Characterization of cell viability in Phaeocystis globosa cultures exposed to marine algicidal bacteria

The bloom of Phaeocystis globosa has occurred frequently in the southern coastal areas of China in recent years, which has led to substantial economic losses. This study investigated the effects of culture broth of strains Y1 and Y4 isolated from algal blooms in Zhuhai, China on physiological characteristics and cell viability of P. globosa. The increase in the levels of reactive oxygen species (ROS) in P. globosa cells exposed to strains Y1 and Y4 culture broth were detected, indicating that the algal cells suffered from oxidative damage. The surplus ROS induced the increase of malondialdehyde (MDA) contents and the activities of antioxidant enzymes, including superoxide dismutase (SOD) and catalase (CAT). The decrease in protein content indicated that strains Y1 and Y4 culture broth inhibited cell growth. The contents of pigments decreased after 96 h treatment, indicated that oxidative stress destroyed pigment synthesis. Furthermore, flow cytometry coupled with the propidium iodide stain and chlorophyll auto-fluorescence was used to investigate cell viability. Results showed that chlorophyll fluorescence intensities and cell integrity decreased with time of exposure, which demonstrated that strains Y1 and Y4 culture broth could change membrane permeability and resulted in the loss of these photosynthetic pigments. The isolated strains were identified as Bacillus sp. by culture morphology, biochemical reactions, and homology research based on 16S rDNA. Overall, these findings suggested that oxidative stress caused from Bacillus sp. potentially destroyed pigment synthesis and cell membrane integrity, and ultimately led to the lysis of the algal cells