Unusual structures in bacteria suggest photosynthesis older than thought

Scientists have discovered unusual structures in rare bacteria that resemble the cellular components that power photosynthesis. The discovery, described this week in the journal Trends in Plant Science, suggests photosynthesis has ancient evolutionary roots.

Plants, algae and some bacteria perform what's known as oxygenic photosynthesis, splitting water into oxygen and hydrogen to power the process that turns solar energy into food. Some bacteria use anoxygenic photosynthesis, splitting other molecules besides water.

Anoxygenic photosynthesis has long been assumed to be the more primitive of the two. Anoxygenic photosynthesis, most scientists agree, emerged 3.5 billion years ago. Oxygenic photosynthesis came a billion years later.

But analysis of rare, ancient bacteria revealed components similar to those that power oxygenic photosynthesis, suggesting oxygenic photosynthesis may have been possible as early as 4.5 billion years ago.

"We're beginning to see that much of the established story about the evolution of photosynthesis is not supported by the real data we obtain about the structure and functioning of early bacterial photosynthesis systems," Tanai Cardona, life scientist at Imperial College London, said in a news release.

Scientists found the unique structures while studying Heliobacterium modesticaldum, an ancient bacteria species that lives in soils, hot springs and waterlogged fields. H. modesticaldum uses anoxygenic photosynthesis, but it is distantly related to cyanobacteria, today's primary performer of oxygenic photosynthesis.

Because the last common ancestor shared by the two types of bacteria can be traced to billions of years ago, scientists can be certain that any traits shared by the two lineages have been around for billions of years.

While analyzing photosynthesis-related structures in both cyanobacteria and H. modesticaldum, researchers found similarities between the structures used to power different types of photosynthesis. Scientists found components inside H. modesticaldum that look like and are positioned in the same place as structures exclusively used by cyanobacteria and plants to split water.

"This result helps explain in fantastic detail why the systems responsible for photosynthesis and oxygen production are the way they are today -- but for it to make sense it requires a change of perspective in the way we view the evolution of photosynthesis," Cardona said. "Under the traditional view -- that anoxygenic photosynthesis evolved first and was the only type for about a billion years or more before oxygenic photosynthesis evolved -- these structures should not exist at all in this type of bacteria."

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