Characteristics of a Microcystin-Degrading Bacterium under Alkaline Environmental Conditions

Abstract

The pH of the water associated with toxic blooms of cyanobacteria is typically in the alkaline range; however, previously only microcystin-degrading bacteria growing in neutral pH conditions have been isolated. Therefore, we sought to isolate and characterize an alkali-tolerant microcystin-degrading bacterium from a water bloom using microcystin-LR. Analysis of the 16S rRNA gene sequence revealed that the isolated bacterium belonged to the genus Sphingopyxis, and the strain was named C-1. Sphingopyxis sp. C-1 can grow; at pH 11.0; however, the optimum pH for growth was pH 7.0. The microcystin degradation activity of the bacterium was the greatest between pH 6.52 and pH 8.45 but was also detected at pH 10.0. The mlrA homolog encoding the microcystin-degrading enzyme in the C-1 strain was conserved. We concluded that alkali-tolerant microcystin-degrading bacterium played a key role in triggering the rapid degradation of microcystin, leading to the disappearance of toxic water blooms in aquatic environments.

Figure 1

Neighbor-joining phylogenetic tree based on 16S rRNA gene sequences indicating the position of isolate C-1 (in bold type) in relation to other related species, including previously isolated MC-degrading bacteria (underlined). Numerical values represent bootstrap support. Scale bar represents expected changes per site.

Figure 2

Effect of pH on growth of MC-degrading bacteria. (a) Sphingopyxis sp. strain C-1, (b) Novosphingobium sp. strain MD-1. Values presented are the means of 3 replicates. ■: pH 7.00, ▲: pH 8.00, □: pH 9.00, ∆: pH, 10.0, ×: pH11.0.

Figure 3

Effect of pH on microcystin-LR degradation activity of Sphingopyxis sp. strain C-1. MC degradation activity was determined as the MCLR concentration at 0 hour and 3 hours. The error bars indicate standard deviation (n = 3).

Figure 4

Mass spectrum of the primary degradation product of Sphingopyxis sp. strain C-1. The raw data obtained using mass spectrometry was treated by MacQuan and the figure shows only the main peaks.

Figure 5

Structure of linearized microcystin-LR.