Narrator: Listen to part of a lecture in a biology class.

旁白：听一段生物学的课程。

Professor: OK.Just before the end of the last class, we started talking about trace metals, metals found in living organisms in very small quantities that serve an important biological, important nutritive function in those organisms.

好的。在我们上堂课结束之前，我们开始讨论了微量元素，在生物体里发现的小量微量元素，对生物体起到了重要的生物的、营养的功能。

And one trace metal that serves a nutritive function is zinc.

其中一个有营养功能的微量元素就是锌。

Zinc assists in a number of processes in humans, but we are going to focus on just one, one that applies to a number of organisms, not just humans.

锌促进了很多人体内部的过程，但是我们将会着重于其中一个，一个适用于很多生物体的，不单单适用于人类的过程。

See, zinc plays a major role in carbon cycling, the conversion of various kinds of molecules with carbon, Iike carbon dioxide, into other kinds of molecules with carbon that organisms can use.

锌在碳循环中起到了主要的作用，对各种各样含碳分子进行转化，比如二氧化碳，转化成其他生物体可以使用的含碳分子。

So, take respiration.

那么，来看一下呼吸作用。

Our bodies, our cells produce carbon dioxide when they break down sugars.

我们的身体，我们的细胞在分解糖分的时候会产生二氧化碳。

We need to get the CO2 out of our bodies, so the CO2 is converted into carbonic acid, which the blood is able to carry to the lungs.

我们需要将二氧化碳排出体外，所以二氧化碳被转化成了碳酸，而血液可以将碳酸带到肺部。

Once the carbonic acid reaches the lungs, it's converted back into carbon dioxide so that we can breathe it out.

一旦碳酸到达了肺部，它又被转化成了二氧化碳，这样我们就可以把它呼出体外。

Now, this whole conversion process relies on a particular enzyme.

这整个转化的过程都依赖于一种特定的酶。

Uh, who remembers what an enzyme is? Bob?

有谁记得酶是什么吗？鲍勃？

Student: Uh, it's a protein, a specific kind of protein, one that speeds up chemical reactions.

学生：这是一种蛋白，一种特定的蛋白，它可以加速化学反应。

Professor: Exactly. Different enzymes assist in different chemical reactions.

教授：非常正确。不同的酶帮助不同的化学反应。

Now, the one that speeds up the conversion of carbon dioxide has zinc in it.

加速二氧化碳转化的酶当中含有锌。

So this zinc enzyme is critical for getting CO2 out of our bodies through the lungs.

所以，这个锌酶对于将二氧化碳通过肺排除体外非常关键。

And it's also extremely important for plants.

并且，这个对于肺部也极其重要。

Bob, can you tell us why?

鲍勃 ，你可以告诉我们为什么嘛？

Student: For making food, for photosynthesis?

学生：为了制造食物，为了光合作用？

Professor: Exactly.For photosynthesis.

教授：是的。为了光合作用。

Plants also convert carbon dioxide into different forms of carbon-containing molecules and the conversion process used relies on the very same enzyme that works in humans.

植物同样也将二氧化碳转化为各种形式的含碳分子，并且使用的转化过程依赖着和人类体内相同的酶。

So zinc is also important for plants.

所以，锌对于植物也很重要。

OK. But zinc is scarce in certain environments.

好的。但是锌在特定环境中是非常稀有的。

And it's particularly scarce in waters near the surface of rivers and lakes and shallower parts of oceans, which might make us wonder how plants could live there at all.

并且它在河流湖泊表面附近的水域及海洋较浅部分的水域里尤其稀少，这个就让我们想知道在那里的植物是怎么存活的。

In fact, there are a lot of marine plants that survive, that grow and reproduce in surface waters.

事实上，那里有很多存活的海洋植物，那些在水域表面生长繁衍的植物。

In particular, there are a lot of diatoms.

尤其是，那里有很多硅藻类。

Diatoms are microscopic, photosynthetic organisms and they are a major source of food for other organisms in the ocean.

硅藻类是细小的、光合的生物体，并且他们是其他海洋生物的主要食物来源。

There are a number of different types of diatoms, and, well, diatoms play a very important role in the carbon cycling process, because they help make carbon available to other organisms in deeper parts of the ocean.

硅藻有很多不同种类，并且他们在碳循环中起到非常重要的作用，因为它们帮主海洋深处的其他生物获得碳。

The carbon that these diatoms use in photosynthesis is transferred to other parts of the ocean when the diatoms are eaten, say, by a fish that absorbs the carbon and then swims to another part of the ocean, or when diatoms die and fall to the ocean floor.

当这些硅藻被吃掉的时候，它们在光合作用中使用的碳就被转移到海洋的其他部分，比如被一条鱼吃掉，吸收了二氧化碳，然后游到海洋的其他部分，或者当硅藻死掉的时候，掉落到海底。

So how did diatoms survive if zinc is so scarce?

那么，如果锌那么稀少，硅藻又是怎么存活下来的呢？

Well, recently researchers discovered that a specific type of diatom makes a different enzyme that serves the same purpose.

最近有研究人员发现，一种特定的硅藻会制造出不同的酶，这种酶起到同一种作用。

But this enzyme doesn't contain zinc.

但是这种酶不含锌。

Instead this new enzyme incorporates another trace metal, cadmium.

取而代之的是，这种新的酶包含了另外一种微量元素，镉。

Kelly, you've got a question?

凯莉，你有问题吗？

Student: Yeah.I thought cadmium was toxic.

学生：是的，我认为镉是有毒的。

Didn't you say that?

你没这样说过吗？

Professor: It is poisonous to humans.

教授：这个对人类是有毒的。

Uh, actually, we used to think that it was toxic to all biological life, that it didn't serve any biological purpose.

嗯，事实上，我们过去认为它对所有生物体都有毒，它不具有任何生物功能。

But new study suggests that cadmium can actually substitute for zinc, that organisms can use it instead of zinc when there isn't enough zinc in their environment.

但是，新的研究表示，镉事实上可以替代锌，当生物体的环境中没有足够锌的时候，生物体可以用它来替代锌。

Now, the discovery of this cadmium—based enzyme is really important for a number of reasons.

因为很多原因，这个以镉为基础的酶的发现是非常重要的。

It's actually the first enzyme we have discovered that uses cadmium.

事实上，它是我们发现的第一个使用镉的酶。

So it's possible that other not so typical trace metals may be used in chemical processes, that marine organisms might make enzymes from other trace metals when the essential one is scarce.

所以，其他一些非典型的微量元素可能在化学过程中被使用，海洋生物可能在必须的微量元素非常稀有的情况下，使用其他微量元素来产生酶。

And there may be other types of diatoms that use cadmium to cycle carbon.

可能会有其他一些酶使用镉来进行碳循环。

But there's something else to think about.

但是，我们有其他一些事情需要思考。

What is one of the most common greenhouse gases in our atmosphere, one of the major culprits in global warming.

什么呢我们大气中最常见的温室气体，全球变暖的主要元凶？

Carbon dioxide, right?

二氧化碳，对吧？

Now, if all these diatoms are taking carbon dioxide from the surface, converting it and transporting it to the bottom of the ocean, well, maybe there's more to that whole process, that cycle, something that we've overlooked.

如果这些酶正在将表面的二氧化碳拿走，将他们转化，并将它们运到海洋底部，可能关于那整个过程及循环，我们忽视了很多东西。

So further research might tell us more about these warming cycles too.

所以，进一步的研究也可能告诉我们这个变暖周期的更多内容。