New research shows that early life on Earth relied on a completely different type of photosynthesis — and that delayed the formation of the atmosphere as we breathe it today.
It’s no understatement to say that life today is wholly dependent on photosynthesis. Not only does it power plants (which directly or indirectly feed everybody else), but it also provides the oxygen we breathe. At least as far as the oxygen-producing photosynthesis of today is concerned. This reaction is what led to the appearance of free oxygen in Earth’s atmosphere, something which was unheard of 2.3 billion years ago (as oxygen is very reactive).
However, we have evidence that oxygen-releasing photosynthesis evolved much earlier in our planet’s history, even as early as 3 billion years ago. New research looking into why Earth’s atmosphere took so long to oxygenate suggests that it may simply have been a case of good ol’ fashioned competition at play.
“The striking lag has remained an enduring puzzle in the fields of Earth history and planetary science,” says Christopher Reinhard, an assistant professor in the School of Earth and Atmospheric Sciences (EAS) and the paper’s corresponding author.
Reinhard and his colleagues, led by EAS postdoctoral researcher Kazumi Ozaki, suggest that an older form of photosynthesis may have delayed the oxygenation of Earth’s atmosphere. Chemical conditions in Earth’s early oceans helped prop-up this competitor, against which oxygen-releasing photosynthesizers could not compete effectively at the time.
Modern photosynthesizers break apart water and release oxygen gas. Primitive ones, the team explains, substitute iron ions for water — and release rust instead of oxygen gas. Through a combination of experimental microbiology, genomics, and large-scale biogeochemical modeling, the team found that these primitive photosynthesizers are “fierce competitors for light and nutrients,” Ozaki explains.
“We propose that their ability to outcompete oxygen-producing photosynthesizers is an important component of Earth’s global oxygen cycle,” Ozaki, now an assistant professor in the Department of Environmental Science at Toho University, in Japan, adds.
The findings help us better understand how geology and the biosphere worked to change the Earth’s atmosphere into what we have today. It also helps us better understand the path life took on our planet; as much as oxygenation was a boon to animals like us, it was an environmental catastrophe for organisms at the time. The findings could also help us refine our search for Earth-like planets, or planets harboring alien life, as they give us a better understanding of how life itself can change a planet — and to what extent.
“Our results contribute to a deeper knowledge of the biological factors controlling the long-term evolution of Earth’s atmosphere,” Ozaki says. “They offer a better mechanistic understanding of the factors that promote oxygenation of the atmospheres of Earth-like planets beyond our solar system.”
The results “yield an entirely new vantage from which to build theoretical models of Earth’s biogeochemical oxygen cycle,” Reinhard adds.
The paper “Anoxygenic photosynthesis and the delayed oxygenation of Earth’s atmosphere” has been published in the journal Nature.