A Little Iron Goes a Long Way
January 26, 2011 
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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:
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- Ships are made almost entirely of metal, much of it iron. If any of that were to find its way into a sample, it would overshadow any measurement of the true iron content. Before scientists learned how to analyze trace amounts of metals, in the 1970s and 1980s, their tests for iron often gave contradictory and confusing results, because they contained metal from ships, cables, sampling equipment, and dust.
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- Scientists like Dr. Chris Measures of the University of Hawaii developed ways to keep iron out of samples and to detect very small amounts of it. Here, Dr. Measures’s special ‘clean’ CTD returns to the surface with 12 bottles of sea water for analysis. The equipment contains no exposed metal.
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- Once the clean CTD and its water samples come back on deck, the team hurries the bottles into their trace metal lab (in nautical lingo, it’s called a ‘van’). Here Dr. Mariko Hatta holds the van door open for Dr. Measures and graduate student Max Grand, so they won’t get iron on their hands by opening the door themselves. Once they’re inside the van, the door is kept closed and the air is constantly filtered to keep dust from creeping in (since dust can contain a lot of iron). This van is not part of the ship—it’s part of Dr. Measures’s scientific equipment. The whole structure and all the equipment inside it travels around the world with the team.
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- In the back of the trace-metal van, Dr. Hatta keeps some 50 samples in a clear, ventilated box called a hood. To her right is a tangle of plastic tubing threading together samples, concentrators, pumps, and analyzers. The team calls it the Spaghetti Factory. The devices concentrate sample water to one-tenth its original volume, extract the iron from it, and then add a chemical that turns pinkish-red when mixed with iron. Finally, a machine called a spectrophotometer analyzes how much red color the sample contains. That measurement lets the team calculate how much iron was in the sample.
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- Dr. Measures is interested in how much iron is dissolved in the water. Dr. Phoebe Lam of Woods Hole Oceanographic Institution studies tiny particles of iron, working inside her own clean room (affectionately called a ‘bubble’). Particles get into the ocean when storms blow dust off the continents or when currents sweep up particles from the seafloor. These particles behave differently from dissolved iron. Phytoplankton can’t use them directly—the particles are too big to get into their cells. But in a patch of seawater, particles can contain several times more iron than the amount that is dissolved. Dr. Lam wants to know whether phytoplankton have discovered ways to use this extra source of iron.
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- Dr. Lam uses battery-powered pumps to filter particles out of the water at specific depths (see Jan. 20 post). Here’s the filter paper that has caught her samples—a few thousandths of a gram that came from thousands of liters of water. All the particles on this paper are 51 microns wide or larger (about half the width of a single hair). Dr. Lam then dries the samples to preserve them. When she’s back home, she’ll dissolve the particles and find out how much iron they contain using a sensitive machine that counts individual atoms, one by one.
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- If you’re wondering where all that iron that Dr. Measures and Dr. Lam work so hard to measure winds up, it’s here. This is your first glimpse of phytoplankton from this expedition—a diatom called Corethron that’s about a tenth of a millimeter long. Those greenish-yellow globules inside the cell are chloroplasts—the places where photosynthesis happens. And there’s something very useful about them…
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- When you shine a fluorescent light on a chloroplast, the chlorophyll inside it starts to glow red. It’s the same process that makes a glow-in-the-dark t-shirt light up. Oceanographers can measure this glow to get an idea of how much phytoplankton is in the water. And since iron is part of the cell’s photosynthetic machinery, that reddish glow is visible proof of where the ocean’s iron goes.
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- Phytoplankton are small, but they’re very numerous. On average, a single phytoplankton cell contains about 100 million atoms of iron. Multiply that by the millions upon millions of phytoplankton in every gallon of seawater, and you can start to imagine why iron is in such high demand in the ocean.
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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