Effects of Temperature on The UV-B Sensitivity of Toxic Cyanobacteria Microcystis aeruginosa CS558 and Anabaena circinalis CS537

Rising global temperatures have been suggested to favor cyanobacteria over eukaryotic algae, but UV-B fluxes are also predicted to remain high and may interact with temperature to affect algal growth. To understand the interactive effects of temperature and UV-B radiation, cultures of Microcystis aeruginosa and Anabaena circinalis were grown at either 25 or 30°C and then exposed to an acute irradiance of UV-B (1.4 W m⁻²). Both species showed differences in growth rates at both temperature regimes. The growth rates of M. aeruginosa (0.41 ± 0.02 day⁻¹) and A. circinalis (0.38 ± 0.01 day⁻¹) were higher at 25 and 30°C, respectively. Rates of damage (k) and repair (r) were calculated from the kinetics of change in effective quantum yield, F_v'/F_m'. Analysis of the estimates of r and k shows that M. aeruginosa exhibited relatively high values for both parameters, compared to A. circinalis, at both growth temperatures. In both species, repair rates were higher at 30°C than at 25°C but in A. circinalis damage was also greater at the higher temperature. In contrast, M. aeruginosa showed a lower damage rate at the higher temperature. For both species, the ratio of r:k was higher at the higher temperature. However, the percent inhibition of effective quantum yield by UV-B was greater in A. circinalis than in M. aeruginosa as the r:k was lower A. circinalis. Therefore, it could be concluded that temperature may influence growth and bloom formation of cyanobacteria and that different species may respond differently to UV-B and temperature interactions.