Nickel is a common heavy metal and often occurs with nitrate (NO3) in effluents from mining and metal-finishing industry. The present study investigates the effects of increasing concentrations (5-200 mg Ni/L) of NiEDTA(2) and NiCl2 on autotrophic denitrification with thiosulfate (S2O32) in batch tests and a fluidized-bed reactor (FBR). In batch bioassays, 50 and 100 mg Ni/L of NiEDTA(2) only increased the transient accumulation of NO2 , whereas 25-100 mg Ni/L of NiCl2 inhibited denitrification by 9-19%. NO3 and NO2 were completely removed in the FBR at feed NiEDTA(2) and NiCl2 concentrations as high as 100 and 200 mg Ni/L, respectively. PCR-DGGE revealed the dominance of Thiobacillus denitrificans and the presence of the sulfate-reducing bacterium Desulfovibrio putealis in the FBR microbial community at all feed nickel concentrations investigated. Nickel mass balance, thermodynamic modeling and solid phase characterization indicated that nickel sulfide, phosphate and oxide precipitated in the FBR during NiCl2 injection. (C) 2017 Elsevier Ltd. All rights reserved.