2020托福听力练习：细菌加工氮气（下载）

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　　Oxygen and water are crucial to most life on Earth, but what about nitrogen?

　　It's in every molecule of DNA in your body, and in all your proteins—you literally can't live without it.

　　But most of Earth's nitrogen exists as an inert atmospheric gas that organisms can't use.

　　Lightning strikes can convert some nitrogen into a bioavailable form.

　　But most of the biosphere's usable nitrogen is the result of bacteria employing an enzyme called nitrogenase to pull nitrogen out of the air.

　　Based on genetic evidence, scientists have thought that nitrogenase first evolved around 2 billion years ago.

　　Before that, life on Earth might have been confined to the oceans and been limited by the crucial substance's inaccessibility.

　　But researchers at the University of Washington now have evidence for the existence of nitrogenase in bacteria going back some 3.2 billion years.

　　The researchers base their argument on the ratios of light to heavy nitrogen isotopes in ancient rocks from Australia.

　　The study is in the journal Nature.

　　The finding indicates that the biosphere more than three billion years ago was much more complex than previously appreciated, and perhaps had already colonized land.

　　An earlier arrival for nitrogenase also may mean that the enzyme evolves more easily than was previously believed.

　　Which could increase the odds that, sooner or later, astrobiologists will find signs of another robust biosphere on some world far away.

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