Effects of dissolved inorganic carbon on competition of the bloom-forming cyanobacterium Microcystis aeruginosa with the green alga Chlamydomonas microsphaera

The availability of dissolved inorganic carbon (DIC) may have a considerable impact on species competition in phytoplankton communities. The growth, photosynthetic characteristics and competition of three strains, the toxic Microcystis aeruginosa FACHB 912, nontoxic M. aeruginosa FACHB 469 and Chlamydomonas microsphaera FACHB 52, were investigated under two different DIC concentrations (0.365 and 7.658mmol l 1 KHCO3). In monoculture, DIC concentration did not affect the specific growth rates of any of the three strains. However, when grown in mixed culture with C. microsphaera, both toxic and nontoxic Microcystis strains showed increased percentages under low DIC concentration but decreased percentages under high DIC concentration. After 12 days’ mixed culture, the percentage ofM. aeruginosa FACHB 912 or FACHB 469 decreased by 9 22% in high DIC medium, but increased by 6–11% in low DIC medium. Low DIC concentration decreased the cell size, cellular chlorophyll fluorescence and photosynthetic capacity of C. microsphaera, but it had little effect on those of M. aeruginosa FACHB 912 and FACHB 469. The apparent photosynthetic affinities for DIC were significantly higher in the two M. aeruginosa strains than in C. microsphaera. From the higher DIC affinity, more efficient photosynthetic capacity and increased percentages in competition experiments under low DIC concentration, it can be concluded that the bloom-forming M. aeruginosa possesses a competitive advantage in DIC-limited conditions.