In situ bacterial mitigation of the toxic cyanobacterium Microcystis aeruginosa: implications for biological bloom control

The algicidal bacterium Xanthobacter autotrophicus HYS0201-SM02 (SM02) was isolated from the surface water of a eutrophic lake (Lake Daechung, Korea). In vivo and in situ experiments showed that SM02 had algicidal activity against both a cultured strain and natural colonial morphs of the toxic cyanobacterium, Microcystis aeruginosa. Both the SM02 bacteria and its culture filtrate showed anti-algal activity against M. aeruginosa, indicating that an algicidal substance was released from SM02. The threshold concentration of SM02 for maximal algicidal activity against a natural bloom of M. aeruginosa was 10(7) CFU/mL. In situ co-culture of SM02 and M. aeruginosa showed that SM02 did not benefit from the massive decay of M. aeruginosa. In fact, repeated inoculations with a low concentration of SM02 were required for optimal algicidal activity, suggesting that water quality worsened during co-culture (i. e., nutrients and microcystin-LR concentration increased). These results suggest a role for the algicidal bacterium X. autotrophicus SM02 in biorestoration but probably not in treating outdoor Microcystis blooms. When developing a biological agent to control M. aeruginosa blooms in the field, it will be important to screen for specific agents with low threshold concentrations and high algicidal activity.