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Seasonal changes in densities of cyanophages infectious to Microcystis aeruginosa were studied in a hypereutrophic pond from March 1997 to January 1998 to elucidate the potential impact of the cyanophage onM. aeruginosa mortality. Densities of M. aeruginosa ranged between 1.8 x 104 and 9.4 x 105 cells ml-1, while those of the cyanophages were between 2.0 x 102 and4.2 x 104 PFU ml-1. Sharp decreases in densities of M. aeruginosa were detected on 10 June and 24 September, as densities of the cyanophages increased, suggesting release of the cyanophages due to the lysis of infected M. aeruginosa. Thus, infection by cyanophages may have a substantial effect on cyanobacterial succession in the pond. Densities of cyanophages became undetectable when those of M. aeruginosa were at low levels during winter. We suggest that there is a tight host-pathogen relationship between M. aeruginosa andthe cyanophage in the pond. [ABSTRACT FROM AUTHOR]

Concentration of dissolved DNA, microbial biomass, and consumption of bacteria by heterotrophic nanoflagellates (HNF) and ciliates were examined in a hypereutrophic pond for over 7 months to elucidate the main factors which influenced the release of dissolved DNA. Changes inconcentration of dissolved DNA correlated well with both abundance of ciliates ( r = 0.788, p < 0.01) and rotifers ( r} = 0.738, p < 0.01). A significant correlation was also found between dissolved DNA concentration and ciliate community ingestion rates ( r = 0.668, p <0.01). These results suggest that consumption of bacteria by ciliates is an important reason for the release of dissolved DNA. [ABSTRACT FROM AUTHOR]