Algicidal bacteria that attack Dolichospermum crassum were isolated from the Karasuhara Reservoir in October 2010. Phytoplankton monitoring was performed from April 2010 to March 2011, and D. crassum was detected from August to November. At its peak frequency (in early October), it accounted for 23% of all phytoplankton cells. Heterotrophic bacteria were isolated from the surface water, and an algicidal assay was conducted. As a result, 3 out of 47 bacterial strains showed strong algicidal activity, and they completely destroyed the trichomes of D. crassum. An initial inoculation dose of only 1.0 × 102 cells ml−1 of these strains was enough to digest D. crassum. These strains were identified as Rheinheimera spp. according to 16S rDNA sequence analyses. This is the first report about algicidal bacteria that attack D. crassum. Algicidal bacteria could be key agents for controlling D. crassum in reservoirs.

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