[1] 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 À ) reduction coupled with pyrite oxidation and its stoichiometry in surface sediments, especially without experimental pyrite addition. In this study, we evaluated NO 3 À reduction coupled with sulfur oxidation by long-term incubation of surface sediments from a sulfiderich 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 À addition increased the SO 4 2À concentration and decreased the NO 3 À concentration. SO 4 2À production (ΔSO 4 2À ) was strongly and linearly correlated with NO 3 À consumption (ΔNO 3 À ) during the incubation period (R 2 = 0.983, P < 0.01, and n = 8), and the slope of the regression (ΔNO 3 À /ΔSO 4 2À) and the stoichiometry indicated sulfur-driven NO 3 À 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 À and ΔSO 4 2À 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 À reduction by dissimilatory NO 3 À 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.Citation: Hayakawa, A., M. Hatakeyama , R. Asano, Y. Ishikawa, and S. Hidaka (2013), Nitrate reduction coupled with pyrite oxidation in the surface sediments of a sulfide-rich ecosystem,