Responses of Different Morphological Cells of Phaeocystis globosa to UV-B Radiation

Phaeocystis globosa is an important member of the plankton community and was considered to be a typical bloom-forming algae. Its life cycle is variable, comprising both solitary and colony cells. The growth process of P. globosa is vulnerable to UV-B radiation. However, the influence of UV-B on photosynthetic activity and the resulting programmed cell death (PCD) process is not entirely understood. Our findings demonstrated that Fv/Fm, rETRmax, Y(II) and α of solitary and colony cells were significantly decreased after UV-B treatment (p < 0.05). The colony cells showed a lower damage rate and higher repair rate than solitary cells (p < 0.05), suggesting that colony cells have better UV-B radiation resistance. After UV-B radiation, we found the characteristic markers of PCD-phosphatidylserine (PS) externalization and DNA fragmentation were discovered in the two cell morphologies, with increased caspase-3-like activity, proving the onset of PCD. In addition, the reactive oxygen species (ROS) content and antioxidant enzyme activities were examined. The results showed that, the ROS content went up, the solitary cells were significantly greater than colony cells under UV-B radiation (p < 0.001). In addition, the superoxide dismutase (SOD) and catalase (CAT) activities increased, and solitary cells always had significantly higher activity than colony cells (p < 0.05), but the changing trend in ROS content did not match the changes in CAT and SOD activities. This may have been due to the necrosis of solitary cells. The findings show that, besides PCD, solitary cells also developed necrosis under UV-B radiation. This study provides evidence that different morphological cells of marine microalgae present different reactions to UV-B radiation. It helps to further improve the knowledge of the environmental adaptation mechanism of P. globosa.