The purpose of this study was to examine the effects of cadmium (Cd) on the daily pattern of energy metabolism and the redox status in Dreissena bugensis. Mussels were exposed to 50 and 100 µg/L CdCl2 for 7 days at 15 °C under constant aeration. At the end of the exposure period, the mussels were collected at different times of the day, ranging from 09:00 to 24:00 for metallothioneins (MT), lipid peroxidation (LPO), peroxidase, thioredoxin reductase (TrxR), NAD(P)H oxidase and NAD+ kinase activity assessments. The results revealed that all of the above followed a circadian rhythm in their activity/levels where MT, TrxR and LPO peaks occurred in the morning between 09:00 to 12:00 and the others peaked at 21:00. Exposure to Cd readily increased MT levels in mussels up to 6‑fold at 09:00, although the daily variation was quenched. LPO levels remained high at 18:00-21:00 compared with the controls and they lost their cycle. Frequency analysis of the oscillatory behavior of the above revealed the presence of circadian (24-h) frequency, with the exception of NAD+ kinase, which showed a 12-14 h frequency. The cyclic activity in TrxR, peroxidase, NAD+ kinase, MT and LPO were altered by Cd exposure while NAD(P)H oxidase and NAD(H)P levels were not affected by Cd. In conclusion, a daily pattern of biomarkers involved in mitochondrial activity and redox status was found in mussels, and exposure to contaminants such as Cd disrupted the oscillatory pattern in redox status during energy metabolism.
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