Morpho-physiological responses of a subtropical strain of Cylindrospermopsis raciborskii (Cyanobacteria) to different light intensities

The toxigenic cyanobacterium Cylindrospermopsis raciborskii previously restricted to tropical latitudes, has been increasingly reported in temperate lakes in recent decades. The causes of its biogeographical expansion are under investigation, but efficient physiological adaptation to changes in temperature and light regimes are likely to be involved. The present study evaluated the morpho-physiological responses of a strain of C. raciborskii from southern Brazil to nine light intensities, from 9 to 250 µmol photons m-2 s-1. Blooms of this cyanobacterium are regularly recorded in the region. Morpho-physiological responses were measured based on growth rate and trichome length. Cylindrospermopsis raciborskii showed slow growth at low light intensities, 9 and 20 µmol photons m-2 s-1, and responded morphologically by increasing the length of trichomes. In turn, the strain displayed constant maximum growth rates at light intensities higher than 50 µmol photons m-2 s-1. These results support the hypothesis that C. raciborskii can survive under low light conditions and continue to produce viable trichomes. Moreover, the strain achieved high growth rates under a relatively wide range of light intensities, a physiological adaptation that can potentially be a competitive advantage in the phytoplankton community.