Fig. 4: Algicidal mechanisms involved in oxidative stress.

a Some substances can enter the interior of algal cells through transport proteins, causing the chloroplasts to produce large amounts of ROS. b High concentrations of metals generate ROS through the Fenton and Haber-Weiss reactions. c With the onset of oxidative stress in algal cells, the antioxidant system is crucial, including antioxidant enzymes and antioxidants. The main antioxidant enzymes include superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD). Due to the dysfunction of the antioxidant system in dying cells, their activity usually increases in the early stages to remove accumulated ROS, then sharply declines in the later stages of the algicidal process. d ROS can lead to lipid peroxidation, which could inhibit photosynthesis by damaging the thylakoid membrane and/or oxidizing photosynthetic pigments (d1), damage the cell membrane and cause leakage of cellular contents (d2), and produce excessive malondialdehyde (MDA) that damages the membrane (d3). e ROS could damage essential biomolecules, such as DNA, RNA, and functional proteins. Red arrows indicate the damage effects to the corresponding cellular structures, whereas black arrows represent the inductive or promotive effects to the corresponding processes.