Microbially Mediated Coupling of Fe and N Cycles by Nitrate-Reducing Fe(II)-Oxidizing Bacteria in Littoral Freshwater Sediments

Abstract

Nitrate-reducing iron(II)-oxidizing bacteria have been known for approximately 20 years. There has been much debate as to what extent the reduction of nitrate and the oxidation of ferrous iron are coupled via enzymatic pathways or via abiotic processes induced by nitrite formed by heterotrophic denitrification. The aim of the present study was to assess the coupling of nitrate reduction and iron(II) oxidation by monitoring changes in substrate concentrations, as well as in the activity of nitrate-reducing bacteria in natural littoral freshwater sediment, in response to stimulation with nitrate and iron(II). In substrate-amended microcosms, we found that the biotic oxidation of ferrous iron depended on the simultaneous microbial reduction of nitrate. Additionally, the abiotic oxidation of ferrous iron by nitrite in sterilized sediment was not fast enough to explain the iron oxidation rates observed in microbially active sediment. Furthermore, the expression levels of genes coding for enzymes crucial for nitrate reduction were in some setups stimulated by the presence of ferrous iron. These results indicate that there is a direct influence of ferrous iron on bacterial denitrification and support the hypothesis that microbial nitrate reduction is stimulated by biotic iron(II) oxidation. The coupling of nitrate reduction and Fe(II) oxidation affects the environment at a local scale, e.g., by changing nutrient or heavy metal mobility in soils due to the formation of Fe(III) minerals, as well as at a global scale, e.g., by the formation of the primary greenhouse gas nitrous oxide. Although the coupling of nitrate reduction and Fe(II) oxidation was reported 20 years ago and has been studied intensively since then, the underlying mechanisms still remain unknown. One of the main knowledge gaps is the extent of enzymatic Fe(II) oxidation coupled to nitrate reduction, which has frequently been questioned in the literature. In the present study, we provide evidence for microbially mediated nitrate-reducing Fe(II) oxidation in freshwater sediments. This evidence is based on the rates of nitrate reduction and Fe(II) oxidation determined in microcosm incubations and on the effect of iron on the expression of genes required for denitrification.

Anaerobic Bacterial Immobilization and Removal of Toxic Sb(III) Coupled With Fe(II)/Sb(III) Oxidation and Denitrification

“…NAFOB oxidation of Fe(II) to Fe(III) is coupled with nitrate reduction, which produces Fe(III) minerals and ammonium, nitrogen or denitrification intermediates (Coby et al, 2011; Nordhoff et al, 2017; Schaedler et al, 2018). Thus, NAFOB play an important role in the process of biomineralization and heavy metal passivation, and thereby affect the migration of heavy metals such as chromium (Cr), arsenic (As), and Sb (Senn and Hemond, 2002; Richmond et al, 2004; Vodyanitskii, 2010).…”

Removal of abamectin and conventional pollutants in vertical flow constructed wetlands with Fe-modified biochar

“…31 It has been previously reported as the main driver of abiotic Fe(II) oxidation during nitrate reduction. 45 It was also proven that Fe 2+ promoted the denitrication process when there was a lack of organic matter in the treatment. 29 The process of oxidation of iron coupled to the reduction of nitrate is as follows: 43,45 10Fe…”

Visualizing Microorganism-Mineral Interaction in the Iberian Pyrite Belt Subsurface: The Acidovorax Case

“…Thus, it was proposed that Fe 2+ is most likely indirectly oxidized by these microorganisms through the reactive nitrogen species produced during the denitrification process (Picardal, 2012;Klueglein and Kappler, 2013;Klueglein et al, 2015). Even so, enzymatic oxidation of iron has not been ruled out and both processes, biotic and abiotic, are accepted today (Carlson et al, 2013;Schaedler et al, 2018). Acidovorax colonies in the IPB subsurface can be found surrounded by ferric iron (Figure 8), indicating that it was oxidized by the microorganism.…”