Nitrate reduction coupled with pyrite oxidation in the surface sediments of a sulfide-rich ecosystem

Abstract

Most studies of denitrification have focused on organic carbon as an electron donor, but reduced sulfur can also support denitrification. Few studies have reported nitrate (NO 3 -) reduction coupled with pyrite oxidation and its stoichiometry in surface sediments, especially without experimental pyrite addition. In this study, we evaluated NO 3 - reduction coupled with sulfur oxidation by long-term incubation of surface sediments from a sulfide-rich ecosystem in Akita Prefecture, Japan. The surface sediments were sampled from a mud pool and a riverbed. Fresh sediments and water were incubated under anoxic conditions (and one oxic condition) at 20°C. NO 3 - addition increased the SO 4 2- concentration and decreased the NO 3 - concentration. SO 4 2- production (δSO 4 2-) was strongly and linearly correlated with NO 3 - consumption (δNO 3 -) during the incubation period (R 2 = 0.983, P < 0.01, and n = 8), and the slope of the regression (δNO 3 - /δSO 4 2-) and the stoichiometry indicated sulfur-driven NO 3 - reduction by indigenous autotrophic denitrifying bacteria. Framboidal pyrite and marcasite (both FeS 2) were present in the sediments and functioned as the electron donors for autotrophic denitrification. Both δNO 3 - and δSO 4 2- were higher in the riverbed sediment than in the mud pool sediment, likely because of the higher amount of easily oxidizable S (pyrite) in the riverbed sediment. Consistently low ammonium (NH 4 +) concentrations indicated that NO 3 - reduction by dissimilatory NO 3 - reduction to NH 4 + was small but could not be disregarded. Our results demonstrate that sulfide-rich ecosystems with easily oxidizable metal-bound sulfides such as FeS 2 near the ground surface may act as denitrification hot spots. © 2013. American Geophysical Union. All Rights Reserved.