Algicidal activity of the bacterium, Pseudomonas fluorescens SK09, to mitigate Stephanodiscus hantzschii (Bacillariophyceae) blooms using field mesocosms

The main goal of this study was to characterize mortality and trophic interactions of Stephanodiscus hantzschii (small centric diatoms) in the presence of an algicidal bacterium, Pseudomonas fluorescens HYK0210-SK09 (SK09) in outdoor mesocosm experiments with an extremely low temperature and also in direct application in a river. SK09 at an inoculation density of 5 x 10(6) cells/mL (Experiment 1) and SK09 cells immobilized in activated carbon polyvinyl alcohol sponge carriers (Experiment 2) did not appear to attack S. hantzschii in a species-specific manner at temperatures close to freezing. Furthermore, SK09 did not control the diatom blooms when SK09 cells immobilized in the carriers were directly applied in the river. The dynamics of both the heterotrophic flagellate and ciliate abundances showed similar trends to the activity of algicidal bacteria; moreover the heterotrophic protists could not control the total bacterial density. Inorganic nutrient concentrations and conductivity remained unchanged compared with those in the control group. The results obtained in this study and previous results are important for assessing biological control using algicidal bacteria in the wild.