Longer days on early Earth set stage for complex life

doi:10.1126/science.373.6555.607

GSTDTAP > 气候变化

DOI	10.1126/science.373.6555.607
	Longer days on early Earth set stage for complex life
	Elizabeth Pennisi
	2021-08-06
发表期刊	Science
出版年	2021
英文摘要	Earth wasn't born with the oxygen-rich atmosphere that fuels life today. Living things supplied the gas, but scientists have struggled to find a satisfying explanation of what triggered the buildup and why it didn't start until well after the first photosynthetic life. Now, based on modeling of Earth's early rotation and evidence from microbial mats coating the bottom of a shallow, sunlit sinkhole in Lake Huron, researchers have identified a surprising potential trigger: the increasing length of a day as ancient Earth's spin slowed. Longer days could have coaxed more photosynthesis from similar mats, allowing oxygen to build up in ancient seas and diffuse up into the atmosphere. That proposal, described this week in Nature Geoscience , has intrigued some other scientists. “The rise of oxygen [on Earth] is easily the most substantial environmental change in the history of our planet,” says Woodward Fischer, a geobiologist at the California Institute of Technology who was not involved with the work. This study offers “a totally new flavor of an idea. It's making a connection that people haven't made before.” ![Figure][1] Light and air on early EarthCREDITS: (GRAPHIC) K. FRANKLIN/ SCIENCE ; (DATA) J. M. KLATT ET AL., NAT. GEOSCI. (2021). DOI: 10.1038/S41561-021-00784-3 By 3.5 billion years ago the planet's vast shallow seas teemed with cyanobacteria, which can form mats on sediments and rock surfaces and today sometimes cause “algal” blooms deadly to fish and other aquatic animals. These microbes had evolved the molecular machinery for photosynthesis, enabling them to convert carbon dioxide and water into sugars and oxygen. They presumably provided Earth's initial supply of oxygen, creating an environment that favored the evolution of aerobic life in all its forms. But that picture left a puzzle: Why did another billion years pass before the first good geological evidence for a buildup of oxygen appears? That puzzle led Judith Klatt, a biogeochemist now at the Max Planck Institute for Marine Microbiology, to a seemingly unrelated phenomenon: the drag that the Moon's gravity exerts on the spinning Earth by tugging at its surface and raising tides. That effect has been slowing Earth's rotation and lengthening days since the beginning. Many agree that 4.5 billion years ago, 1 day was only about 6 hours long. By about 2.4 billion years ago, computer models suggest, the pull of the Moon had slowed that spin to about a 21-hour day. The models predict Earth's rotational speed then stayed constant for about 1 billion years, as other forces countered the Moon's pull on Earth. Those forces fell out of balance about 700 million years ago, models suggest, and the planet's spin resumed slowing until it reached its current speed, creating a 24-hour day. In 2016, after a chance suggestion, Klatt realized those slowdowns in Earth's rate of spin mirrored big leaps in atmospheric oxygen. For example, oxygen first jumped during what's called the Great Oxygenation Event, some 2.4 billion years ago, and then again during the Neoproterozoic era, more than 1 billion years later. The Paleozoic, about 400 million years ago, brought a final major increase in atmospheric oxygen. As a postdoc at the University of Michigan, Ann Arbor, Klatt had studied the Middle Island Sinkhole in Lake Huron, where oxygen-depleted water and sulfur gas bubble up from the lake floor, creating anoxic conditions that roughly approximate conditions of early Earth. The shallow sinkhole also hosts microbial mats, rich in cyanobacteria, that get enough sunlight for photosynthesis. Scuba divers collected samples of the microbial mats and in the lab, Klatt tracked the amount of oxygen they released under various day lengths simulated with halogen lamps. The longer the exposure to light, the more of the gas the mats released. Excited, Klatt and Arjun Chennu, a modeler from the Leibniz Centre for Tropical Marine Research, set up a numerical model to calculate how much oxygen ancient cyanobacteria could have produced on a global scale. When the microbial mat results and other data were plugged into this computer program, it revealed a key interaction between light exposure and the microbial mats. Typically, microbial mats “breathe” in almost as much oxygen at night as they produce during the day. But as Earth's spin slowed, the additional continuous hours of daylight allowed the simulated mats to build up a surplus, releasing oxygen into the water. As a result, atmospheric oxygen tracked estimated day length over the eons: Both rose in a stepped fashion with a long plateau (see graphic, above here). This “elegant” idea helps explain why oxygen didn't build up in the atmosphere as soon as cyanobacteria appeared on the scene 3.5 billion years ago, says Timothy Lyons, a biogeochemist at the University of California, Riverside. Because day length was still so short back then, oxygen in the mats never had a chance to build up enough to diffuse out. “Long daytimes simply allow more oxygen to escape to the overlying waters and eventually the atmosphere,” Lyons says. Still, Lyons and others say, many factors likely contributed to the rise in oxygen. For example, Fischer suspects free-floating cyanobacteria, not just those in rock-affixed mats, were big players. Benjamin Mills, an Earth system modeler at the University of Leeds, thinks the release of oxygen-binding minerals by ancient volcanoes likely countered the early buildup of the gas at times and should be factored into oxygen calculations. Nonetheless, changing day length “is something that should be considered in more detail,” Mills says. “I'll try to add it to our Earth system models.” [1]: pending:yes
领域	气候变化 ; 资源环境
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文献类型	期刊论文
条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/335552
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Elizabeth Pennisi. Longer days on early Earth set stage for complex life[J]. Science,2021.
APA	Elizabeth Pennisi.(2021).Longer days on early Earth set stage for complex life.Science.
MLA	Elizabeth Pennisi."Longer days on early Earth set stage for complex life".Science (2021).