The potential of winter biofilm for microcystin-LR (MCLR) biodegradation was comparatively evaluated under various nutrient conditions. Results indicated that MCLR was completely biodegraded by Day 7 without nutrient addition. MCLR-biodegradation was inhibited in the presence of phosphate or glucose addition, with complete MCLR removal observed by Day 10. MCLR was totally biodegraded by Day 7 with dual nutrients comprising glucose and nitrate, suggesting that additional nitrate alleviated the inhibitory effect of glucose alone on the biodegradation. Simultaneously, MCLR-degrading gene (mlrA) abundance were detected to increase with increasing amount of MCLR being degraded under the respective conditions, implying that MCLR-biodegradation depended on the population of indigenous MCLR-degrading bacteria (MCLRDB), which was related to the population of non-degrading bacteria in the biofilm. MCLRDB was found to primarily use MCLR for proliferation rather than other nutrients. This is the first report verifying MCLR as a primary substrate for bacteria under various nutrient conditions. (C) 2011 Elsevier Ltd. All rights reserved.