The Oyster’s Garter

Rebecca Horridge asked a really good question over on the “Contact Us” page, and I wanted to give it the in-depth explanation it deserved:

My Dad is a professor of physiology and a marine scientist. He reckons that if everyone ate oysters this would help global warming through shell sequestration. Do you know of anyone who works on this idea? ie could we use oysters, corals or possibly (excuse me if this is blasphemy) genetically engineered organisms to make calcium carbonate which we could then bury in the soil.? I am having trouble finding info about this.

I hereby dub this proposal “Oyster World,” and I sure do wish it were feasible. I LOVE to eat oysters (not to mention mussels, clams, and shelled critters of all kind. I am a bad, bad Jew.) However, Oyster World would not work, for the reasons I’ve listed below in order of least to most scary.

1) Oysters and corals are animals that respire. That is, they breathe in oxygen and breathe out CO2. So growing lots and lots of oysters would actually add some CO2 to the water. I don’t know how this balances with the amount of carbon stored in the shell matrix, but it would definitely lessen the carbon storage impact of Oyster World.

2) Oyster World would also be thwarted by the balance of carbon ions in the ocean. In order to form calcium carbonate shells, critters need both calcium ions and carbonate ions. Here’s the chemical equation:

Ca⁺ + CO₃^- –> CaCO₃

There’s a lot of calcium ions in the ocean, so that’s usually not a problem. But carbonate ions are a little harder to come by. That’s because most of the carbon ions in the ocean exist in the form of bicarbonate (HCO₃^-), which organisms cannot use. Here’s the equation:

CO₂ + CO₃^2- + H2O <-> 2 HCO₃^-

This is an oversimplification of a complicated set of reactions, but essentially it means that CO₂ and CO₃^2- are inversely related. If you’ve got a lot of CO₂, you’re only going to have a little CO₃^2-, and vice versa.

In Oyster World, we would make a lot of CaCO₃ shells,  and then remove them from the ocean. This would remove an important source of  CO₃^2- ions, which would increase the proportion of CO₂ ions. Unfortunately, increasing CO₂ in the ocean has a very nasty side effect…

3) Ocean Acidification (Or, the Terrifying Other Effect of Carbon Emission.)

So remember how increasing CO₂ decreases the CO₃^2- that our heroic oysters need to make shells? That’s what is happening right now. As more and more CO₂ is absorbed into the ocean, less and less CO₃^2- is available to anything that make a shell. This isn’t just oysters, but also corals, sea urchins, snails, lobsters, tiny shelled phytoplankton (foraminifera) - a vast swath of ocean life. Various studies have shown that critters have a hard time make shells when there isn’t enough carbonate around. And here’s the really scary part - in 2099, nothing that needs a shell may be able to live in huge areas of the ocean.

To illustrate this, here’s a map from Woods Hole Oceanographic Institute. (This map also appeared in “An Inconvenient Truth,” as some of you might recall.) The purple, red, and orange is enough carbonate to make shells for oysters & friends, yellow is tough but possible, green and blue means not enough carbonate to live.

Not only is Oyster World an unlikely solution to global warming, but the oysters themselves could be in future trouble.

Here are some resources to get you started. I am not sure how much science training you have, so I’ve included a selection of different sources.

• “A Chemical Imbalance”, from the excellent Altered Oceans series in the LA Times.
• A technical but accessible summary of the acidification chemistry from RealClimate
• The Ocean Acidification Network
• If you want to get into the scientific literature, this is a tiny sample of what’s out there:
  • Caldeira, K., and Wickett, M.E. Anthropogenic carbon and ocean pH. Nature: 425, 365, 2003.
  • Orr et. al. Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437, 681-686 (29 September 2005).