe(II) by reactive nitrogen species in cultures of the nitrate-reducing Fe(II) oxidizer Acidovorax sp BoFeN1 - questioning the existence of enzymatic Fe(II) oxidation

Nitrate-reducing, Fe(II)-oxidizing bacteria were suggested to couple with enzymatic Fe(II) oxidation to nitrate reduction. Denitrification proceeds via intermediates (NO2 , NO) that can oxidize Fe(II) abiotically at neutral and particularly at acidic pH. Here, we present a revised Fe(II) quantification protocol preventing artifacts during acidic Fe extraction and evaluate the contribution of abiotic vs. enzymatic Fe(II) oxidation in cultures of the nitrate-reducing, Fe(II) oxidizer Acidovorax sp. BoFeN1. Sulfamic acid used instead of HCl reacts with nitrite and prevents abiotic Fe(II) oxidation during Fe extraction. Abiotic experiments without sulfamic acid showed that acidification of oxic Fe(II) nitrite samples leads to 5.6-fold more Fe(II) oxidation than in anoxic samples because the formed NO becomes rapidly reoxidized by O2, therefore leading to abiotic oxidation and underestimation of Fe(II). With our revised protocol using sulfamic acid, we quantified oxidation of approximately 7mm of Fe(II) by BoFeN1 within 4days. Without addition of sulfamic acid, the same oxidation was detected within only 2days. Additionally, abiotic incubation of Fe(II) with nitrite in the presence of goethite as surface catalyst led to similar abiotic Fe(II) oxidation rates as observed in growing BoFeN1 cultures. BoFeN1 growth was observed on acetate with N2O as electron acceptor. When adding Fe(II), no Fe(II) oxidation was observed, suggesting that the absence of reactive N intermediates (NO2 , NO) precludes Fe(II) oxidation. The addition of ferrihydrite [Fe(OH)3] to acetate/nitrate BoFeN1 cultures led to growth stimulation equivalent to previously described effects on growth by adding Fe(II). This suggests that elevated iron concentrations might provide a nutritional effect rather than energy-yielding Fe(II) oxidation. Our findings therefore suggest that although enzymatic Fe(II) oxidation by denitrifiers cannot be fully ruled out, its contribution to the observed Fe(II) oxidation in microbial cultures is probably lower than previously suggested and has to be questioned in general until the enzymatic machinery-mediating Fe(II) oxidation is identified.