(单词翻译:单击)
听力文本
Earth's Longer Days May Have Produced More Oxygen
A group of scientists has proposed a new theory about how Earth's atmosphere got its oxygen: The planet slowed down and days got longer.
A recent study proposes that longer, continuous 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 laboratory 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 released their study in the publication Nature Geoscience this month. It suggests that Earth's slowing rotation 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 photosynthesis.
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 the planet's rotation slowed because of tidal friction 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.
Divers 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.
I'm Alice Bryant.
重点解析
重点讲解：
1. slow down 减速；放慢速度；使...慢下来
There was a sign telling motorists to slow down.
有一个让汽车司机减速的指示牌。
2. At first 起初；一开始
At first the place was strange to me.
起先我对这个地方不熟悉。
3. In the case of 至于；在...情况下
I dread to think what will happen in the case of a major emergency.
我不敢想像万一出现重大紧急情况会怎么样。
4. Over a long period 长期以来；日积月累
Smoking a lot of cigarettes can harm and even kill over a long period of time.
长期大量吸烟会危害身体健康，甚至导致死亡。
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参考译文
地球白昼变长导致氧气激增
一组科学家提出了一种关于地球大气如何获得氧气的新理论：地球（自转）速度变慢，白昼变长。
最近的一项研究提出，连续不断的日照时间延长会促使细菌产生大量氧气。氧气让我们所知的大部分生命得以存活。
科学家从休伦湖一处深坑中挖出了一些气味难闻，颜色艳丽的细菌，休伦湖是北美五大湖之一。在实验室进行的实验中，研究人员用不同时长的光照照射细菌。他们发现，这类细菌被连续照射的时间越久，产氧的速度就越快。
科学领域最大的谜团之一：地球如何从一个氧气稀薄的行星变成如今可供我们呼吸空气的星球。科学家们一直认为，被称为蓝藻细菌的微生物与此相关。但是他们不知道是什么引发了大氧化事件。
本月，研究人员在《自然·地球科学》期刊上发表了这一研究。研究表明，地球自转速度放缓使蓝藻细菌产生氧气来维持生命存活成为可能。在很长一段时期内，地球的自转周期从6小时逐渐变成如今的24小时。
科学家们表示，24亿年前，地球大气中几乎没有氧气。我们所知的动植物无法在这种条件下生存。而很多微生物可以吸入二氧化碳气体。以蓝藻细菌为例，它们以最早的光合作用形式产生氧气。
起初大气的氧气含量不高。但是在大约4亿年后，地球大气层中的含氧量达到了目前水平的十分之一。该研究的主要作者朱迪斯·克拉特（Judith Klatt）称，这是一个巨大的飞越。她是德国马克斯·普朗克研究所的专家。随着时间的推移，氧气含量的增加使得动植物能够不断进化。她说，其他植物也开始加入制氧的队伍中。
但是为什么细菌开始制造越来越多的氧气呢？布莱恩·阿比克（Brian Arbic）是密歇根大学的海洋专家。他研究了地球上的潮汐力以及其如何使地球放缓自转速度。阿比克表示，地球自转速度变慢是因为潮汐摩擦以及地球与月球（相互作用）的关系。
美国密歇根大学和德国的研究人员用类似于24亿年前的细菌来验证他们的设想。他们使用的白紫相间的蓝藻细菌来自于休伦湖一处深24米的落水洞。
克拉特称，“实际上，我们假设地球在过去的大多数时间里就像是（休伦湖）中岛的落水洞”。
潜水员带回了多块细菌垫，它们的味道闻起来像臭鸡蛋。克拉特和其他同事将它们暴露在不同时长的光照下，最长可达26小时。他们发现，持续光照的时间越长，微生物产生的氧气就越多。
这项研究的作者以及没有参与该研究的科学家们表示，这只是解释地球氧气增加的可能性之一。蒂姆·莱昂斯（Tim Lyons）表示，这个设想之所以有趣，是因为它不需要细菌或海洋发生任何重大的生物变化。他是加州大学河滨分校(University of California, Riverside)的教授，并不是该研究小组的成员。
爱丽丝·布莱恩特为您播报。
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