Circadian rhythm regulation of the oxidation–antioxidant balance in Daphnia pulex

Rhythmic changes in reactive oxygen species (ROS) levels with time have been previously reported, but circadian rhythmic changes in ROS levels in Daphnia pulex remain unexplored. Here, in light–dark (L/D 16:8, L/D 8:16), continuous light (LL), and continuous dark (DD) conditions were set up to analyze the accumulation of ROS and activity of antioxidant enzymes in D. pulex at different time points. We observed that ROS levels significantly varied from day to night. By fluorescence probe labeling, we noted that ROS majorly accumulated in the thoracic limbs, brood chamber, and rectum in D. pulex. In addition, in the L/D 16:8 and 8:16 groups, ROS level was negatively correlated to the gene expression level of catalase, glutathione S-transferase, and glutathione peroxidase. In the LL and DD groups, food supply emerged as a zeitgeber, driving D. pulex to maintain a specific biological rhythm. The ability to scavenge hydroxyl radicals and activity of antioxidant enzymes were also affected by L/D cycles. To summarize, we indicate that different L/D cycles can induce different oxidation–antioxidant system responses in D. pulex and that Daphnia can adapt to the living environment by circadian rhythm regulation of the oxidation–antioxidant balance.