Step into your new, microscopic time machine. Scientists at the University of Colorado Boulder have discovered that a single-celled organism that lives in today’s oceans may be very similar to life forms that existed since billions of years ago – and fundamentally changed the planet.
The new research, which will appear January 6 in the journal Advances in science, the latest to study the life of the hardest possible microbes in nature: cyanobacteria.
These single-celled, photosynthetic organisms, also known as “blue-green algae” are found in almost a large body of water today. But more than 2 billion years ago, they played an additional important role in the history of life on Earth: During a period known as the “Great Oxygen Event,” ancient cyanobacteria caused a sudden, and dramatic, rise in oxygen gas.
“We see this overall shift in ocean and atmospheric chemistry, which has changed the evolution of life, too,” said study lead author Sarah Hurley, a postdoctoral research associate in the departments of Geological Sciences and Biochemistry. “Today, all animals need higher oxygen to survive.”
Until now, scientists still don’t know what these establishment microbes looked like, where they lived or what inspired the transformation of the universe.
But Hurley and her colleagues believe they may have gotten closer to responding by drawing on studies of naturally occurring and genetically engineered cyanobacteria. The team reports that these ancient microbes may have floated freely in an open ocean and resembled a modern life form called beta-cyanobacteria.
Studying them, the researchers said, offered a window into a time when single-celled organisms ruled the Earth.
“This research gave us a unique opportunity to create and test ideas about what the ancient Earth looked like, and what these ancient organisms would have been like,” said co-author Jeffrey Cameron, associate professor of biochemistry.
Take a breath
You can still argue that cyanobacteria rule the planet. Hurley noted that these organisms currently make up about a quarter of the oxygen that comes from the world’s oceans.
One secret to their success may be carboxysomes – or tiny particles with proteins floating within all living cyanobacteria. These pockets are vital to the life of these organisms, allowing them to accumulate molecules of carbon dioxide in their cells.
“Being able to accumulate carbon will allow cyanobacteria to survive at what is, in the context of Earth’s history, very low carbon dioxide concentrations,” Hurley said.
Before the Big Ox Event, it was a different story. Carbon dioxide levels in the atmosphere may have been as high as 100 times what they are today, and there was almost no oxygen. For this reason, many scientists have long accepted that microorganisms do not need carboxysomes to accumulate carbon dioxide.
“Cyanobacteria have persisted in some form over two billion years of Earth’s history,” she said. “They could be quite different than the cyanobacteria of today.”
To find out how similar they were, the researchers grew clumps full of bright green cyanobacteria under conditions similar to those on Earth 2 billion years ago.
Hurley explained that different types of cyanobacteria prefer to digest different forms, or “isotopes,” of carbon atoms. As a result, as they grow, die and decompose, the organisms leave different chemical names in old sedimentary rocks.
“We believe cyanobacteria existed about billions of years ago,” she said. “Now, we can find out what they were doing and where they lived at that time because we have a record of their metabolism.”
Picking up zombie microbes
In particular, the team studied two types of cyanobacteria. They contained beta-cyanobacteria, which are common in the oceans today. But the researchers also added a new version to the study. They tried to bring old cyanobacterium back from the dead. Hurley and her colleagues used genetic engineering to design a specific type of microorganism that lacked carboxysomes. Think of it as a zombie cyanobacterium.
“We were able to do what was essentially a psychology of laboratory resuscitation,” said Boswell Wing, research coauthor and associate professor of geological sciences.
But when the researchers examined the metabolism of their cultures, they found something remarkable: Their zombie cyanobacterium did not appear to bear a chemical name that coincided with the carbon signatures. isotope previously seen by scientists in the rock plate. In fact, beta-cyanobacteria may be the best cure for these old symptoms – still very much alive today.
The team, in other words, appears to have fallen on a living fossil that was hidden in plain sight. And, they said, it is clear that cyanobacteria that lived around the Great Oxygen Event had a carboxysome-like structure. This structure may have helped cells to protect themselves from the buildup of oxygen in the air.
“Today’s organisms could look like those old cyanobacteria – that was real,” Wing said.
Scientists, they note, have now gained a much better idea of what ancient cyanobacteria looked like and where they lived. And that means they can start running experiments to dig deeper into life like they were in the 2 billion year old ocean.
“This is solid evidence from the geological record and a model of an organ that can shed new light on life on ancient Earth,” said Cameron.
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Other coauthors on the new paper included Claire Jasper, CU Boulder undergraduate student and Nicholas Hill graduate student.