Listen to part of a lecture in a biology class.

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 important biological and important nutritive function in those organisms.

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

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.

See, zinc plays a major role in carbon cycling, the conversion of various kinds of molecules with carbon, ...

... like carbon dioxide into other kinds of molecules with carbon that organisms can use.

So, take respiration, our bodies—our cells produce carbon dioxide when they break down sugars.

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

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

Now, this whole conversion process relies on a particular enzyme. Um, who remembers what an enzyme is? Bob?

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

Exactly, different enzymes assist in different chemical reactions.

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

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

And it's also extremely important for plants. Bob, can you tell us why?

Bob: For making food, for photosynthesis?—Exactly! For photosynthesis.

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. So, zinc is also important for plants.

OK, but zinc is scarce in certain environments, 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.

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

In particular, there are a lot of diatoms. Diatoms are microscopic, photosynthetic organisms and they are a major source of food for other organisms in the ocean.

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.

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 the diatoms die and fall to the ocean floor.

So, how do diatoms survive if zinc is so scarce?

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

But this enzyme doesn't contain zinc. Instead, this new enzyme incorporates another trace metal—cadmium.

Kelly, you've got a question?—Kelly: Yeah, I thought cadmium was toxic, didn't you say that? It is poisons to humans.

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

But new study suggests that cadmium can actually substitute for zinc, that organism can use it instead of zinc when there is not enough zinc in their environment.

Now the discovery of this cadmium-based enzyme is really important for a number of reasons; it's actually the first enzyme we discovered that uses cadmium.

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.

And there're maybe other types of diatoms that use cadmium to cycle carbon.

But there is something else to think about, ...

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

Carbon dioxide, right? 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 overlooked, so further research might tell us more about these warming cycles, too.

enzyme   n. [生化] 酶

protein   n. 蛋白质；朊 adj. 蛋白质的

photosynthesis   n. 光合作用

marine plants   海洋植物

microscopic   adj. 微观的；用显微镜可见的 

photosynthetic   adj. [生化] 光合的；光合作用的

culprit   n. 犯人，罪犯；被控犯罪的人，罪魁祸首