When the sun goes down, our bodies start to go into sleep mode, and when the sun rises again, we normally wake up refreshed and ready for a new day. This process is regulated by the body's circadian clock. But this ‘clock’ doesn't just tell us when to wake up or when to go to bed; it also tells the cell's powerhouses – your mitochondria – when to increase their activity. This, in turn, gives you more energy when you need it and less energy when you don't; like when you're asleep. However, the mitochondria can only make a lot of energy if they have enough oxygen to do so. This led Chiara Ciccone and a group of researchers from the Arctic University of Norway to ask whether the mitochondria in deep-diving animals, such as the hooded seal (Cystophora cristata), also have this daily rhythm even though they experience long bouts without oxygen when they dive during the day.
First, the team had to make the harrowing journey from Norway to Greenland aboard a ship during stormy seas and return with the seals in tow. After returning to Norway, the team collected cells from the seals. Ciccone and colleagues were then able to culture the cells in an incubator and mimic a circadian rhythm by increasing the temperature in the incubator to 39.5°C for 12 h during the day and decreasing the temperature at night to 36.5°C. After measuring the levels of two genes associated with the circadian clock, arntl and per2, the researchers found that levels of these genes alternate every 12 h: one being high during the day and the other high at night. This happened even if the temperature of the incubator didn't change, suggesting that the seal's cells are showing a circadian rhythm. But could these genes also be affecting how well the mitochondria generate energy?
So, the team measured how much oxygen the mitochondria of the cells were using. Focusing on time points when either arntl or per2 had high expression levels, the researchers found that the first of five complexes that form a chain reaction, generating energy within the mitochondria, was more efficient during the day when arntl levels were low. This means that the mitochondria can turn oxygen into energy more efficiently during the day. It seems that the circadian rhythm of the mitochondria does indeed help the seals just when they need it most, during deep dives and while foraging for food.
