Oxygen is essential for life on Earth, but this was not always the case. Before about 2.4 billion years ago, Earth was a virtually oxygen-free environment. The appearance of cyanobacteria, or blue-green algae, changed all that.
Cyanobacteria injected the atmosphere with oxygen, setting the scene for the development of complex life as we know it. But a funny thing happened: Although conditions were ripe for algae to pump more oxygen into the atmosphere, oxygen levels remained low. And they stayed low for the next 2 billion years.
Researchers have proposed many theories to explain the oxygen lag, including a lack of nutrients for the cyanobacteria or limited supplies of nitrogen. But the exact mechanism that kept oxygen production low remains a mystery.
In a new paper in Trends in Plant Science, scientists suggest that an enzyme contained only in cyanobacteria may have acted as a regulator of oxygen for billions of years. The enzyme may have essentially capped the amount of oxygen produced by the algae until the evolution of complex plants overrode its limits about 450 million years ago.
The Origins of Oxygen
Until about 2.4 billion years ago, bacteria lived in an environment with no oxygen. To photosynthesize, bacteria would use electrons from hydrogen sulfide, hydrogen, or iron to trigger photosynthesis.
But around that time, “the cyanobacteria really discovered something that changed everything,” said John Allen, a biochemist at University College London. They “invented a brilliant new way of doing photosynthesis, which was to take electrons from water,” he explained.
The by-product of photosynthesis is oxygen, and the new gas accumulated in the atmosphere. This event, called the Great Oxidation Event, marked the end of the Archean.
Although cyanobacteria cracked the photosynthesis code and introduced oxygen to the atmosphere, atmospheric oxygen levels were stagnant and rose to only about 10% of our present-day levels.
Those low levels persisted for almost 2 billion years, even though blue-green algae had everything they needed to thrive, said Allen. “They should be unstoppable, because water is everywhere,” he added.
What Held Oxygen Back?
Researchers have long been trying to uncover why the oxygen levels stayed low. Some scientists think that the availability of metals in early Earth limited cyanobacteria producing oxygen.
But Allen was unconvinced.
“These trace metals act like lubricants,” he said. “They’re not being used for fuel.” He compares trace metals to the oil in a car—it just lubricates the engine, whereas gasoline actually powers the car. Allen and his colleagues were looking for a model that had a simpler solution with a feedback mechanism.
Coauthor Brenda Thake, of Queen Mary University of London, noted that when cyanobacteria are grown in laboratory conditions in which light and trace elements abound but combined nitrogen is limited, the cyanobacteria fix their own nitrogen by using an enzyme called nitrogenase.
The cyanobacteria photosynthesize and produce oxygen, explained Allen, but the oxygen feeds back into the system to inhibit the enzyme nitrogenase. He pointed out that the process is like a thermostat telling a heater to shut off instead of heating a room indefinitely.
The result is that lab-grown cyanobacteria will produce oxygen but to no more than 10% of our present levels—exactly the amount of oxygen produced in the Proterozoic.
Allen said, “This was too much of a coincidence to be a coincidence.” He noted that the team thought the very low oxygen levels “could simply be that the oxygen being produced inhibited nitrogenase, which prevented cell growth.”
Their hypothesis is an interesting one, said Christopher Junium, a stable isotope geochemist at Syracuse University in New York. He was not involved with the study.
“I think what’s key is that they present a broadly testable hypothesis” about why oxygen was limited after the Great Oxidation Event, said Junium. He said that this study focuses on one particular organism, but there’s room for scientists to test other cyanobacteria to see how they respond in a laboratory setting.
Junium also noted that the organic microfossil record is pretty limited. But expanded investigations might shed light on the evolutionary process even more. “Just because we’ve only found heterocysts from 408 million years ago (and younger) doesn’t mean that they don’t exist in deeper time,” said Junium.
After the Oxygen
After the first appearance of oxygen, there’s quite a stretch of time where oxygen levels stayed the same—what scientists have dubbed the “Boring Billion.” But Allen takes umbrage with that term.
“I think the Boring Billion, under the surface, was not at all boring,” Allen said. “There were all sorts of very interesting things going on in this world of very low oxygen concentration.”
He explained that complex, multicellular organisms evolved, eventually dying and accumulating in organic-rich deposits. Allen says plants becoming firmly rooted on land allowed for the evolution of a physical separation between aerial oxygen and nitrogen fixing in soil.
“The bigger apparent increase in oxygen content really does coincide with the evolution of land plants,” Junium said. He added that there was a “pretty broad range of evidence” that atmospheric oxygen levels rose during this time.
The connection between plants and oxygen makes sense, said Junium. “The consequence of carbon burial, when it’s produced by oxygenic photosynthetic organisms like vascular plants (Devonian ferns, for example), is increasing oxygen.”
The new paper will generate a lot of scientific discussions, said Junium. A theory-focused paper is a rarity these days, but he added that “it was neat—I feel like there should be more papers written like this.”
—Sarah Derouin (@Sarah_Derouin), Freelance Journalist