The impact of gamma-Al2O3 nanoparticles (NP) on specific methanogenic activity (SMA) and humus reducing activity (HRA) in an anaerobic consortium was evaluated. SMA in sludge incubations without gamma-Al2O3 was always higher compared with those performed in the presence of 100 g/L of gamma-Al2O3. Nevertheless, the SMA in incubations with gamma-Al2O3 was not completely inhibited, indicating that some methanogenic microorganisms were physiologically active even in the presence of gamma-Al2O3 NP during the incubation period (similar to 400 h). SMA and HRA of the anaerobic consortium were also conducted in the presence of gamma-Al2O3 NP coated with humic acids (HA). Microbial HA reduction occurred 3.7-fold faster using HA immobilized on gamma-Al2O3 NP (HA(Imm)), compared with the control with suspended HA (HA(Sus)). Furthermore, immobilized HA decreased the toxicological effects of gamma-Al2O3 NP on methanogenesis. Scanning electron microscopy (SEM) images revealed cell membrane damage in those sludge incubations exposed to uncoated gamma-Al2O3 NP. In contrast, cell damage was not observed in incubations with HA-coated gamma-Al2O3 NP. Methanogenesis out-competed microbial humus reduction regardless if HA was HA(Imm) or HA(Sus). The present study provides a clear demonstration that HA immobilized in gamma-Al2O3 NP are effective terminal electron acceptor for microbial respiration and suggests that HA could mitigate the toxicological effects of metal oxide NP on anaerobic microorganisms.