Impact of reactive surfaces on the abiotic reaction between nitrite and ferrous iron and associated nitrogen and oxygen isotope dynamics
Authors: Visser Anna-Neva , Wankel Scott D. , Niklaus Pascal A. , Byrne James M. , Kappler Andreas A. , Lehmann Moritz F. 
Published: August 2020

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.