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.

Five strains (HYY0510-SK04, HYY0511-SK09, HYK0512-SK12, HYK0512-PK04 and HYY0512-PK05) of algicidal bacteria against the harmful bloom forming diatom Stephanodiscus hantzschii and dinoflagellate Peridinium bipes, were isolated. Among these strains, HYY0510-SK04, HYY0511-SK09 and HYK0512-SK12 have an effective algicidal activity for S. hantzschii, while HYK0512-PK04 and HYY0512-PK05 have an algicidal effect against P. bipes. Sequence analysis of 16S rDNA showed that HYY0510-SK04 and HYY0511-SK09 were closely related to Acidovorax delafieldii ATCC 17505 T . HYK0512-SK12, HYK0512-PK04 and HYY0512-PK05 showed high homology with Variovorax paradoxus IAM 12373 T (98.9%), Hydrogenophaga palleronii ATCC 49743 T (98.8%) and Pseudomonas plecoglossicida ATCC 700383 T (98.3%), respectively. HYY0510-SK04, HYY0511-SK09 and HYK0512-SK12 degraded S. hantzschii cells within two weeks when those bacteria were inoculated at densities of ≥10 7 cells mL −1 to the lag or logarithmic growth phase of the algal culture. HYK0512-PK04 and HYY0512-PK05 degraded more than 90% of P. bipes cells within 14 and 8 days, respectively, when these bacteria were inoculated at densities of ≥10 7 cells mL −1 . Among the five bacterial strains, HYK0512-SK12 and HYY0512-PK05 showed the most effective growth inhibition of all the algae and cyanobacteria tested. Biochemical assays revealed that the main algicidal substance from all isolates were likely to be extracellular substances. These results indicate that the bacterial strains isolated for this study are potential agents for the control of harmful algal blooms in eutrophic reservoirs.

The abundance, growth, and grazing loss rates of picophytoplankton were investigated in August 2002 in Barguzin Bay, Lake Baikal. Water samples for incubation were taken once at a near-shore station and twice at an offshore station. Contributions of picophytoplankton to total phytoplankton were high (56.9-83.9%) at the offshore station and low (5.8-6.8%) at the near-shore station. The picophytoplankton community in the offshore station comprised mainly phycoerythrin (PE)-rich cyanobacteria, with eukaryotic picophytoplankton being less abundant. In contrast, as well as PE-rich cyanobacteria and eukaryotic picophytoplankton, phycocyanin (PC)-rich cyanobacteria were found in the near-shore station. At the offshore station, growth and grazing loss rates on 25 August were 0.56 and 0.43 day )1 , respectively, and on 29 August, 0.69 and 0.83 day )1 , respectively. At the near-shore station, growth and grazing loss rates were 1.61 and 0.70 day )1 , respectively. These results show that there is a difference in the abundance, composition, and ecological role in the microbial food web of picophytoplankton between the near-shore and the offshore areas in Barguzin Bay.

Nutrient limitation of the primary production of phytoplankton at some stations in southern and central Lake Baikal was studied by nutrient enrichment experiments in August 2002. Chlorophyll (Chl.) a concentrations ranged from 0.7 to 5.8 µg l −1 . Inorganic nutrient concentrations were low: soluble reactive phosphorus ranged from 0.05 to 0.20 µmol l −1 , ammonia from 0.21 to 0.41 µmol l −1 , and nitrite plus nitrate from 0.33 to 0.37 µmol l −1 . In the five enrichment experiments, phosphate spikes and phosphate plus nitrate spikes always stimulated primary production. Nitrate spikes also stimulated primary production in four of the experiments. Significant differences were detected between the controls and phosphate spikes and between the controls and phosphate plus nitrate spikes. Thus, the first limiting nutrient is thought to be phosphorus, but once phosphorus is supplied to the surface water, the limiting nutrient will quickly shift from phosphorus to nitrogen.

