Impact of reactive surfaces on the abiotic reaction between nitrite and ferrous iron and associated nitrogen and oxygen isotope dynamics

Abstract

Abstract. Anaerobic nitrate-dependent Fe(II) oxidation (NDFeO) is widespread in various aquatic environments and plays a major role in iron and nitrogen redox dynamics. However, evidence for truly enzymatic, autotrophic NDFeO remains limited, with alternative explanations involving the coupling of heterotrophic denitrification with the abiotic oxidation of structurally bound or aqueous Fe(II) by reactive intermediate nitrogen (N) species (chemodenitrification). The extent to which chemodenitrification is caused (or enhanced) by ex vivo surface catalytic effects has not been directly tested to date. To determine whether the presence of either an Fe(II)-bearing mineral or dead biomass (DB) catalyses chemodenitrification, two different sets of anoxic batch experiments were conducted: 2 mM Fe(II) was added to a low-phosphate medium, resulting in the precipitation of vivianite (Fe3(PO4)2), to which 2 mM nitrite (NO2-) was later added, with or without an autoclaved cell suspension (∼1.96×108 cells mL−1) of Shewanella oneidensis MR-1. Concentrations of nitrite (NO2-), nitrous oxide (N2O), and iron (Fe2+, Fetot) were monitored over time in both set-ups to assess the impact of Fe(II) minerals and/or DB as catalysts of chemodenitrification. In addition, the natural-abundance isotope ratios of NO2- and N2O (δ15N and δ18O) were analysed to constrain the associated isotope effects. Up to 90 % of the Fe(II) was oxidized in the presence of DB, whereas only ∼65 % of the Fe(II) was oxidized under mineral-only conditions, suggesting an overall lower reactivity of the mineral-only set-up. Similarly, the average NO2- reduction rate in the mineral-only experiments (0.004±0.003 mmol L−1 d−1) was much lower than in the experiments with both mineral and DB (0.053±0.013 mmol L−1 d−1), as was N2O production (204.02±60.29 nmol L−1 d−1). The N2O yield per mole NO2- reduced was higher in the mineral-only set-ups (4 %) than in the experiments with DB (1 %), suggesting the catalysis-dependent differential formation of NO. N-NO2- isotope ratio measurements indicated a clear difference between both experimental conditions: in contrast to the marked 15N isotope enrichment during active NO2- reduction (15εNO2=+10.3 ‰) observed in the presence of DB, NO2- loss in the mineral-only experiments exhibited only a small N isotope effect (<+1 ‰). The NO2--O isotope effect was very low in both set-ups (18εNO2 <1 ‰), which was most likely due to substantial O isotope exchange with ambient water. Moreover, under low-turnover conditions (i.e. in the mineral-only experiments as well as initially in experiments with DB), the observed NO2- isotope systematics suggest, transiently, a small inverse isotope effect (i.e. decreasing NO2- δ15N and δ18O with decreasing concentrations), which was possibly related to transitory surface complexation mechanisms. Site preference (SP) of the 15N isotopes in the linear N2O molecule for both set-ups ranged between 0 ‰ and 14 ‰, which was notably lower than the values previously reported for chemodenitrification. Our results imply that chemodenitrification is dependent on the available reactive surfaces and that the NO2- (rather than the N2O) isotope signatures may be useful for distinguishing between chemodenitrification catalysed by minerals, chemodenitrification catalysed by dead microbial biomass, and possibly true enzymatic NDFeO.

