Identification of algicidal bacterium Sp37

Abstract:

[Background] Phytoplankton blooms are a worldwide problem that brings a serious threat to environment and human health. It is urgent to control bloom through effective methods. Algicidal bacteria have been considered as an effective strategy to control bloom because they can kill algae. [Objective] To isolate and identify Microcystis aeruginosa and its algicidal bacteria from Dianchi Lake, and further analyze the algicidal effect and inhibition mechanism of algicidal bacteria against M. aeruginosa. [Methods] Bacteria were isolated by dilution separation method and identified based on 16S rRNA genes. Algae were isolated by dilution separation and capillary separation methods, and identified based on cpcBA and 18S rRNA genes. Algicidal efficiency was calculated based on the chlorophyll-a concentration of algal culture, which was measured with heat-ethanol extraction method. Determination of CAT (catalase), GSH (glutathione) and MDA (malondialdehyde) were used to investigate the algal response of antioxidant system to the algicidal bacteria. [Results] In total 11 Microcystis and 17 algicidal bacteria strains were isolated, M. aeruginosa DCM4 and Bacillus siamensis Sp37 were selected for the further research. Sp37 showed a strong algicidal activity against M. aeruginosa DCM4 (Ae=92.4%±1.5%, t=4 d), and it also can kill M. flos-aquae and M. wesenbergii of Cyanophyta, but not Monoraphidium and Scenedesmus of Chlorophyta; there is no significant difference of algicidal ratio (P>0.05) between the culture and cell-free filtrate of Sp37 (86.8%±4.3% and 81.1%±2.2% (t=4 d), respectively), while that of washed cells were much lower (25.4%±7.3%); Sp37 cell-free filtrates that treated at different temperature and pH conditions displayed no obvious difference of algicidal effects; Cell-free filtrate of Sp37 caused great changes of CAT, GSH and MDA content of DCM4. [Conclusion] Sp37 had strong algicidal effect on M. aeruginosa DCM4, and it is likely can kill Microcystis selectively. Sp37 kills M. aeruginosa DCM4 by releasing algicidal substance(s) which has (have) good thermal and acid-base stability. Cell-free filtrate of Sp37 can activate antioxidant systems and destroy cell membrane of DCM4, and ultimately cause algal cell death.