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Community N and O isotope fractionation by sulfide-dependent denitrification and anammox in a stratified lacustrine water column
Journal
Geochimica et cosmochimica acta
Volume
125
Pages / Article-Number
551-563
Abstract
We investigated the community nitrogen (N) and oxygen (O) isotope effects of fixed N loss in the northern basin of Lake Lugano, where sulfide-dependent denitrification and anammox are the main drivers of suboxic N-2 production. A decrease in nitrate (NO3-) concentration toward the redox transition zone (RTZ) at mid-water depth was paralleled by an increase in delta N-15 and delta O-18 from approximately 5 parts per thousand to > 20 parts per thousand and from 0 parts per thousand to > 10 parts per thousand, respectively. Ammonium (NH4+) concentrations were highest in the near-bottom water and decreased toward the RTZ concomitant with an increase in delta N-15-NH4+ from similar to 7 parts per thousand to > 15 parts per thousand. A diffusion-reaction model yielded N and O isotope enrichment factors that are significantly smaller than isotope effects reported previously for microbial NO3 reduction and NH4+ oxidation ((15)epsilon(NO3) approximate to 10 parts per thousand, (18)epsilon(NO3) approximate to 7 parts per thousand, and (15)epsilon(NH4) approximate to 10-12 parts per thousand). For the Lake Lugano north basin, we constrain the apparent under-expression of the N isotope effects to: (1) environmental conditions (e. g., substrate limitation, low cell specific N transformation rates), or (2) low process-specific (chemolithotrophic denitrification and anammox) isotope fractionation. Our results have confirmed the robust nature of the co-linearity between N and O isotope enrichment during microbial denitrification beyond its organotrophic mode. However, the ratio of O-18 to N-15 enrichment ((18)epsilon(NO3) : (15)epsilon(NO3)) associated with NO3- reduction in the RTZ was similar to 0.89, which is lower than observed in marine environments and in most culture experiments. We propose that chemolithotrophic NO3- reduction in the Lake Lugano north basin was partly catalyzed by the periplasmic dissimilatory nitrate reductase (Nap) (rather than the membrane-bound dissimilatory Nar), which is known to express comparably low 18eNO3 : 15eNO3 ratios in the ambient NO3- pool. However, NO2- re-oxidation, e. g., during anammox or microaerobic nitrification, could have contributed to the lowered O-18 to N-15 enrichment ratios. Although we do not yet understand the exact controls on the observed N (and O) isotope fractionation in the Lake Lugano north basin, our study implies that caution is advised when assuming canonical (i.e., high) N isotope effects for NO3- reduction and NH4+ oxidation in natural environments. In Lake Lugano, the community N (and O) isotope effects by sulfide-dependent denitrification and anammox in a natural ecosystem appear to be significantly lower than for organotrophic denitrification and aerobic ammonium oxidation. (C) 2013 Elsevier Ltd. All rights reserved.
