Abstract:
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 (103 cells ml–1) together with
Microcystis species, the bacterium proliferated to 108 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.
