Competitiveness of alga Microcystis aeruginosa co-cultivated with cyanobacterium Raphidiopsis raciborskii confirms its dominating position

Microcystis aeruginosa has always been regarded as the main culprit of cyanobacterial blooms in freshwater. However, in recent years, Raphidiopsis raciborskii has gradually replaced M. aeruginosa as the culprit of cyanobacterial blooms in some tropical and subtropical shallow lakes. To reveal which one plays a more dominant role, interactions between cylindrospermospin (CYN)-producing R. raciborskii and microcystins (MCs)-producing or non-MCs-producing M. aeruginosa strains were studied using bialgal cultures at different initial ratios of biomasses of the two species at 25 °C. During the co-cultivation, the M. aeruginosa strains inhibited the growth and heterocyst formation of R. raciborskii filaments, and thus occupied a dominant position during the co-cultivation regardless of the initial biomass ratios in the cultures. In addition, the MCs-producing M. aeruginosa strain contributed to a higher portion of the total biomass and exerted a stronger inhibitory effect on R. raciborskii compared with the non-MCs-producing strain. However, the growth of both MCs-producing and non-MCs-producing M. aeruginosa strains was stimulated by R. raciborskii in the co-cultures compared with M. aeruginosa monoculture, indicating that M. aeruginosa could outcompete R. raciborskii if given enough time, enabling it to develop into the dominant species even in very low initial concentration. To our best knowledge, this is the first report on the loss of heterocyst formation by a species of cyanobacteria that resulted from interactions between two different species of cyanobacteria. These findings indicate that it is difficult for R. raciborskii to replace the dominant position of M. aeruginosa under the experimental environmental condition, and the allelopathic effects of M. aeruginosa on R. raciborskii could significantly contribute to the success of M. aeruginosa .