Aims:  The study of an algicidal activity and mechanism of the isolated Pseudomonas fluorescens HYK0210‐SK09 (SK09) against a winter bloomed harmful diatom, Stephanodiscus hantzschii. Methods and Results:  SK09 was isolated from the Paldang reservoir, Korea and used to biological control of S. hantzschii. The inoculation of SK09 at the final density of 5 × 106 cells ml−1 caused degradation of >90% of S. hantzschii cells within 5 days. The algal cell lysis was achieved by a direct attack of the bacteria to the diatom cells, and the algicidal compound was located in the cytoplasm of the cell. As SK09 did not suppress Microcystis aeruginosa, Anabaena cylindrica, Coelastrum astroideum or Cyclotella meneghiniana, it appeared to attack S. hantzschii in a species‐specific manner. Testing in an indoor mesocosms confirmed that SK09 effectively reduced S. hantzschii cells by 88% within 9 days. Conclusions:  This bacterium is useful in regulating blooms of S. hantzschii. However, it should be studied in the future that their impact in shaping phytoplankton community and their activity in natural environments. Significance and Impact of the Study:  The bacterium enabled us to develop a new strategy, to understand the interaction for anthropogenic control of harmful algal blooms in nature.

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Two different morphs of the small cryptomonad, Plagioselmis prolonga var. nordica with a posterior tail, were observed during summer and fall in the hypertrophic lake, Lake Kasumigaura, Japan. The tail shortened in mesocosms stocked with planktivorous silver carp ( Hypophthalmichthys molitrix Val.) and elongated by more than 50% in mesocosms from which silver carp were removed. The density of Plagioselmis cells increased significantly upon fish stocking and decreased upon fish removal. The tail length was negatively correlated with algal abundance and positively correlated with crustacean densities, but there was no correlation with nutrient levels or physical environmental parameters in the mesocosms. The variation in tail length was induced by the presence/absence of fish, but was not related to their density. However, silver carp manipulation strongly affected the density of the majority of zooplankters and, interestingly, there was a strong correlation between zooplankton density and tail-length change in Plagioselmis. We propose a possible herbivore-induced defense mechanism triggered by the top predator, silver carp.

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