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Everything Is Linked To The Rotation of Earth, Day and Night Cycles


Researchers at USC were surprised recently to discover just how much the rising and setting of the sun drives life on Earth – even in unexpected places.

Their findings, which appear this month in the Proceedings of the National Academy of Sciences, “speak volumes to the evolution of life on Earth,” according to USC scientist Andrew Y. Gracey.

“Everything is tied to the rotation of the planet,” he said.

Human as well as most living organisms on earth possess circadian a (24-hour) life rhythm. This rhythm is generated from an internal clock that is located in the brain and regulates many bodily functions, including the sleep-wake cycle and eating.

Although the evidence for their existence is obvious and they have been studied for more than 150 years, only recently the mechanisms that generate these rhythms have begun to be unraveled.

Recent findings suggest that circadian rhythms are intricately tied to weight gain. When Carla Green and her collaborators at the University of Virginia fed high-fat diets to mice with and without a protein involved in circadian rhythms, they found that the mice lacking the protein gained only a modest amount while the mice with the protein nearly doubled their body weight. The findings suggest that circadian rhythms regulate metabolic processes involved in diet-induced weight gain.

In all organisms, a certain amount of gene expression (the process by which products are created from the blueprint contained in genes) is rhythmic.

Mussels – which Gracey chose to study – instead spend their entire lives in dark shells in an area between the land and the sea, submerged or exposed depending on the tide.

Most of their physical activity is based on the tidal cycle; when mussels are exposed to the air, they close their shells and switch to an anaerobic metabolism, starving for oxygen, and when submerged they breathe and feed.

“It’s a really profound change in their biology as they go from sea to land,” said Gracey, assistant professor of biological sciences at the USC Dornsife College of Letters, Arts and Sciences.

One would expect, then, that the tidal cycle would be the clock that drives their gene expression. But in fact, as Gracey’s tests discovered, while a “tidal clock” probably does exist for mussels, the lion’s share of their gene expression is instead driven by the circadian cycle.

“The circadian cycle is trumping the tidal cycle,” Gracey said.

Gracey and USC graduate student Kwasi Connor constructed an aquarium with an artificial tide by pumping water in and out every six hours. For four days straight, Connor collected samples every two hours – getting excellent data, but not much sleep.

“That’s why this paper is so good; we have such a high resolution,” Gracey said.

Connor shrugged off the sacrifice in the name of science: “It’s critical that you get up and do measurements in a precise manner, otherwise you lose the value of the data,” he said.

Later, Gracey and Connor ran a similar simulation in a more natural environment by suspending cages of mussels off of a dock, this time for 50 hours.

The results were unexpected. Of the genes that showed rhythmic expression, between 80 and 90 percent were driven by the circadian cycle.


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