Physiological response of the toxic and non-toxic strains of a bloom-forming cyanobacterium Microcystis aeruginosa to changing ultraviolet radiation regimes.

Microcystis aeruginosa, a common bloom-forming cyanobacterium with both non-toxic and toxic strains, experiences a variable light environment due to buoyancy regulation and the variable mixing of the water columns. However, little is known on how non-toxic and toxic strains respond to changing photosynthetically active radiation (PAR) and ultraviolet radiation (UVR). Here, the non-toxic and toxic strains of M. aeruginosa were exposed to simulated solar radiation for 6 h, and their physiological changes were investigated at different irradiance levels of UVR (295-400 nm) in combination with PAR (400-700 nm). Our results showed that UVR at each level resulted in a larger inhibition on the maximum photochemical yield of Photosystem II (PSII) in the toxic strain. The non-toxic strain showed a quicker rise in the non-photochemical quenching when PAR + UVR were below 40.8 + 5.0 W m−2 and higher exopolysaccharide content at each radiation level, while the toxic strain exhibited stronger recovery capacity and superoxide dismutase (SOD) activity compared with the non-toxic strain, particularly for cells treated in the presence of UVR. In addition, UVR induced much higher content of microcystin in the toxic strain with the increase of irradiance levels, but decreased it when UVR was higher than 7.3 W m−2. Although UVR led to growth inhibition in both strains, the toxic strain showed much higher specific growth rate under UVR in comparison with the non-toxic strain. Our results indicate that the toxic strain has a competitive advantage relative to the non-toxic strain in a changing light environment with increase of UVR and PAR via stronger antioxidant capacity (higher SOD activity and the synthesis of microcystin) and quicker PSII recovery capacity. [ABSTRACT FROM AUTHOR]