Abstract
Microcystin-LR (MC-LR) and microcystin-RR (MC-RR) produced by harmful cyanobacterial blooms (HCBs) pose substantial threats to the ecosystem and public health due to their potential hepatotoxicity. Degradation of microcystins (MCs) by indigenous bacteria represents a promising method for removing MCs from fresh water without harming the aquatic environment, but only a few microcystin (MC)-degrading bacteria have been isolated and had their mechanisms reported. This study aimed to isolate indigenous bacteria from Lake Taihu, and investigate the capability and mechanism of MC degradation by these bacteria. During a Microcystis bloom, an indigenous MC-degrading bacterium designated MC-LTH2 was successfully isolated from Lake Taihu, and identified as Stenotrophomonas acidaminiphila based on phylogenetic analysis. In the presence of MC-LR together with MC-RR, the strain MC-LTH2 was capable of totally degrading both simultaneously in 8 days, at rates of 3.0 mg/(L⋅d) and 5.6 mg/(L⋅d), respectively. The degradation rates of MCs were dependent on temperature, pH, and initial MC concentration. Adda (3-amino-9-methoxy-2, 6, 8-trimethyl-10-phenyldeca-4, 6-dienoic acid) was detected as an intermediate degradation product of MCs using high performance liquid chromatography coupled with time-of-flight mass spectrometry (HPLC-TOF-MS). To the best of our knowledge, this is the first report of Stenotrophomonas acidaminiphila capable of degrading two MC analogues and other compounds containing Adda residue completely under various conditions, although the mlrA gene in the strain was not detected. These results indicate the Stenotrophomonas acidaminiphila strain MC-LTH2 possesses a significant potential to be used in bioremediation of water bodies contaminated by MC-LR and MC-RR, and is potentially involved in the degradation of MCs during the disappearance of the HCBs in Lake Taihu.
Highlights
Harmful cyanobacterial blooms (HCBs) have occurred frequently in eutrophic lakes all over the world in the past two decades, producing a series of cyanotoxins, such as microcystins (MCs), nodularins and saxitoxins [1,2]
An indigenous bacterium capable of degrading MC-LR and MC-RR simultaneously was isolated from Lake Taihu during a Microcystis bloom, and the capability and mechanism of MC degradation by this strain were investigated
Simultaneous degradation of MC-LR and MC-RR by S. acidaminiphila has not been reported before, MC degradation has been addressed for a few different bacterial strains including Sphingomonas sp. [3,27,28], Pseudomonas sp. [29], Arthrobacter, Brevibacterium and Rhodococcus sp. [30], Methylobacillus sp. [7,8], Sphingopyxis sp. [31,32], and Bacillus sp. [16]
Summary
Harmful cyanobacterial blooms (HCBs) have occurred frequently in eutrophic lakes all over the world in the past two decades, producing a series of cyanotoxins, such as microcystins (MCs), nodularins and saxitoxins [1,2]. More than 90 analogues of MCs have been identified until now [4,5]. They have the same genetic structure cyclo-(D-Ala-X-D-MeAsp-Z-AddaDGlu-Mdha-), where X and Z represent variable L-amino acids, and Adda refers to the b-amino acid residue of 3-amino-9methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid [6,7,8]. The World Health Organization (WHO) recommends that MC-LR concentration in drinking water should not exceed 1 mg/L [11]
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