Effect of nitrate on sulfur transformations in sulfidogenic sludge of a marine aquaculture biofilter

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.