Soluble ions released by zinc oxide (ZnO) and copper (Cu-0) nanoparticles (NPs) have been associated with toxicity to methanogens. This study evaluated the role of biogenic sulfide in attenuating ZnO and Cu-0 NP toxicity to methanogens. Short- and long-term batch experiments were conducted to explore ZnO and Cu-0 NPs toxicity to acetoclastic methanogens in sulfate-containing (0.4 mM) and sulfate-free conditions. ZnO and Cu-0 were respectively 14 and 7-fold less toxic in sulfate-containing than in sulfate-free assays as indicated by inhibitory constants (K-i). The K-i with respect to residual soluble metal indicated that soluble metal was well correlated with toxicity irrespective of the metal ion source or presence of biogenic sulfide. Long-term assays indicated that ZnO and Cu-0 NPs caused different effects on methanogens. ZnO NPs without protection of sulfide caused a chronic effect, whereas Cu-0 NPs caused an acute effect and recovered. This study confirms that biogenic sulfide effectively attenuates ZnO and Cu-0 NPs toxicity to methanogens by the formation of metal sulfides. (C) 2014 Elsevier B.V. All rights reserved.