Algicidal bacteria in particle-associated form and in free-living form during a diatom bloom in the Seto Inland Sea, Japan

To analyze the interrelated dynamics of microalgae and algicidal bacteria (i.e. bacteria that kill and utilize microalgal cells) in a seawater environment, we investigated particle-associated and free-living algicidal bacteria during a diatom bloom in the Seto Inland Sea (Japan) in the summer of 2005. A conventional most probable number (MPN) protocol revealed that bacteria that are algicidal against the harmful microalga Chattonella antiqua (Raphidophyceae) increased in number towards the end of the diatom bloom, during which only a few cells of Chattonella species were observed. Relatively abundant heterotrophic bacteria were present during the bloom period, indicating that the bloom promoted the growth and activity of the ambient bacterial population, including algicidal bacteria. The algicidal abilities of 487 strains isolated from the particle-associated bacteria (PAB) fraction and 249 strains isolated from the free-living bacteria (FLB) fraction were tested against axenic cultures of 3 dinoflagellates and 3 raphidophycean flagellates. About half of the PAB isolates (231/487 strains) showed algicidal activity against 1 or more tested microalgae, while only 22% of the FLB isolates (55/249 strains) were algicidal. Eighty percent of algicidal bacteria from the PAB fraction could kill multiple species of tested microalgae, whereas most of the algicidal bacteria from the FLB fraction killed only a single species. These observations suggest that diatom blooms provide a habitat for algicidal bacteria with a wide prey range that potentially limit harmful algal blooms caused by dinoflagellates and raphidophycean flagellates.