Relationship between photosynthetic processes and microcystin in Microcystis aeruginosa grown under different photon irradiances

Microcystis aeruginosa is well known for its ability to produce the group of hepatotoxic microcystins (MCYST) but the mechanisms that control their synthesis are not clearly understood. Possible effect of light and nutrients on MCYST content was already proposed, but contradictory results were reported. In this study, we have grown M. aeruginosa (UTCC299) under a wide range of light conditions (24-820 μmol photons (PAR) m−2 s−1) and showed that MCYST content decreased (106 fg (SE ± 7) to 47 fg (SE ± 3) MCYST cell−1) with increasing photon irradiance. Our data showed that MCYST content was positively linked to Chl a content and inversely correlated to cell specific division rate. Chlorophyll fluorescence kinetics was used to assess photosynthetic activity and the relative electron transport rate (ETR). The latter was negatively correlated with MCYST content and was the best predictor of MCYST content compared to light and growth. Moreover, the redox state of the photosynthetic apparatus estimated by the unquenched fluorescence parameter (UQF(REL)) was also negatively correlated to MCYST content per cell. Therefore, our data demonstrated that photosynthetic light reactions, through electron transport rate (ETR) and PSII redox state, modulate MCYST content at the cellular level. Knowing that environmental conditions affect photosynthetic light reactions, our results indicated that future work should also pay closer attention to the involvement of photosynthesis in the modulation of MCYST.