For the first two billion years of Earth’s history, there was little oxygen in the air. Although some microorganisms were already photosynthesizing during the second half of this period, oxygen had not yet accumulated to the level that would affect the global biosphere.
But about 2.3 billion years ago, that stable, low-oxygen balance changed, and oxygen began to accumulate in the atmosphere, eventually reaching the levels that support life breathing today. This rapid rise in oxygen levels is known as the Great Oxidation Event (GOE). What triggered the event that pulled the earth out of its oxygen-starved morass? This has been one of the great mysteries of science.
A team of researchers has proposed a new hypothesis that oxygen eventually began to accumulate in the atmosphere due to interactions between certain Marine microbes and minerals in ocean sediments. These interactions helped prevent oxygen from being consumed, and they set off a self-amplifying process that allowed more and more oxygen to accumulate in the atmosphere.
The scientists used mathematics and evolutionary analysis to come up with their hypothesis, suggesting that microbes did exist before the GOE and evolved the ability to interact with sediment. This is the first study to link the co-evolution of microbes and minerals to earth’s oxygen levels. The paper was recently published in Nature Communications.
Lift a step
One of the most important biogeochemical changes in earth’s history has been the oxidation of the atmosphere. Oxygen levels in the atmosphere today are a stable balance between processes that produce oxygen and those that consume it. Before GOE, the atmosphere was in a very different equilibrium, with producers and consumers of oxygen equally in equilibrium, but in a way that left little extra oxygen in the atmosphere.
If you look at earth’s history, there seem to have been two distinct jumps in oxygen levels, from stable low oxygen to stable high oxygen, one in the Paleoproterozoic and one in the Neoproterozoic. These jumps cannot be due to the build-up of excess oxygen, and there must be some feedback loop that brings about this sudden change in stability.
The team wondered if such a positive feedback loop might come from a process in the ocean that prevents some organic carbon from being used by its consumers. Organic carbon is consumed primarily by oxidation, which is usually accompanied by oxygen depletion. Microbes in the ocean use oxygen to break down organic matter, such as debris that settles in sediments. Could there be some process where the presence of oxygen stimulates further accumulation?
The researchers developed a mathematical model that predicted that if microbes had only partial capacity to oxidize organic matter, some of the oxidized material (POOM) would become “sticky” and chemically bind to minerals in the sediment, protecting it from further oxidation. As a result, oxygen that would otherwise be consumed to degrade the material completely would accumulate freely in the atmosphere. The team found that this process can act as a positive feedback, providing a natural pump that pushes the atmosphere into a new high-oxygen equilibrium.
Hidden in the genes
This further raises the question, is there really a class of microbes whose metabolism produces POOM in the ocean?
To answer this question, the team searched the scientific literature and identified a group of microbes that are partially oxidizing organic matter in the deep ocean today. The microbes, which belong to the bacterial group SAR202, do some of their oxidation by an enzyme called Bayer-Veliger monooxygenase (BVMO).
The team performed a phylogenetic analysis to see how far back in the past the microbes and the genes for the enzyme could be traced. They found that the bacteria did indeed have ancestors that could trace back to GOE, and that the gene for the enzyme could be traced back to pre-Goe times in various microbial species.
What’s more, the diversity of this gene, or the number of species that acquired it, increased significantly during periods when the atmosphere experienced oxygen surges, including one in the Paleoproterozoic GOE and another in the Neoproterozoic. This temporal link supports the overall hypothesis proposed by the study.
In recent years, the theory of co-evolution of organisms and minerals has become more and more popular in the field of earth science. As we explore the story of earth’s past, we look less and less at the elements in isolation, but as a whole, together writing the pages of earth’s history. The research is showing how interactions between microbial, mineral and geochemical environments work together to raise oxygen levels in the atmosphere.
Proposing a new approach, and presenting evidence of its validity, is a first but important step. To confirm this hypothesis, more follow-up studies will be needed, ranging from laboratory experiments to field investigations. But now, new research has created a new suspect in the explosion of oxygen levels in the earth’s atmosphere.