One of the greatest mysteries in science is just how Earth went from a planet with minimal oxygen to the breathable air we have now. Scientists long figured that cyanobacteria (蓝细菌) were involved, but couldn’t tell what started the great oxygen-producing cycle.
A new study published in Monday’s Nature Geoscience may provide the key. It theorizes that Earth’s gradually lengthened day from six hours to the current 24 hours kick-starts cyanobacteria into producing lots of oxygen, making most of life as we know it possible. Here are the takeaways of the study.
About 2. 4 billion years ago, there was so little oxygen in Earth’s atmosphere that it could barely be measured, so no animal or plant could live. Instead, lots of bacteria breathed in CO2, and in the case of cyanobacteria, produced oxygen in the earliest form of photosynthesis (光合作用).
At first it wasn’t much. But in about 400 million years, as Earth’s rotation (旋转) gradually slowed down and the day was lengthened from six hours to the present 24 hours, the cyanobacteria breathed more and more oxygen into Earth’s atmosphere until it reached one-tenth the amount of oxygen we have now. The increased oxygen allowed plants to join in the oxygen-making party.
The authors of the study put their theory to test with the bacteria found on a 24-meter-deep seabed in Lake Huron. They exposed the smelly bacteria, which are very similar to the cyanobacteria living around 2. 4 billion years ago, to varying amounts of light, and found that the more continuous light the bacteria got, the more oxygen they produced.
“What makes the idea so convincing is that it doesn’t require any big biological changes in bacteria or the world’s oceans,” said Tim Lyons, a professor at the University of California, Riverside, who wasn’t part of the research team. “This may be a simple but plausible explanation for Earth’s oxygen increase.”
【小题1】What started the oxygen-producing cycle according to the new study?| A．Earth’s faster rotation. | B．The lengthened daytime. |
| C．The bacteria in Lake Huron. | D．The appearance of plants on Earth. |
| A．To protect the cyanobacteria. | B．To study the similarities of the bacteria. |
| C．To provide evidence for the theory. | D．To improve the oxygen level on seabed. |
| A．Reasonable. | B．Sincere. | C．Complex. | D．Unacceptable. |
| A．The Rotation of Earth Is Slowing Down |
| B．Cyanobacteria’s Role Has to Be Re-examined |
| C．A Breakthrough Is Achieved in Ocean Protection |
| D．Earth’s Slowing Rotation Increased Oxygen Production |
