A Little Iron Goes a Long Way

We love getting questions from people reading along on our blog or in classrooms. Yesterday we heard from Shiquan at Monument School, who asked why iron makes plants grow. That’s a great question that gets right to the heart of biological oceanography, so let’s take a day and explore the answer.

If you remember from the MCDW post, we’re out here in the Ross Sea to find out whether Modified Circumpolar Deep Water carries iron to the surface waters and sets off great blooms of phytoplankton. But why would iron do that? What’s so important about it?

In a single word, the answer is photosynthesis. Iron is a very important ingredient in the molecular machinery that plants use to turn sunlight into food. Inside the plant’s chloroplasts, iron helps carry the energy from sunlight along a series of proteins, putting it to work.

But here’s the catch: iron is very, very rare in the ocean. It almost seems like a cruel twist of fate: about 5 percent of the entire Earth is iron, but almost none of it is in the ocean. It’s a simple matter of solubility. For example, sugar is very soluble in water—you can make lemonade pretty much as sweet as you can stand. So is salt. But iron simply isn’t. The acidity, temperature, and oxygen content of ocean water means that almost all iron settles out of it rather than going into solution.

So phytoplankton face a big problem. They need iron to make chloroplasts so that they can grow, but they are surrounded by ocean water with almost no iron in it. The water might have plenty of other nutrients in it, but if iron is missing they won’t be able to grow. And it turns out that very large portions of the world’s oceans—including the Southern Ocean around Antarctica—fall into this category.

Because phytoplankton make the food that feeds the entire ocean, many oceanographers are interested in how iron gets moved around through ocean waters. But studying iron is a tall order. There’s so little of it that analysis techniques must be very carefully designed. Read on through the slideshow to see how two scientists do it:

Thanks to Shiquan for asking an important question—please be sure to send your own questions in. We’re looking forward to answering them!

Many thanks to Dr. Angelicque White for help preparing the plankton slides.

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About Hugh Powell

Hugh is a staff writer at the Cornell Lab of Ornithology and is on special assignment with the Rutgers University Institute of Marine and Coastal Sciences. He has previously written for the Woods Hole Oceanographic Institution.

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