Isotopic signatures of biotic and abiotic N2O production and consumption in the water column of meromictic, ferruginous Lake La Cruz (Spain)

“…In the unfiltered 15 NO À 2 incubation experiments with Fe 2+ , we observed a relatively high N 2 O production rate, whereas in both the filtered NO À 2 + Fe 2+ (no cells and particles) experiment and in the unfiltered NO À 2 -only (biological denitrification) experiment N 2 O production was just above detection limit, suggesting that reactive surfaces of bacterial or other cells and/or particulate iron catalyze the reaction of Fe 2+ with NO À 2 . This effect has been previously demonstrated in various experiments under controlled lab conditions (Sutka et al 2006;Kopf et al 2013;Visser et al 2020). In experiments with NO À 3 and Fe 2+ , we did not observe catalyzing effects of reactive surfaces on N 2 O production as they were possibly masked by microbial N 2 O production through denitrification.…”

“…Both oxic and anoxic incubation bottles were incubated at $16 C in the dark. At each of three preassigned time points ($12, 24, and 36 h), three bottles were sacrificed by introducing a 40-mL He headspace and terminating microbial activity, as well as potential chemodenitrification by adding 5 mL of 10 M NaOH, which precipitates ferrous iron and strongly decreases the reactivity of nitrite (Braida and Ong 2000;Visser et al 2020). One bottle was sacrificed directly after starting the experiment.…”

“…1, pathway 7; Burgin and Hamilton 2007 and references therein). It can be accelerated by surface catalysts, such as mineral surfaces (Buchwald et al 2016; Grabb et al 2017), or dead bacterial detritus (Visser et al 2020), and therefore indirectly connected to microbial activity, as microbes provide reactive N metabolites and catalyzing surfaces.…”

“…For instance, they may help to identify AOB‐associated N 2 O production from ammonium via nitrification (Δδ 15 N NH4+→N2O ≈ 7‰) and nitrifier denitrification (Δδ 15 N NH4+→N2O ≈ 57‰) (Frame and Casciotti 2010). Contrarily, the N and O isotopic offsets with nitrite or nitrate as substrate can hardly distinguish between denitrification (Δδ 15 N NO3−→N2O ≈ 10–39‰, Δδ 18 O NO3−→N2O ≈ −40 to −4‰; Casciotti et al 2002; Toyoda et al 2005; Sutka et al 2006) and chemodenitrification (Δδ 15 N NO2−→N2O ≈ 2–30‰, Δδ 18 O NO2−→N2O ≈ −30 to −17‰; Jones et al 2015; Buchwald et al 2016; Visser et al 2020).…”

“…For instance, they may help to identify AOB-associated N 2 O production from ammonium via nitrification i Sutka et al (2006) j Frame and Casciotti (2010) k Sutka et al (2003Sutka et al ( , 2004 l Yamazaki et al (2014) m Heil et al (2014) n Buchwald et al (2016) o Jones et al (2015) p Visser et al (2020) q Grabb et al (2017) r Stanton et al (2018) s Toyoda et al (2005) t Casciotti et al (2002) u Ostrom et al (2007) v Yamagishi et al (2007) w Yoshida (1988) Reference list a Moews and Audrieth (1959) b Bonner et al (1978) c Döring and Gehlen (1961) d Schreiber et al (2012) e Wrage et al (2001) f Burgin and Hamilton (2007) g Yoshida and Toyoda (2000) h (Δδ 15 N NH4+!N2O ≈ 7‰) and nitrifier denitrification (Δδ 15 N NH4+-!N2O ≈ 57‰) (Frame and Casciotti 2010). Contrarily, the N and O isotopic offsets with nitrite or nitrate as substrate can hardly distinguish between denitrification (Δδ 15 N NO3À!N2O ≈ 10-39‰, Δδ 18 O NO3À!N2O ≈ À40 to À4‰; Casciotti et al 2002;Toyoda et al 2005;Sutka et al 2006) and chemodenitrification (Δδ 15 N NO2À!N2O ≈ 2-30‰, Δδ 18 O NO2À!N2O ≈ À30 to À17‰; Jones et al 2015;Buchwald et al 2016;Visser et al 2020). The δ 18 O-vs.-δ 15 N relationship of N 2 O may be particularly helpful in constraining N 2 O sources and sinks in low O 2 / anoxic environments.…”