Aerobic biodegradation has been identified as the main attenuation mechanism for microcystin, but the role of anaerobic microcystin biodegradation remains unclear. To elucidate this process, we assessed the potential for anaerobic microcystin LR biodegradation by sediment microbial community from Dianchi Lake and evaluated the effects of environmental factors and additional nutrient sources on the rates of anaerobic biodegradation. The results showed that microcystin LR was rapidly degraded from 5 mg/L to below detection limit within 2 days, demonstrating that the indigenous microorganisms can efficiently degrade microcystin LR under anaerobic conditions and can use microcystin LR as a sole nitrogen source. The rates of anaerobic microcystin LR biodegradation increased with increasing incubation temperature within the experimental range of 15-30 degrees C. Anaerobic microcystin LR biodegradation was slower (pH = 5.0) or even ceased (pH = 3.0) at acidic pH, but there was no difference in the rates at neutral (pH = 7.0) and alkaline (pH 9.0, 11.0) conditions. The addition of glucose decreased pH of the culture by producing acidic compounds and therefore significantly inhibited the anaerobic biodegradation of microcystin LR, but with the addition of NO3-, this inhibition disappeared. NO3- amendment also retarded the biodegradation of microcystin LR, demonstrating that NO3- was not used as a terminal electron acceptor. These findings suggest that anaerobic biodegradation might be another main attenuation mechanism for microcystin LR in sediments and present a significant bioremediation potential.