About 2.4 billion years ago, molecular oxygen accumulated in the Earth's atmosphere (seen here), sustaining complex life on Earth.
Credit: NASA
About 2.4 billion years ago, molecular oxygen accumulated in the Earth's atmosphere (seen here), sustaining complex life on Earth.
Credit: Wikimedia
NEW YORK: Oxygen may have been used and produced by marine creatures millions of years before it accumulated in the Earth's atmosphere, according to U.S. scientists.
The broad consensus in the geoscience community regarding oxygen is that before about 2.4 billion years ago, there was very little of it in the environment. Then, during the 'Great Oxidation Event' (GOE), oxygen levels in the atmosphere ramped up.
Scientists have long debated how much oxygen might have been around before the big rise and whether organisms were producing it through photosynthesis.
The results from a new study, published online ahead of print this week in the Proceedings of the National Academy of Sciences have shown that small amounts of oxygen were present on Earth before it permanently filled our atmosphere. This oxygen was enough to sustain eukaryotic life and was probably produced by photosynthesis. The evidence helps geochemists understand the origins of oxygen, which is crucial to learning how life on Earth evolved.
"It gives us new context for interpreting … the evolution of the Earth's atmosphere system," said geochemist and lead author Jacob Waldbauer from the University of Chicago.
Oxygen and the fossil record
A few years ago, scientists found evidence of molecular biomarkers of eukaryotic cells, known as steroids, in 2.7 billion year old fossils of ancient marine organisms. What's interesting about these steroids is that they cannot be formed without oxygen.
Because of this research, scientists concluded that even millions of years before the GOE, there must have small amounts of oxygen in the atmosphere.
But how small were these amounts? Using substitutes for ancient marine eukaryotes, Waldbauer and his colleagues at the Massachusetts Institute of Technology and the Woodhole Oceanographic Institute in Massachusetts determined how much oxygen was needed to produce the steroid biomarkers found in the ancient fossils.
Predicting dissolved oxygen levels
They confirmed that steroids can be produced in the presence of extremely low levels of oxygen (in the nanomolar range), similar to the microscopic levels found in surface waters 2.7 billion years ago.
To obtain these results, they conducted experiments on Saccharomyces cerevisiae, a common yeast distantly related to the eukaryotic organisms thought to have produced the now-fossilized steranes. By growing the yeast cells at different concentrations of oxygen, and labelling the steroids they produced, Waldabuer's team was able to trace the oxygen levels required for yeast to produce steroids de novo.
Next they created a simple model to calculate global sea-air gas exchange rates and thus predict the concentration of dissolved oxygen present in the ocean surface in that time period. This concentration was found to be far greater than the concentration required to support steroid synthesis.
What oxygen's history revealed
The experiments revealed that steroids could be produced at very low concentrations of dissolved oxygen, which were probably present in the oceans 300 million years before the GOE. This shows the molecular fossil record is consistent with other evidence for an anoxic atmosphere at the time.
"Our research suggests that potentially a lot of aerobic biochemistry might have evolved when oxygen was a simply a trace constituent of the environment," Waldbauer explained. It also strengthens the idea that photosynthesis existed before the rise of atmospheric oxygen.
"When we want to look for life on new planets, we go about looking for oxygen in their atmosphere," said Ariel Anbar, a geochemist at Arizona State University who wasn't involved in the research. Since oxygen is the primary indicator of life, detailed knowledge about the abundance of oxygen and its rise is crucial for those interested in exploring extraterrestrial life.
"This is a very important piece of work that pushes the field forward," said Anbar. He added that scientists have no idea of how much oxygen was around before it became an abundant atmospheric gas. With this new evidence, at least there are some quantitative constraints on that number.