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Uchiumi Bay experiences intermittent physical events of 'Kyucho ' and bottom intrusion.A Kyucho is an intrusion of warm surface water from the Kuroshio in the Pacific Ocean. Bottom intrusion, which contains a large amount of nitrates, phosphates, and silicates, slips through just above the continental shelf. We investigated seasonal changes in the abundance of Prochlorococcus, Synechococcus, and eukaryotic picophytoplankton while monitoring Kyucho and bottom intrusion from March to October 2002. Kyucho and bottom intrusion frequently occurred from June to September. Relatively high concentrations of nitrate + nitrite (> 0.8 µmol N l -1 ) and phosphate (> 0.1 µmol P l -1 ) were found when bottom intrusion occurred. The cell densities of Prochlorococcus were relatively high (>1 × 10 4 cells ml -1 ) when Kyucho occurred. Those of Synechococcus were high (2 to 30 × 10 4 cells ml -1 ) during the period of thermal stratification except in July, when bottom intrusion occurred. The cell densities of eukaryotic picophytoplankton were high (2 to 8 × 10 4 cells ml -1 ) in May and July. To examine the effects on picophytoplankton growth of the nutrients supplied by bottom intrusion, we conducted nutrient-enrichment experiments. The growth rates of Prochlorococcus and Synechococcus were not stimulated by the addition of any kinds of nutrients. The growth rates of Prochlorococcus were negative in most cases. In July, the growth rate of eukaryotic picophytoplankton was stimulated by nitrate and phosphate additions. Thus, Prochlorococcus detected in Uchiumi Bay might have been transported by Kyucho from the Pacific Ocean and could therefore not grow vigorously. Synechococcus may have been flushed out by bottom intrusion, and its growth was not limited by the nutrient concentrations. Eukaryotic picophytoplankton was abundant in spring, and its growth might have been limited by the nutrient concentrations in some cases. These results suggest that Kyucho and bottom intrusion have different effects on the abundance and growth rate of the 3 picophytoplankton groups.

The red tide of dinoflagellate, Cochlodinium polykrikoides has frequently occurred in coastal waters, causing severe damage to fisheries. In the present study, the algicidal bacterium Alteromonas sp. A14 isolated from the southern coast of Korea was applied to a red tide of C. polykrikoides in a laboratory experiment. In the experiment, the abundance of the strain A14 was monitored using fluorescence in situ hybridization. Inoculation of the A14 at a final cell density of 9.0 x 10(5) cells/ml caused a significant decrease in C. polykrikoides abundance from 1,830 to 700 cells/ml during 2 days, while abundances of harmless diatoms rapidly increased from 3 days. Abundances of both A14 and other bacteria increased to 1 day. After 1 day, with flagellate abundance increased, bacterial abundance decreased. Finally, algicidal bacterial abundance decreased to 3.5 x 10(4) cells/ml. In the biological control of harmful algal blooms, in addition to decrease in target algal abundance and not occurrence of other harmful blooms, decrease in abundance of utilized organism is also important. This study emphasizes the importance of monitoring the inoculated bacterium when applying bacterium to natural seawater.

Various measurements of microbial productivity in streambed pebble biofilms were analyzed almost monthly for 1 year to quantify the importance of primary production as an autochthonous source of organic matter utilized to support heterotrophic bacterial production in the dynamic food web within this natural microbial habitat. Bacterial density varied from 0.3x10(8) to 1.4x10(8) cells cm-2, and chlorophyll a concentration ranged from 0.7 to 25.9 microg cm-2, with no coupled oscillation between seasonal changes in these two parameters. In bottle incubation experiments, the instantaneous bacterial growth rate of bacteria was significantly correlated with their production rate [measured by frequency of dividing cells (FDC)] as follows: ln mu=0.138FDC-3.003 (n=15, r2=0.445, p<0.001). FDC values in the pebble biofilms increased with fluctuations during the study period, ranging from 3.6% to 9.2%. Bacterial production rates largely fluctuated between 0.15 to 0.92 microg C cm-2 h-1, and its seasonal pattern was similar to that of bacterial density. Net primary production measured between May 2002 to November 2002 attained minimum level (0.5 microg C cm-2 h-1) in June and maximum level (1.9 microg C cm-2 h-1) in August. Percentages of bacterial production to net primary production ranged between 21% and 120%. Because this ratio extends both below and above 100% for these parameters, it is likely that both autochthonous and allochthonous supplies of organic matter are important for production of bacteria in the pebble biofilms that develop in rapidly flowing fresh water streams.