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Abstract

The effect of NO3− addition on dissimilatory SO42− reduction and sulfide conversion in organic-rich sludge from the digestion basin of a recirculating marine aquaculture system was studied. SO42− reduction could only explain a minor fraction (up to 4–9%) of the observed total sulfide production (up to 35 mmol L−1 day−1), indicating that the main source of sulfide in the sludge was not SO42− reduction, but desulfuration during the decomposition of organic matter. Although NO3− inhibited SO42− reduction, but not desulfuration, the primary NO3− mitigation effect was the onset of NO3−-mediated sulfide oxidation (up to 75 mmol L−1 day−1), partially to elemental sulfur (S0). Above NO3− concentrations of 0.6 mM in the bulk water, the net sulfide production and oxidation zones were moved deeper into flocs and sludge cores, which effectively prevented sulfide from entering the water column. However, the sulfide efflux from the sludge instantly recovered after NO3− depletion. Thus, the NO3− level in the water column controls the zonation and magnitude of sulfur transformations in the sludge. The effect of NO3− relies therefore on its sustained presence in the water column, which in turn depends on a well-functioning nitrification in the mariculture system.