Scientists propose a new mechanism by which oxygen may have first built up in the atmosphere
For the first 2 billion years of Earth's history, there was barely any oxygen in the air. The time between 2.4 billion to 400 million years ago represents an important chapter in the development of life on Earth. Banded ironstone formations - fossilized microbial mats made up from silica and iron-oxides - dating in that period show how oxygen levels rose from almost zero to significant amounts in the atmosphere, with concentrations fluctuating but eventually reaching modern-day concentrations.
Eventually, oxygen fueled a more effective metabolism, based on "burning" nutrients inside cells, allowing for more complex multicellular organisms to evolve.
A new hypothesis, proposed by Massachusetts Institute of Technology scientists, suggests that oxygen finally started accumulating in the atmosphere thanks to interactions between certain marine microbes and minerals in ocean sediments. These interactions helped prevent oxygen from being consumed, setting off a self-amplifying process where more and more oxygen was made available to accumulate in the atmosphere.
"Probably the most important biogeochemical change in the history of the planet was oxygenation of the atmosphere," says study co-author Daniel Rothman, professor of geophysics in MIT's Department of Earth, Atmospheric, and Planetary Sciences (EAPS). "We show how the interactions of microbes, minerals, and the geochemical environment acted in concert to increase oxygen in the atmosphere."
Their study, appearing in Nature Communications, is the first to connect the co-evolution of microbes and minerals to Earth's oxygenation.
Today's oxygen levels in the atmosphere are a stable balance between processes that produce oxygen - like photosynthesis by plants and microorganisms - and those that consume it - like rock weathering and oxygen-breathing organisms. On early Earth, the atmosphere maintained a different kind of equilibrium, with producers and consumers of oxygen in balance, but in a way that didn't leave much extra oxygen for the atmosphere.
"If you look at Earth's history, it appears there were two jumps, where you went from a steady state of low oxygen to a steady state of much higher oxygen, once in the Paleoproterozoic (2.5 billion years ago), once in the Neoproterozoic (0.5 billion years ago)," Gregory Fournier, co-author and associate professor of geobiology in MIT's Department of Earth, Atmospheric, and Planetary Sciences, notes. "These jumps couldn't have been because of a gradual increase in excess oxygen. There had to have been some feedback loop that caused this step-change instability."
The scientists wondered whether such a positive feedback loop could have come from a process happening on early Earth and involved microbes, inhabiting the oceans at the time.
In our modern oceans, organic carbon is mainly consumed through oxidation, a process by which microbes in the ocean use oxygen to break down organic matter, such as detritus that has settled in sediment. Curiously enough, models show how such an oxygen-consuming process could lead to a positive feedback loop increasing oxygen levels in the atmosphere.
The scientists identified a group of microbes that partially oxidizes organic matter in the deep ocean today. The partially oxidize organic matter becomes "sticky," and chemically binds to minerals in sediment in a way that would protect it from further oxidation. The excess oxygen that would otherwise have been consumed to fully degrade the organic matter would instead be free to build up in the atmosphere. This process, they found, could serve as positive feedback, providing a natural pump to push the atmosphere into a new, high-oxygen equilibrium.
A phylogenetic analysis of genes associated with the ability of the microbes to partially oxidizes organic matter shows that not only do the genes date back 2 billion years, but the gene's diversification, or the number of microbe species that acquired the gene, increased significantly during times when the atmosphere experienced spikes in oxygenation.
"We found some temporal correlations between the diversification of partially oxidized organic matter-producing genes, and the oxygen levels in the atmosphere," lead author Haitao Shang, a former MIT graduate student, says. "That supports our overall theory."
To confirm this hypothesis will require far more follow-up, from experiments in the lab to surveys in the field, and everything in between. With their new study, the team has introduced a new suspect in the age-old case of what oxygenated Earth's atmosphere.