VOA慢速英语2021--地球白昼变长使氧气激增

Earth's Longer Days May Have Produced More Oxygen

A group of scientists has proposed¹ a new theory about how Earth's atmosphere got its oxygen: The planet² slowed down and days got longer.

A recent study proposes³ that longer, continuous⁴ daylight caused bacteria to produce lots of oxygen. That oxygen made most of life as we know it possible.

The scientists dug up bad smelling, colorful bacteria from a deep sinkhole in Lake Huron, which is one of the Great Lakes in the northern United States. In laboratory⁵ experiments, the researchers gave the bacteria different amounts of light. They found that the bacteria produce oxygen at a higher rate the longer they receive continuous light.

One of the great mysteries in science is how Earth went from a planet with little oxygen to the breathable air we have now. Scientists long believed that microorganisms called cyanobacteria were involved. But they do not know what started the great oxygenation event.

The researchers released⁶ their study in the publication⁷ Nature Geoscience this month. It suggests that Earth's slowing rotation⁸ made it possible for the cyanobacteria to make the atmosphere able to support life. Over a long period, Earth went from a six-hour rotation to its current 24-hour rotation.

The scientists say that 2.4 billion years ago there was almost no oxygen in Earth's atmosphere. No animal or plant that we know of could live in such conditions. Instead, lots of bacteria breathed carbon dioxide gas. In the case of cyanobacteria, they produced oxygen in the earliest form of photosynthesis⁹.

At first it was not much. But after about 400 million years, Earth's atmosphere reached one-tenth the amount of oxygen we have today. That is a huge jump, said the study's lead writer, Judith Klatt. She is an expert at the Max Planck Institute in Germany. The increased oxygen permitted plants and animals to develop over time. And plants began joining the oxygen-making party, she said.

But why did bacteria begin to make more oxygen? Brian Arbic is an ocean expert at the University of Michigan. He studies tidal forces on Earth and how they have slowed Earth's rotation. Arbic says planet's rotation slowed because of tidal friction¹⁰ and Earth's relationship with the moon.

Researchers from the University of Michigan and from Germany tested their ideas with bacteria similar to what would have existed 2.4 billion years ago. They used white and purple mats of cyanobacteria living 24 meters underwater in the sinkhole in Lake Huron.

"We actually imagine that the world looked kind of like the Middle Island sinkhole for most of its history," Klatt said.

Divers¹¹ brought up the mats of bacteria, which smell like very old eggs. Klatt and others exposed them to different amounts of light, up to 26 straight hours. They found that more continuous light causes the bacteria to produce more oxygen.

The writers of the study and outside scientists said this is just one possible reason for Earth's oxygen increase. What makes the idea interesting is that it does not require any big biological changes in bacteria or the world's oceans, said Tim Lyons. He is a professor at the University of California, Riverside and was not part of the research team.

Words in This Story

sinkhole –n. a low area in the ground or underwater that is formed by flowing water removing underlying¹² soil and rocks

rotation –n. a complete turn around a central point; a complete turn of the Earth

photosynthesis –n. the process by which a green plant turns water and carbon dioxide into food and releases¹³ oxygen

tidal –adj. related¹⁴ to tides; the push and pull caused by the gravity of the sun and moon that affects the world's oceans

friction –n. the force that causes moving objects to slow down

mat –n. a thick mass of something that is stuck together

1 proposed   

被提议的

参考例句：

• There is widespread discontent among the staff at the proposed changes to pay and conditions. 员工对改变工资和工作环境的建议普遍不满。
• an outcry over the proposed change 对拟议的改革所发出的强烈抗议

2 planet   

n.行星

参考例句：

• Neptune is the furthest planet from the sun. 海王星是离太阳最远的行星。
• Rubbish, however, is only part of the problem of polluting our planet. 然而, 垃圾只是我们这个星球的污染问题的一个方面。

3 proposes   

v.提议，建议( propose的第三人称单数 )；打算；提供（解释）；提出（行动，计划或供表决的方案等）

参考例句：

• He proposes a short rest before we continue the work. 在继续这项工作之前，他建议暂休一会儿。 来自辞典例句
• Aristotle proposes to cure such hardships by means of equity. 亚里士多德提出用衡平的方法来解决这样的困难。 来自辞典例句

4 continuous   

adj.继续的，连续的，持续的，延伸的

参考例句：

• She finally got in after 10 years'continuous effort.坚持不懈地努力了十年后，她终于当选了。
• We must be continuous to study.我们必须不断学习。

5 laboratory   

n.实验室，化验室

参考例句：

• She has donated money to establish a laboratory.她捐款成立了一个实验室。
• Our laboratory equipment isn't perfect,but we must make do.实验室设备是不够理想，但我们只好因陋就简。

6 released   

v.释放( release的过去式和过去分词 )；放开；发布；发行

参考例句：

• He was released on bail pending committal proceedings. 他交保获释正在候审。
• With hindsight it is easy to say they should not have released him. 事后才说他们本不应该释放他，这倒容易。

7 publication   

n.出版，发行；出版；公布，发表

参考例句：

• They don't think this article is suitable for publication.他们认为这篇文章不宜发表。
• The government has delayed publication of the trade figures.政府已将贸易统计数字延后公布。

8 rotation   

n.旋转；循环，轮流

参考例句：

• Crop rotation helps prevent soil erosion.农作物轮作有助于防止水土流失。
• The workers in this workshop do day and night shifts in weekly rotation.这个车间的工人上白班和上夜班每周轮换一次。

9 photosynthesis   

n.光合作用

参考例句：

• In apple trees photosynthesis occurs almost exclusively in the leaves.苹果树的光合作用几乎只发生在叶内。
• Chloroplasts are the structures in which photosynthesis happens.叶绿体就是光合作用发生的地方。

10 friction   

n.摩擦，摩擦力

参考例句：

• When Joan returned to work,the friction between them increased.琼回来工作后，他们之间的摩擦加剧了。
• Friction acts on moving bodies and brings them to a stop.摩擦力作用于运动着的物体，并使其停止。

11 divers   

adj.不同的；种种的

参考例句：

• He chose divers of them,who were asked to accompany him.他选择他们当中的几个人，要他们和他作伴。
• Two divers work together while a standby diver remains on the surface.两名潜水员协同工作，同时有一名候补潜水员留在水面上。

12 underlying   

adj.在下面的，含蓄的，潜在的

参考例句：

• The underlying theme of the novel is very serious.小说隐含的主题是十分严肃的。
• This word has its underlying meaning.这个单词有它潜在的含义。

13 releases   

v.释放( release的第三人称单数 )；放开；发布；发行

参考例句：

• Nuclear fission releases tremendous amounts of energy. 核裂变释放出巨大的能量。 来自《现代汉英综合大词典》
• Zemel says that when calcium levels are low, the body releases a hormone that helps squeeze the most out of every available milligram of the mineral. 泽莫尔博士说，当人体中的钙含量偏低时，身体里就会产生一种荷尔蒙，它能帮助肌体最大限度地吸收所摄入的钙。 来自《简明英汉词典》

14 related   

adj.有关系的，有关联的，叙述的，讲述的

参考例句：

• I am not related to him in any way.我和他无任何关系。
• We spent days going through all related reference material.我们花了好多天功夫查阅所有有关的参考资料。