Isolation and identification of a native bacterium <i>Citrobacter farmeri</i> against microcystin-LR in anaerobic environments

Microcystin-LR (MC-LR), produced by cyanobacterial harmful algal blooms, poses a serious threat to aquatic ecosystems and human health. Biodegradation is an important method for MC-LR elimination. Aerobic biodegradation has been extensively studied and many bacteria were identified. However, few MC-degrading bacteria have been isolated from anaerobic environments, and these degradation mechanisms are poorly understood. The aim of this study was to collect anaerobic MC-degrading bacteria from lake sediments in Lake Taihu using acclimation culture to assess biodegradation. Five strains with MC-LR degradation ability were isolated, with strain A4 belonging to <i>Citrobacter farmeri</i> (<i>C. farmeri</i>). exhibiting the highest efficiency at 0.486 μg/ml/d. High-performance liquid chromatography (HPLC) identified two novel MC-LR degradation products. Further polymerase chain reaction (PCR) analysis suggested that <i>C. farmeri</i> A4 did not possess the known MC-degrading gene <i>mlrABCD</i>, suggesting the involvement of an <i>mlrABCD</i>-independent anaerobic degradation pathway. Data demonstrated that the bacterial strain <i>C. farmeri</i> A4 found in Lake Taihu exhibited high anaerobic MC-LR degradation properties, which indicated that anaerobic biodegradation may constitute an important biological method for MC-LR removal in natural environment.