Geochemical and isotopic study of abiotic nitrite reduction coupled to biologically produced Fe(II) oxidation in marine environments

Abstract

Estuarine sediments are often characterized by abundant iron oxides, organic matter, and anthropogenicnitrogen compounds (e.g., nitrate and nitrite). Anoxic dissimilatory iron reducing bacteria (e.g.,Shewa-nella loihica) are ubiquitous in these environments where they can catalyze the reduction of Fe(III) (oxyhydr)oxides, thereby releasing aqueous Fe(II). The biologically produced Fe(II) can later reduce ni-trite to form nitrous oxide. The effect on nitrite reduction by both biologically produced and artificiallyamended Fe(II) was examined experimentally. Ferrihydrite was reduced byShewanella loihicain a batchreaction with an anoxic synthetic sea water medium. Some of the Fe(II) released by S. loihica adsorbedonto ferrihydrite, which was involved in the transformation of ferrihydrite to magnetite. In a second setof experiments with identical medium, no microorganism was present, instead, Fe(II) was amended. Theamount of solid-bound Fe(II) in the experiments with bioproduced Fe(II) increased the rate of abiotic NO2- reduction with respect to that with synthetic Fe(II), yielding half-lives of 0.07 and 0.47 d, respec-tively. The d18O and d15N of NO2- was measured through time for both the abiotic and innoculated ex-periments. The ratio of ε18O/ε15N was 0.6 for the abiotic experiments and 3.1 when NO2- was reduced by S. loihica, thus indicating two different mechanisms for the NO2-reduction. Notably, there is a wide rangeof the ε18O/ε15N values in the literature for abiotic and biotic NO2- reduction, as such, the use of this ratioto distinguish between reduction mechanisms in natural systems should be taken with caution.Therefore, we suggest an additional constraint to identify the mechanisms (i.e. abiotic/biotic) controllingNO2 reduction in natural settings through the correlation of d15N-NO2- and the aqueous Fe(II)concentration.