The Oceans and Iron: A Quick Fix or Heavy Metal Siren Song?

by Bill Chameides | February 10th, 2010
posted by Erica Rowell (Editor)

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Phytoplankton might be experiencing iron deficiency from ocean acidification; I suspect eating lots of broccoli won’t help.

Rising atmospheric concentrations of carbon dioxide (CO2) from fossil fuel-burning and deforestation are a problem. That’s not news. Most everyone knows that CO2 is a greenhouse gas and increasing concentrations are warming the globe and disrupting the climate. But there’s more than just climate change.

Ocean Acidification Threatens World’s Fisheries

Less appreciated but potentially just as (or even more) serious is ocean acidification. As CO2 concentrations increase, more CO2 is dissolved in the ocean, and, because dissolved CO2 forms an acid, the ocean water becomes more acidic. Acidification of the ocean could have disastrous effects.

Myriad marine species, such as corals, mollusks, and certain phytoplankton species that use calcium in their shells and exoskeletons, will find survival more challenging in an acidifying ocean because the calcium compounds that make up their outer frames will become more soluble and thus harder to make and harder to keep. (More on ocean acidification here.)

That’s not all. Undermining those species undermines the entire marine food web upon which more than a billion people depend for their primary source of protein. Ouch.

New Paper Finds That Acidification May Leave More CO2 in the Atmosphere

Well, now we may have a double-ouch. In a paper published in the journal Science last week, Dalin Shi of Princeton University and co-authors reported the results of lab experiments and field studies that suggest that ocean acidification may inhibit the ability of phytoplankton — the tiny organisms that are the source of virtually all food in the ocean — to carry out the photosynthesis that produces that food.

The researchers’ work has implications for large swaths of the ocean where iron limits the rate of photosynthesis — in such places, adding more iron increases photosynthesis, and taking away iron slows photosynthesis.

Shi’s team found that as the acidity of seawater increases, the ability of phytoplankton to take up or absorb iron decreases.

The researchers speculate that this occurs because with increasing acidity, a critical form of iron in seawater (referred to as Fe(III), meaning it is an iron atom that has lost three of its electrons) becomes increasingly bonded to organic compounds or ligands and thus less available for uptake by phytoplankton.

The Shi et al work is sound and plausible, but questions remain. For example, while increasing acidity might make Fe(III) less available, it should make another form of iron, Fe(II), more available. Might these two effects counteract each other? Clearly, more research is needed to confirm and explain their results. That’s the good news. The bad news: they might be right.

The Bad News Part

Recall that a significant fraction of the CO2 we emit is transferred to the ocean and a significant fraction of CO2 in the surface ocean is transported to the deep ocean via the so-called biological pump — whereby phytoplankton take up CO2 during photosynthesis and then carry that carbon to the deep ocean when they sink and die. Without that pump, the atmospheric concentrations of CO2 would be a lot higher than they are today.

But if ocean acidification causes less iron uptake in regions where iron is the limiting nutrient, it could slow photosynthesis and slow the biological pump, which would in turn leave more CO2 in the atmosphere, exacerbating climate and, you got it, ocean acidification. A classic positive feedback effect.

Might Be a Good Time to Rethink Geo-Engineering With Iron

It’s a little bit, ah, ironic.

Recognizing the difficulties in getting the world to reduce CO2 emissions (blogged about here and here), scientists have been hunting for quick fixes. Their geo-engineering efforts are designed to counteract the effects of CO2 emissions rather than reduce the emissions themselves. One proposed fix is “iron fertilization,” the idea being to dump iron into the ocean, triggering more photosynthesis and thus enhancing the biological pump and drawing CO2 out of the atmosphere.

On paper a great idea. In practice, maybe not. There are a number of practical problems as well as a cautionary principle. We don’t know what the long-term consequences might be of spiking the ocean with iron. To go ahead with iron fertilization would be akin to carrying out a global experiment with the one and only ocean we have. The cure could turn out to be worse than the disease. So most scientists and policy makers agree we’d better hold off on such experiments.

If the work of Shi and colleagues is correct, it turns out that the joke is on us, for we have already unwittingly been carrying out a grand global experiment involving iron and ocean fertilization. It’s just been going in the wrong direction. We haven’t been doing iron fertilization, we’ve been doing iron de-fertilization. Are you laughing?

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