Cylindrospermopsis raciborskii and Microcystis aeruginosa competing under different conditions of pH and inorganic carbon

Based on the fact that the intense photosynthetic activity of blooms causes a marked decrease in free CO2 concentrations and increase in pH of the water, the aim of the present study was to investigate the competitive relationship between Cylindrospermopsis raciborskii and Microcystis aeruginosa under different pH and inorganic carbon conditions. Our hypothesis is that C. raciborskii is a better competitor than M. aeruginosa in alkaline pH and when bicarbonate (HCO3−) is the main source of carbon. Semi-continuous cultures were conducted in a factorial experiment with three different pH conditions (free [unbuffered], 6.8 and 8.2), with and without aeration, and with and without the addition of a bicarbonate source. Both species demonstrated a good performance under high pH, but the interaction between the factors determined significant differences in the competitive responses of the C. raciborskii and M. aeruginosa strains, with a change in dominance in the different scenarios. Whilst C. raciborskii was favoured by aeration, the addition of bicarbonate improved the growth of M. aeruginosa. The results suggest that affinity and efficiency in bicarbonate use may be a determinant of dominance and competitive success in potentially toxic cyanobacteria, such as the genus Microcystis.