ABSTRACT: Phaeocystis globosa causes severe marine pollution by forming harmful algal blooms and releasing hemolytic toxins and is therefore harmful to marine ecosystems and aquaculture industries. In this study, Microbulbifer sp. YX04 exerted high algicidal activity against P. globosa by producing and secreting metabolites. The algicidal activity of the YX04 supernatant was stable after exposure to different temperatures (-80 to 100°C) and pH values (4 to 12) for 2 h, suggesting that algicidal substances could temporarily be stored under these temperature and pH value conditions. To explore the algicidal process and mechanism, morphological and structural changes, oxidative stress, photosynthesis, autophagic flux, and global gene expression were investigated. Biochemical analyses showed that the YX04 supernatant induced reactive oxygen species (ROS) overproduction, which caused lipid peroxidation and malondialdehyde (MDA) accumulation in P. globosa. Transmission electron microscopy (TEM) observation and the significant decrease in both maximum photochemical quantum yield (Fv/Fm) and relative electron transfer rate (rETR) indicated damage to thylakoid membranes and destruction of photosynthetic system function. Immunofluorescence, immunoblot, and TEM analyses indicated that cellular damage caused autophagosome formation and triggered large-scale autophagic flux in P. globosa. Transcriptome analysis revealed many P. globosa genes that were differentially expressed in response to YX04 stress, most of which were involved in photosynthesis, respiration, cytoskeleton, microtubule, and autophagosome formation and fusion processes, which may trigger autophagic cell death. In addition to P. globosa, the YX04 supernatant showed high algicidal activity against Thalassiosira pseudonana, Thalassiosira weissflogii, Skeletonema costatum, Heterosigma akashiwo, and Prorocentrum donghaiense. This study highlights multiple mechanisms underlying YX04 supernatant toxicity toward P. globosa and its potential for controlling the occurrence of harmful algal blooms. IMPORTANCE P. globosa is one of the most notorious harmful algal bloom (HAB)-causing species, which can secrete hemolytic toxins, frequently cause serious ecological pollution, and pose a health hazard to animals and humans. Hence, screening for bacteria with high algicidal activity against P. globosa and studies on the algicidal characteristics and mechanism will contribute to providing an ecofriendly microorganism-controlling agent for preventing the occurrence of algal blooms and reducing the harm of algal blooms to the environment. Our study first reported the algicidal characteristic and mechanism of Microbulbifer sp. YX04 against P. globosa and demonstrated that P. globosa shows different response mechanisms, including movement ability, antioxidative systems, photosynthetic systems, gene expression, and cell death mode, to adapt to the adverse environment when algicidal compounds are present.
