Isolation Of A Microcystin-degrading Bacterium From Sediment Of Taihu Lake And The Biodegradation Of Microcystin

The removal of microcystin in natural water is mainly caused by biodegradation.At present, numerous studies have focused on the microorganasims which are able tobiodegrade MC. However, the species of microcystin-degrading bacteria which havebeen reported are relatively few. Only one MC degradation pathway was reported. Inorder to futher study the biodegradation pathways for MCLR, we need to isolate moreMC-degrading bacteria.In this paper, the MC degradation bacterium was isolated from Taihu Lakesediment using MCLR as the sole carbon and nitrogen source. The strain wasidentified based on morphological and16S rDNA Gene sequence. The degradationefficiency of MC by this strain was investigated. Furthermore, the influence ofenvironmental factors, such as temperature、pH value, and nutrition sources on thisdegradation process were studied. At last, we preliminary deduced the MCLRdegradation pathway by studying the degradation products and detecting thedegrading gene cluster. The specific research contents and results are as follows:(1)The microcystins degradation bacterium was isolated from Taihu lakesediment. The strain TH is a gram-negative bacteria. The colony morphology of strainTH in inorganic medium is white, circular, edge tidy, with smooth surface. The strainTH is rod-shaped, bacteria cells were1~3μm in length and0.3~0.5μm in width.16S rDNA sequence homology analysis result show that the bacteria TH was close tothe Rhizobium sp.W29bacteria (KF560402). The phylogenetic tree indicate that THwas belongs to the Rhizobium genera under α-proteobacteria.(2)When incubated at30℃and pH=7, the strain TH can degrading97%MCLRin12h.Temperature influenced the degradation of MCLR, strain TH show thehighest efficiency of MC degradation when incubated at30℃and35℃, which cancompletely degraded3μg/ml MCLR in12h. However, when TH incubated at20℃,the efficiency of MC degradation at the later period is slow. The results of influenceby pH value confirm that strain TH show the best activity at pH value of8and9,while the degradation rate were slow at other pH values. The degradation reaction rateof the strain TH which added carbon sources was0.22h-1. While the degradationreaction rate of the control was0.15h-1. The result confirms that it will promote theefficiency of MCLR degradation and shorten the lag phase when added glucose ascarbon sources and added sodium nitrate as nitrogen sources. (3)Two kinds of products were detected when the strain TH degraded MCLR.The UV spectra of biodegradation products are very similar to that of MCLR. Theλmaxof biodegradation products and MCLR are all238nm. The products weretetrapeptide (NH2-Adda-Glu-Mdha-Ala-OH) and Adda respectively. There was nofinal product during the degrading. MCLR (4μg/ml) were completely degraded bythe intracellular enzyme of strain TH in8h. The products detected during the enzymedegradation were the same as the products of strain degradation. However the finalproduct of enzyme degradation was Adda. Furthermore, the homologous genes of themlrB, mlrC and mlrD previously associated with the degradation of MCLR werefound in strain TH. These findings suggest that strain TH might degrade MCLRthrough the same pathway from that have been reported.