On the control of Microcystis aeruginosa and Synechococccus species using an algicidal bacterium, Stenotrophomonas F6, and its algicidal compounds cyclo-(Gly-Pro) and hydroquinone

Cyanobacterial blooms occur annually in Lake Taihu, causing large ecological and economic losses. As part of efforts to enhance the strategies explored to control cyanobacterial blooms, an algicidal bacterium, F6, was isolated from Lake Taihu and then identified as a Stenotrophomonas species by 16S rDNA sequence analysis. The results showed that Stenotrophomonas F6 exhibited strong algicidal activity against dominant cyanobacterial bloom-forming species in Lake Taihu, mainly via excretion of extracellular algicidal compounds. The algicidal compounds were subsequently extracted with ethyl acetate, purified by high-performance liquid chromatography (HPLC) and identified as cyclo-(Gly-Pro) and hydroquinone by gas chromatography-mass spectrometry (GC-MS) and H-1 nuclear magnetic resonance (NMR) analysis. Cyclo-(Gly-Pro) exerted a strong algicidal effect against Microcystis aeruginosa 9110 with a 50 % effective concentration (EC50,24h) value of 5.9 mg L-1, but had no algicidal effect on Synechococcus sp. BN60. Hydroquinone showed stronger algicidal activity against both M. aeruginosa 9110 and Synechococcus sp. BN60, with EC50,24h values of 0.96 and 5.6 mg L-1, respectively. Through characterization of algicidal process, the algicidal activity of Stenotrophomonas F6 was found to be dependent on the concentration of algicidal compounds, and the secreted algicidal compounds could be detected only when the cell density of F6 was above a threshold of 2-4 x 10(8) CFU mL(-1). Taken together, these results suggest that the algicidal bacterium F6 and its two algicidal compounds have the potential for use as bio-agents for controlling cyanobacterial blooms in Lake Taihu.