An Insight into Algicidal Mechanism of Bacillus Altitudinis G3 from Dysfunctional Photosystem and Overproduction of Reactive Oxygen Species

Cyanobacterial blooms frequently occur in eutrophic waters and negatively affect aquatic ecosystems and human health. Algicidal bacteria can efficiently control the growth of bloom-causing algae. Bacillus altitudinis G3, which was isolated from the bloom area of Dianchi Lake, has high algicidal activity against Microcystis aeruginosa. In this study, we investigated algicidal characteristics including attack mode, and algicidal mechanisms based on algicidal substance characteristics, photosynthesis responses, source, and the contribution of reactive oxygen species (ROS). The results showed that G3 efficiently and specifically killed M. aeruginosa by releasing algicidal substances, and the <1000 Da and >1000 Da fractions were partially thermolabile and thermostable, respectively. The G3 filtrate from the mid-logarithmic growth phase exhibited the highest algicidal activity. The algicidal ratio under full-light conditions was significantly higher than that under dark conditions (P < 0.001). The G3 filtrate caused photosystem dysfunction by significantly decreasing photosynthetic efficiency (Fv/Fm and PIABS) and inhibiting photosynthetic electron transfer (rETR), especially in QA- downstream (φEo, Mo, and ψo). This indicated that the algicidal activity of the G3 filtrate relied on light, and the algal photosystem is an important attack target. ROS and malondialdehyde contents were significantly increased in case of algae exposed to the G3 filtrate. The ROS decreased in the presence of diuron (3-(3-4-dichlorophenyl)-1, 1-dimethylurea) which blocked photosynthetic electron transport from QA to QB. This indicates that excessive ROS were produced from disrupted photosynthesis, especially the inhibited electron transport area in QA- downstream, and caused severe lipid peroxidation and oxidative stress in algae. The ROS scavenger N-acetyl-L-cysteine significantly decreased algal ROS and algicidal ratio (P < 0.05). This indicated that the excessive ROS attributed to the G3 filtrate significantly contributed to the algicidal effect. Bacillus altitudinis G3 is expected to be an algicide that efficiently controls M. aeruginosa blooms in waters under suitable light conditions.