Biochemical and Physiological Responses of Harmful Karenia mikimotoi to Algicidal Bacterium Paracoccus homiensis O-4

Abstract

Harmful algal blooms caused by <i>Karenia mikimotoi</i> frequently occur worldwide and severely threaten the marine environment. In this study, the biochemical and physiological responses of <i>K. mikimotoi</i> to the algicidal bacterium <i>Paracoccus homiensis</i> O-4 were investigated, and the effects on the levels of reactive oxygen species (ROS), malondialdehyde content, multiple antioxidant systems and metabolites, photosynthetic pigments, and photosynthetic index were examined. The cell-free supernatant in strain O-4 significantly inhibited <i>K. mikimotoi</i> cell growth. The bacterium caused the <i>K. mikimotoi</i> cells to activate their antioxidant defenses to mitigate ROS, and this effect was accompanied by the upregulation of intracellular antioxidant enzymes and non-enzyme systems. However, the overproduction of ROS induced lipid peroxidation and oxidative damage within <i>K. mikimotoi</i> cells, ultimately leading to algal death. In addition, the photosynthetic efficiency of the algal cells was significantly inhibited by O-4 and was accompanied by a reduction in photosynthetic pigments. This study indicates that O-4 inhibits <i>K. mikimotoi</i> through excessive oxidative stress and impaired photosynthesis. This research into the biochemical and physiological responses of <i>K. mikimotoi</i> to algicidal bacteria provides insights into the prophylaxis and control of harmful algal blooms <i>via</i> interactions between harmful algae and algicidal bacteria.