Nano-scale zero-valent iron particles (NZVI) are increasingly being used to treat sites contaminated with chlorinated solvents. This study investigated the effect of NZVI on dechlorinating microorganisms that participate in the anaerobic bioremediation of such sites, NZVI can have a biostimulatory effect associated with water-derived cathodic H-2 production during its anaerobic corrosion (730 +/- 30 mu mol H-2 Was produced in 166 h in abiotic controls with 1 g/L NZVI) or an inhibitory effect upon contact with cell surfaces (assessed by transmission electron microscopy). Methanogens, which are known to compete for H-2 with dechlorinators, were significantly biostimulated by NZVI and methane production increased relative to NZVI-free controls from 58 +/- 5 to 275 +/- 2 mu mol. In contrast, bacteria dechlorinating TCE were inhibited by NZVI, and the first-order degradation rate coefficient decreased from 0.115 +/- 0.005 h(-1) (R-2 = 0.99) for controls to 0.053 +/- 0.003 h(-1) (R-2 = 0.98) for treatments with 1 g/L NZVI. Ethene production from TCE was initially inhibited by NZVI, but after 331 h increased to levels observed for an NZVI-free system (7.6 +/- 0.3 mu mol ethene produced in 502 h compared to 11.6 +/- 0.5 mmol in the NZVI-free system and 3.8 +/- 0.3 mu mol ethene for NZVI alone). Apparently, cathodic H-2 was utilized as electron donor by dechlorinating bacteria, which recovered following the partial oxidation and presumably passivation of the NZVI. Overall, these results suggest that reductive treatment of chlorinated solvent sites with NZVI might be enhanced by the concurrent or subsequent participation of bacteria that exploit cathodic depolarization and reductive dechlorination as metabolic niches. (C) 2009 Elsevier Ltd. All rights reserved.