Electron Shuttles Enhance Anaerobic Ammonium Oxidation Coupled to Iron(III) Reduction

Abstract

Anaerobic ammonium oxidation coupled to iron­(III) reduction, termed Feammox, is a newly discovered nitrogen cycling process. However, little is known about the roles of electron shuttles in the Feammox reactions. In this study, two forms of Fe­(III) (oxyhydr)­oxide ferrihydrite (ex situ ferrihydrite and in situ ferrihydrite) were used in dissimilatory Fe­(III) reduction (DIR) enrichments from paddy soil. Evidence for Feammox in DIR enrichments was demonstrated using the 15N-isotope tracing technique. The extent and rate of both the 30N2–29N2 and Fe­(II) formation were enhanced when amended with electron shuttles (either 9,10-anthraquinone-2,6-disulfonate (AQDS) or biochar) and further simulated when these two shuttling compounds were combined. Although the Feammox-associated Fe­(III) reduction accounted for only a minor proportion of total Fe­(II) formation compared to DIR, it was estimated that the potentially Feammox-mediated N loss (0.13–0.48 mg N L–1 day–1) was increased by 17–340% in the enrichments by the addition of electron shuttles. The addition of electron shuttles led to an increase in the abundance of unclassified Pelobacteraceae, Desulfovibrio, and denitrifiers but a decrease in Geobacter. Overall, we demonstrated a stimulatory effect of electron shuttles on Feammox that led to higher N loss, suggesting that electron shuttles might play a crucial role in Feammox-mediated N loss from soils.

Biochar Addition Increases the Rates of Dissimilatory Iron Reduction and Methanogenesis in Ferrihydrite Enrichments

“…(A,B) ferrous iron production, (C,D) acetate turnover, (E,F) CO 2 production, and (G,H) CH 4 production. Data (A–D) of these three setups (CK abiotic, CK biotic, and powdered biochar biotic) in both the enrichments has been described before (Zhou et al, 2016). …”

“…This might result from the higher concentration of salts contained in the in situ ferrihydrite enrichment when preparing the in situ ferrihydrite (Zhou et al, 2016). In addition, other elements (e.g., P, Ni, Al, Se, et al, from the medium) might be incorporated into in situ ferrihydrite during its formation (Zhou et al, 2016).…”

“…This might result from the higher concentration of salts contained in the in situ ferrihydrite enrichment when preparing the in situ ferrihydrite (Zhou et al, 2016). In addition, other elements (e.g., P, Ni, Al, Se, et al, from the medium) might be incorporated into in situ ferrihydrite during its formation (Zhou et al, 2016). This difference in conductivity between two enrichments could explain the higher iron(III) reduction extents and rates in all setups in the in situ ferrihydrite enrichment compared with those in the ex situ ferrihydrite enrichment.…”

“…Granulated biochar (1.2–2 mm) and powdered biochar (<0.15 mm) were sieved by mesh size of 2 mm and 0.15 mm, respectively. The basic properties of biochar were described previously (Xu et al, 2014b; Zhou et al, 2016). The detailed information were listed as following: pH 10.3, electrical conductivity (mS cm -1 ) 5.3, ash content (%) 29.3, total C (%) 48.6, total N (%) 1.7, K (%) 2.1, Ca (%) 0.8, Si (%) 29.4, Cl (%) 0.019, Mg (%) 1.1, P (%) 0.26, Fe (%) 4.4, S (%) 0.1, Mn (%) 0.06, Na (%) 0.6, Al (%) 8.2, Zn (%) 0.01, Rb (%) 0.01, Ba (%) 0.06, Ti (%) 0.5, Cr (%) 0.01, and Sr (%) 0.01.…”

Thermodynamic energy of anaerobic microbial redox reactions couples elemental biogeochemical cycles

“…It was reported that AQDS can function as an electron shuttle to facilitate feammox, which indicates that bacterial AQDS reduction is also coupled to ammonia oxidation (Zhou et al 2016). Therefore, humic substances, which are rich in quinone moieties like AQDS, are likely to be potential electron acceptors of bacterial ammonia oxidation despite no evidence yet.…”