Algicidal effect of the bacterium Alcaligenes denitrificans on Microcystis spp.

A freshwater gliding bacterium, Alcaligenes denitrificans, was isolated from a hypereutrophic pond in Japan. This bacterium caused cell lysis and death of some cyanobacterial species, but showed no algicidal effects on the species of chlorophyceae tested. Microcystis aeruginosa, M. viridis and M. wesenbergii were susceptible to the bacterial attack and the growth-inhibiting effect of the bacterium was significant on M. aeruginosa, particularly when the alga was in the exponential growth phase. When A. denitrificans was inoculated at low densities (10(3) cells ml(-1)) together with Microcystis species, the bacterium proliferated to 10(8) cells ml(-1) and caused algal cell lysis. M. aeruginosa died when A. denitrificans was added to the algal culture but not when only the filtrate from the bacterial culture was added. This suggests that extracellular products are not inhibitory to M. aeruginosa and that only direct contact between A. denitrificans and M. aeruginosa was lethal. Thus, we suggest that A. denitrificans plays an important role in influencing the growth of Microcystis spp. and contributes to the death of Microcystis spp. in freshwater environments.