Aims: Identification of bacterium HYK0203-SK02 and its lysis of Stephanodiscus hantzschii. Methods and Results: In an effort to identify a bio-agent capable of controlling S. hantzschii blooms, we used the algal lawn method to identify 76 bacteria in relevant water samples. Of these, the seven isolate showed algicidal activity against S. hantzschii; isolate HYK0203-SK02 exhibited the strongest algicidal activity, and was used for further analysis. 16S rDNA sequencing of this isolate allowed us to identify HYK0203-SK02 as a strain of Pseudomonas putida (99·2%). Growth of S. hantzschii was strongly suppressed by bacteria in all growth phases, with the strongest algicidal activity noted against diatoms in the exponential stage (5–18 days). Host range assays revealed that isolate HYK0203-SK02 also strongly inhibited the growth of Microcystis aeruginosa, but stimulated growth of the diatom Cyclotella sp., which has a similar structure to that of S. hantzschii. Biochemical assays revealed that the algicidal substance seemed to be localized in the cytoplasmic membrane of this newly identified algicidal bacterium. Conclusion: The algicidal bacteria P. putida HYK0203-SK02 caused cell lysis and death of not only diatom S. hantzschii but also cyanobacteria M. aeruginosa, dramatically. Algicidal substance might be located at the compartment of cytoplasmic membrane. Significance and Impact of the Study: Taken together, our results indicate that P. putida HYK0203-SK02 may be a potential bio-agent for future use in controlling freshwater diatomic blooms.

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.

Effective sustainable algal cultivation techniques are essential for mass production of the marine microalga Tetraselmis for biofuel and array of co-products. The phycospheric communities affect the microalgal growth and metabolism through various allelochemical and nutrient interactions; hence, their potential to affect the quantity and quality of both biomass and bioproducts is significant. In the present study, we have screened the phycospheric communities of biofuel producing Tetraselmis striata (KCTC1432BP). A total of 26 bacterial strains were isolated and identified from the phycosphere of Tetraselmis striata mass culture. Then, each bacterial strain was tested in co-cultivation conditions with Tetraselmis striata for evaluating its growth promoting and inhibitory effects. Among these all strains, two promising strains (Pelagibaca bermudensis KCTC 13073BP and Stappia sp. KCTC 13072BP) were selected because of their maximum growth promoting effects and mutualistic interactions. The growth rate, biomass productivity, lipid contents and fatty acids were analyzed during their combined growth in O3 media and compared with axenic growth of Tetraselmis striata. Later, growth promoting mechanisms in the co-cultivation environment were investigated for these promising bacterial strains under replete and limited conditions of nutrients (nitrate, phosphate and vitamin B12). The growth promoting potential of P. bermudensis was illustrated by the two fold enhancement in biomass productivity. These bacteria are promising for microalgal cultivation without any negative effects on the native seawater bacterial communities, as revealed by NGS analysis. This study represents, to date, the first report highlighting the role of phycospheric growth promoting bacteria of promising biofuel feedstock Tetraselmis striata.