Photoinhibition and recovery after selective short-term exposure to solar radiation of five chlorophyll c-containing marine microalgae

Abstract

The differential sensitivity of five chlorophyll c-containing marine microalgae to different components of solar radiation, e.g. photosynthetic active radiation (PAR), ultraviolet-A (UV-A) and ultraviolet-B (UV-B), was investigated in an exclusion experiment involving exposure to PAR, PAR+UV-A and PAR+UV-A+UV-B (P-, PA- and PAB-treatment, respectively) for 20 min and subsequent recovery for up to 24 h in dim light. The decrease in the variable to maximal chlorophyll fluorescence ratio (Fv/Fm) was used as indicator of photoinhibition. Changes in photosynthetic oxygen production, cell densities and pigment contents were also ascertained. The ratio Fv/Fm decreased in all the algae after exposure, but differences were found between the algae and the treatments. In relation to the inhibition extent, Chaetoceros sp. was the least affected alga in each treatment, while Phaeodactylum tricornutum was the most sensitive. Data of Fv/Fm during recovery fitted well to a sigmoid exponential function, and calculated constants were used to quantify the particular recovery rate of each alga. In general, recovery time did not show a direct relationship with the extent of inhibition. The highest recovery rate was shown by P. tricornutum cells exposed to only PAR, and the lowest by Isochrysis galbana cells exposed to PAB. There were no losses of cell density at the end of the recovery period in relation to the initial cell density in any of the algae. Photosynthetic oxygen production dropped in the five algae in all treatments and showed a similar evolution pattern to Fv/Fm during recovery, except for P. tricornutum and Amphidinium sp. Results of this study point out that capacity for photosynthesis inhibition is specific for each alga and, consequently, it should be taken into consideration for a reliable assessment of differential sensitivity to ultraviolet radiation among diverse marine phytoplanktonic species.