The mechanism of competition between two bloom-forming Microcystis species

1. Cyanobacteria, a phylum of bacteria that obtains energy by oxygenic photosynthesis, can be found in almost every terrestrial and aquatic habitat on Earth and play an important role in global carbon and oxygen cycles and as food sources for fish. However, common bloom-forming cyanobacteria in the genus Microcystis produce microcystins that are toxic to human beings and aquatic animals and can have disastrous effects in aquatic ecosystems. 2. Algal species follow seasonal successions of bloom formation and Microcystis spp. have been reported to be more competitive than other algal species in lakes. However, so far there has been no clear demonstration of competition among Microcystis species. 3. By means of growth experiments, we demonstrate that two common cyanobacterial species Microcystis aeruginosa FACHB-905 and Microcystis flos-aquae FACHB-1028, which are the dominant toxigenic species in Lake Taihu, exhibit a competitive relationship under coculture condition, the former outcompeting the latter. 4. We show that M. aeruginosa is a superior competitor to M. flos-aquae regardless of temperature, nutrients and initial abundance ratios. Moreover, both unicultured filtrates of M. aeruginosa and cocultured filtrates of both species strongly inhibit the growth of M. flos-aquae. 5. An analysis using gas chromatography mass spectrometry of extracts of filtrates from unicultures and cocultures indicates that some extracellular allelopathic compounds produced by M. aeruginosa, such as D-limonene and 1-chlorine heptacosane, might play important roles in competition among the species.