How microcystin‐degrading bacteria express microcystin degradation activity

Alkali tolerance and the mechanism of microcystin (MC) degradation were investigated in the MC‐degrading bacterial species, Sphingopyxis sp. C‐1, to better understand the increased MC degradation under the alkaline conditions that arise during the disappearance of water blooms. MC‐degrading bacteria harbour mlrA, mlrB and mlrC that encode MC‐degrading enzymes. Sphingopyxis sp. C‐1 also possesses these genes, as well as the mlrD gene that has been assumed to encode MC and its degradation transporter. This study demonstrated that MC degradation activity was promoted by the intermittent addition of microcystin‐LR (MCLR) to cultures of strain C‐1. That the expression of mlrA, mlrB and mlrC is induced by MCLR also was indicated, whereas that of mlrA and mlrB is induced by the MCLR degradation products linear MC, H‐Adda‐Glu‐Mdha‐Ala‐OH (tetra peptide) and 2S, 3S, 8S, 9S‐3‐amino‐9‐methoxy‐2, 6, 8‐trimethyl‐10‐phenyldeca‐4E, 6E‐dienoic acid (Adda). Adda played a key role in the induction of mlrA and mlrB gene expression, and the cyclic structure of MCLR was closely associated with the induction of mlrC gene expression. It is suggested, therefore, that Adda is an essential part of a signalling molecule involved in cell‐to‐cell communication. Finally, the MC‐degrading bacteria responded to MCLR and its degradation products by degrading the MlrA, MlrB and MlrC enzymes through a sequential chain reaction for the expression of each.