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.

Twitter Digg Delicious Stumbleupon Technorati Facebook Email

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.

14 Responses to “A Little Iron Goes a Long Way”

  1. Hello from Eisenhower Middle School in Wyckoff, NJ. Our grade 8 science classes tested diatom growth in distilled water, water with nitrogen and phosphorus, water with iron, and water with nitrogen, phosphorus, and iron. The diatoms grew best in the flask that had nitrogen, phosphorus, and iron. Our pictures are at our Ross Sea webpage. Thanks for the great explanation.

    • Dear Loris – wow, I’m impressed to hear about your experiments—your students are doing work in the same vein as the experiments that Dr. Kustka and his team are doing. Glad to hear that the diatoms with all three major nutrients did the best… that’s how we expect these ocean ecosystems to work!

  2. The corethron diatoms are beautiful–thanks for posting the photos! Does the scant supply of iron in the water limit the size of the population in other places (where there’s not a “bloom”)? What happens to the iron in diatoms that have died? It is picked up by new diatoms?

    • Hi Kelly, those are great questions. Lack of iron is definitely thought to be one reason why blooms don’t occur everywhere in the Ross Sea. There are large areas of the world’s oceans, such as the Southern Ocean and the eastern equatorial Pacific, where this is the case. The work we’re doing on this ship is helping to decide whether it’s true here.

      As for whether iron in dead diatoms gets recycled, that’s a very keen suggestion. The answer is yes, but only a little bit. Dr. Kustka told me he has done experiments that have shown that several generations of diatoms use the same iron. But each time a diatom (or other organism) dies, there’s a good chance the iron will sink out of the surface water. Iron is unusual in this regard—most nutrients stay in the surface ecosystem for years. But iron that winds up in seawater quickly sticks onto particles (chemists say it gets ‘adsorbed’). These particles get heavy and then sink out of the surface ocean. So iron tends to stay in surface water for only a matter of months before it sinks away.

  3. Is the iron in the phytoplankton essential to the marine life that consumes them? The photos are awesome!

    • Dear Fran. Great question about physiology. The answer is yes—animals (and humans) need iron just as much as plants do. We use it to make hemoglobin, which carries oxygen around our bodies. And iron is an indispensable ingredient in mitochondria—the cellular structures that almost all life uses to convert sugar into energy.

  4. Mrs. Worth's student's from Intermediate South January 31, 2011 at 10:17 am

    You stated in the article that Earth is made up of 5% iron. We know from science class that Earth’s inner core is mostly made of iron and nickle. Our question is, how much of the 5% of iron found on Earth is actually found on earth’s crust?

    • Dear students: thank you for catching my mistake! I should have said that the Earth’s crust is made up of about 5% iron. You’re correct that the core contains much more iron. Mrs. Worth should be proud of you!

  5. What exactly is the compound that causes the red glow in the phytoplankton? You also mentioned that the same compound is used in glow up T-shirts, how do they make the shirt glow a different color? Do they use a different compound?

    • Dear Dan, those are great questions. Several proteins that help out with photosynthesis are what glows red in those phytoplankton pictures. Glow-in-the-dark T-shirts don’t use the same compounds, they just take advantage of the same process, which is called fluorescence. But you’re right, the different colors of glow-in-the-dark products come from different compounds that fluoresce differently. (Basically, they absorb light at one wavelength and re-emit it at a different wavelength, sometimes quite a while after the lights go out. Light’s wavelength corresponds to its color. So if you want a different color of glow, you find a compound that re-emits, or fluoresces, at a different wavelength.)

  6. Hi, my question is if there is iron found in the mountains on antarctica is there iron in the earth under water? If so why cant the pyhtotplanktonuse that iron?

    • Hi Matt, great idea. You’re right, there is certainly iron in the earth under the water. The catch is that the phytoplankton are all at the surface of the sea, and the iron is all at the bottom. So the thing that is keeping phytoplankton from growing in areas like the Ross Sea and Southern Ocean is the difficulty of moving iron from the bottom of the ocean to the surface. That’s what this cruise is all about—you can read a bit more in our Feb. 8 post.

  7. Skye from Mrss. Worth's 7th grade class February 10, 2011 at 7:50 pm

    hi i was wondering, since you verified that the earth’s crust is made up of 5% of iron, how much of that 5% of the iron is usable to the phytoplankton of the Ross Sea?

    • Dear Skye, that’s the hard part! Iron is very insoluble in water, so only a very tiny fraction of the iron in the earth’s crust will ever be in the water at any one time. Once it’s in the water, there have to be the right kinds of currents and water mixing to bring the iron into the reach of the phytoplankton.

Skip to toolbar