To better understand the circadian clock in modern cyanobacteria, a Japanese research team has investigated ancient time-keeping systems. They studied the oscillation of the clock proteins KaiA, KaiB, and KaiC (Kai proteins) in modern cyanobacteria and compared it with the function of the Kai proteins of their ancestors. Their research findings were published in Nature Communications.

Better Understanding of the Physiological Origin of Circadian Clock Systems

“Today’s cyanobacteria use a circadian clock to predict the Earth’s light-dark cycle based on its rotation, thereby achieving efficient photosynthetic reactions. We wanted to know when ancient bacteria developed the circadian clock and how this trait was passed on to today’s cyanobacteria,” explains Atsushi Mukaiyama, associate professor at Fukui Prefectural University.

Cyanobacteria, sometimes called blue-green algae, are photosynthetic bacteria that have an important influence on the Earth’s oceans and atmosphere. Scientists know that the last common ancestor of cyanobacteria emerged around 3 billion years ago. It evolved into today’s ecosystem during the Great Oxidation Event, which took place around 2.3 billion years ago when the oxygen content in the Earth’s atmosphere increased. This development continued during at least two snowball Earth events about 2.4 and 0.7 billion years ago, when the planet was covered in ice, as well as during the oxygen enrichment in the Neoproterozoic era, when the Earth’s oxygen content rose a second time. The oxygen enrichment in the Neoproterozoic era took place between 800 and 540 million years ago.

Based on fossils and molecular evolutionary models, scientists suspect that the most recent common ancestor of cyanobacteria already possessed primitive oxygen photosynthesis systems. The efficiency of photosynthesis is strongly influenced by light-dark cycles in the environment. The research team wanted to investigate whether primitive cyanobacteria had a time system when photosynthesis became active during the Great Oxidation Event. This could help scientists understand the physiological origin of circadian clock systems.

The Circadian Clock of Cyanobacteria

Scientists have identified circadian clocks, i.e., internal timers that cause an organism to function according to a 24-hour rhythm, in various organisms such as bacteria, fungi, plants, and mammals. The research team studied the circadian clock of cyanobacteria using the cyanobacterial strain Synechococcus elongatus. They reconstructed the clock oscillator in a test tube using the clock protein KaiC. They also studied the function and structure of the original Kai proteins to determine how self-sustaining Kai protein oscillators evolved over time.

Since it is known that light-dark cycles influence the efficiency of photosynthesis in cyanobacteria, the team wanted to find out whether ancient cyanobacteria already had a self-sustaining circadian clock when the ancient oxidation processes took place and photosynthetic systems first emerged. They discovered that faster rhythmic phenomena were encoded in the proteins of the primordial clock. “The clock of ancient cyanobacteria was synchronized to a cycle of 18 to 20 hours. This means that the history of the Earth’s rotation period could be reconstructed by tracing the evolution of clock protein molecules,” explained Yoshihiko Furuike, assistant professor at the Institute of Molecular Sciences.

Faster Evolution

The team’s findings show that the oldest KaiC in the ancestral bacteria did not have the function and structure necessary for rhythmic properties. Through molecular evolution, the Kai proteins of the ancestors acquired the necessary function and structure around the time of global oxidation and the snowball Earth. Finally, the most recent common ancestor of photosynthetic cyanobacteria inherited this self-sustaining circadian oscillator. These findings are extremely helpful to scientists in understanding chronobiology. “Our ultimate goal is to develop modified cyanobacteria that can adapt to the rotation periods of planets and satellites other than Earth by shortening or lengthening the period of the Kai protein oscillator. Cyanobacteria took a long time to synchronize their clock to 24 hours, but with modern knowledge and technology, we could achieve even faster evolution,” said Shuji Akiyama, professor at the Institute of Molecular Science.