Anaerobic Biodegradation Of MCLR By An MC-degrading Bacterium CJ5

Microcystins(MCs) is a group of heptapeptide hepatotoxin produced by cyanobacteria, which are widely distributed in eutrophic water. Its potent hepatotoxicity and tumor promotion activity have posed risks to ecosystem and human health. It has been reported that biodegradation is the main pathway that leads to the decrease of MCs in eutrophic water environment, and biodegradation process is considered as a potential method to get rid of MCs contamination from water bodies. To date, about40MC-degrading bacteria have been isolated and identified from different environmental samples, and the mechanism of MCs degradation by pure isolate also have been investigated to some extent. However, most of studies on MCs biodegradation have been restricted to under aerobic conditions and all MC-degrading bacteria isolated from environmental samples are aerobic bacteria. Little information is available about the MCs anoxic or anaerobic degradation and its mechanisms.Recent studies confirmed that MCs can be degraded by the sediment samples under anoxic and anaerobic conditions, suggesting that anoxic or anaerobic degradation maybe an important pathway to remove MCs in natural environment. But until now no futher study on anaerobic degradation of MCs was reported. An anaerobic MCs-degrading baterium CJ5was isolated from the sediment of Lake Dianchi previously. It can degrade MCs effectively under anaerobic condition and was identified as Acidaminobacter sp. through the analysis of16S rDNA sequence. In present study, the anaerobic MCs degradation process by CJ5was further investigated. The effects of initial inoculum, temperature, pH value, extra carbon and nitrogen source and initial concentration of MCLR on the degradation of MCs were studied in the static experiments. The position of degradation enzyme which responsible for the MCLR degradation was investigated through enzymatic degradation experiments. PCR amplification was performed to testify whether strain CJ5contains mlrA gene. The main results are as followed:(1) Temperature and pH value have more significant influence on anaerobic degradation of MCLR. MCLR can be rapidly degraded at20℃-30℃, while the degradation of MCLR was slow at low temperatures (10℃,15℃), with a6days or8days lag time before degradation, respectively. MCLR was easily degraded under neutral and alkaline (pH=7.0-10.0) conditions and was degraded completely within12days without obvious lag phase. However, about20days lag time were needed prior to the onset of microcystin degradation under acid conditions (pH=4.0and5.0), and. MCLR was completely degraded untill day40. In addition, extra carbon or nitrogen sources have no significant effect on the degradation ability of CJ5.(2) The results of enzyme assay showed that the MCs-degradation enzymes belong to intracellular enzymes. A new kind of degradation product was detected during the degradation of MCLR by the intracellular enzyme of CJ5, which was different both from the final product Adda produced during the degradation of MCLR by CJ5strain and from the products produced during aerobic MCLR degradation. The fact that this product possess the characteristic UV absorption spectra (λmax=238nm) of Adda suggested that it contains the intact Adda group. The protease inhibitor experiments showed that this product might be the initial product poduced during anaerobic degradation of MCLR by CJ5. These results indicated that the enzymatic degradation pathway of MCLR by CJ5was different from that by other aerobic MC-degraders.(3) No mlrA gene homologue was detected in CJ5, suggesting that CJ5might degrade MCs through a new pathway different from aerobic bacteria.