Scientists Uncover Ancient Oxygen-Breathing Microbes: A New Twist on the Oxygen Mystery
The air we breathe today is rich in oxygen, but for most of Earth's history, it was scarce. Scientists have long puzzled over why it took so long for oxygen to become a significant component of the atmosphere, with the Great Oxidation Event (GOE) marking a pivotal moment around 2.33 billion years ago. However, a recent study by geobiologists at MIT and the University of Oregon challenges this timeline, suggesting that some microbes may have been using oxygen hundreds of millions of years earlier.
The research, led by Fatima Husain and Gregory Fournier from MIT, along with Haitao Shang and Stilianos Louca from the University of Oregon, focused on a crucial oxygen-using enzyme found in most oxygen-breathing life today. Their findings indicate that this enzyme originated during the Mesoarchean era, between 3.2 and 2.8 billion years ago, well before the GOE.
This discovery raises intriguing questions about the early Earth's oxygen dynamics. The first oxygen producers were cyanobacteria, which use sunlight and water to generate energy, releasing oxygen as a byproduct. However, the accumulation of oxygen in the atmosphere was slow, with many scientists attributing this to reactions with rocks and dissolved chemicals that rapidly removed it. The MIT team introduces a new factor: early life forms that could 'eat' oxygen.
Fatima Husain, a postdoc in MIT's Department of Earth, Atmospheric and Planetary Sciences, explains, "This does dramatically change the story of aerobic respiration. Our study adds to this very recently emerging story that life may have used oxygen much earlier than previously thought. It shows us how incredibly innovative life is at all periods in Earth's history."
The enzyme in question is heme-copper oxygen reductase, which plays a vital role in the final step of aerobic respiration, converting oxygen into water and helping cells build a proton gradient for energy production. The researchers focused on the enzyme's core, using subunit I as a marker, which contains metal centers and histidine building blocks essential for its function.
Oxygen reductases are grouped into A, B, and C families based on proton movement and oxygen affinity. A-type enzymes have low affinity, while B- and C-type enzymes have higher affinity. This distinction is crucial because early Earth likely provided oxygen in localized bursts, and low-affinity enzymes could function at very low oxygen levels, allowing microbes to utilize oxygen without a significant atmospheric impact.
The study involved a massive genomic hunt, gathering 35,984 subunit I sequences from various oxygen reductase types and related nitric oxide reductases. The researchers aligned and trimmed these sequences to create a 'family tree' of respiration, removing repeats and partial sequences. They used a rooting method to place nitric oxide reductases as an outgroup to oxygen reductases, and then dated the tree with further downsampling.
The molecular clock analyses revealed that major heme-copper oxygen reductase lineages likely emerged before the GOE, with key ancestors dating back to around 3.4 to 3.6 billion years ago. For A-type oxygen reductases, estimates clustered around 3.19 to 3.21 billion years ago. Removing two deeply branching archaeal sequences near the base of the A-type group further supported this timing.
The study also highlighted a deep duplication among cyanobacteria with A-type enzymes, suggesting that aerobic respiration was an early trait in these microbes, possibly present in stem cyanobacteria. Pre-duplication cyanobacterial A-type oxygen reductases dated back to around 2.36 to 2.40 billion years ago, close to the GOE, and supported diversification during the transition to a more oxygenated world.
This research has practical implications for understanding ancient rock signals and searching for life beyond Earth. It also underscores the rapid adaptability of life to new energy sources, providing valuable insights into early metabolism, microbial evolution, and the intricate relationship between biology and planetary chemistry.
