Learning to live with oxygen on early Earth

The finding is the first concrete validation of a long-held hypothesis that oxygen was being produced and consumed by that time and that the transition to an oxygenated atmosphere was long term. The results are published in the on-line early edition of the Proceedings of the National Academy of Science, to appear the week of October 16th.

It is generally believed that before 2.4 billion years ago, Earth's atmosphere was essentially devoid of oxygen. Exactly when and how oxygen-producing photosynthesis evolved and began fueling the atmosphere with the gas that much of life depends on has been hotly debated for some time. Plants, algae, and cyanobacteria (blue-green algae) emit oxygen as a waste product of photosynthesis–the process by which sugar, essential for nutrition, is made from light, water, and carbon dioxide.

“Our evidence points to the likelihood that Earth was peppered with small 'oases' of shallow-water, oxygen-producing, photosynthetic microbes around 2.7 billion years ago,” stated lead author Jennifer Eigenbrode of Carnegie's Geophysical Laboratory, who collected the data while pursuing her Ph.D. at Penn State. “Over time these oases must have expanded, eventually enriching the atmosphere with oxygen. Our data record this transition.”

The researchers discovered changes in fossil isotopes of the life-essential element carbon in a 150 million-year section of rock that included shallow and deepwater sediments from the late Archean period (the Archean lasted from 3.8 to 2.5 billion years ago) in Hamersley Province in Western Australia. Isotopes are different forms of an element's atoms. The relative proportions of carbon and other isotopes in organic matter depend on chemical reactions that happen as the carbon wends its way through an organism's metabolism. There are two stable isotopes of carbon found in nature–12C and 13C–which differ only in the number of neutrons in the nucleus. By far the most abundant variety is in the lighter, 12C. About 1% is 13C, a heavier sibling with an additional neutron; it is the key to understanding photosynthetic organisms.

“Photosynthetic microbes evolved in the shallow water where light was plentiful,” explained Eigenbrode. “They used light and CO2 to produce their food, like cyanobacteria do today. They gobbled up 12C and 13C, which became part of the organisms. The results are recorded in the rocks containing the remains for us to find billions of years later. Organisms leave behind different mixes of 12C and 13C depending on what they eat and how they metabolize it. Changes in these chemical fingerprints tell us about changes in how organisms got their energy and food.”

In the Archean, microbes that could not live with oxygen–anaerobic organisms–ended up with relatively small amounts of 13C. As oxygen became available in shallow water due to oxygen-producing photosynthesis, anaerobic organisms were out-competed by microbes that had adapted to oxygen. As a result, the amount of 13C increased–first in shallow water, then in deeper water. Changes in the mix of carbon isotopes in these late Archean rocks indicate microbes were learning to live with oxygen well before the atmosphere began accumulating noticeable amounts of oxygen.