Competitive dominance of the cyanobacterium Microcystis aeruginosa in nutrient-rich culture conditions with special reference to dissolved inorganic carbon uptake

The aim of the present study was to determine what factors contribute to the competitive advantage of cyanobacteria in eutrophic conditions. Mixed species batch culture experiments were conducted at three pHs (8.2, 8.8, 10.2) and irradiances (30, 90, 180 μmol photons m^?2 s^_1) between Microcystis aeruginosa Kützing and Staurastrum dorsidentiferum W. et West or Synedra ulna (Nitzsch) Ehrenberg, which always resulted in the dominance of M. aeruginosa. The final yields of competitors were often significantly lower than when cultured singly. The dominance of the surface‐loving M. aeruginosa appears to be related to its advantageous C0₂ uptake. To clarify the importance of dissolved inorganic carbon (DIC) as a selective factor, the effect of aeration was examined with M. aeruginosa. The growth of M. aeruginosa quickly stopped in nonaerated conditions, while it continued growing throughout the culture when aerated. This result indicates that DIC limitation easily occurs in static conditions and the viewpoint of C0₂ exchange efficiency would be helpful in discussing the competitive advantage of M. aeruginosa. When the three species were cultured at various ratios of surface area to volume (s/v), postulating it as an index of the gas exchange efficiency, the increases in population densities strongly correlated with s/v, and the relationships between the specific growth rate and s/v corresponded well to a Monod type saturation function. We found that M. aeruginosa had the lowest half‐saturation constant among the three, reflecting its high affinity for DIC.