Biosphere 2 officially collapsed in 1995, and Columbia University took
over the management of the building. The “ocean,” a tank the size of an
Olympic swimming pool, was by this point a wreck: most of the fish it had
been stocked with were dead, and the corals were just barely hanging on.
A marine biologist named Chris Langdon was assigned the task of figuring
out something educational to do with the tank. His first step was to adjust
the water chemistry. Not surprisingly, given the high CO 2 content of the
air, the pH of the “ocean” was low. Langdon tried to fix this, but strange
things kept happening. Figuring out why became something of an
obsession. After a while, Langdon sold his house in New York and moved
to Arizona, so that he could experiment on the “ocean” full-time.
Although the effects of acidification are generally expressed in terms
of pH, there’s another way to look at what’s going on that’s just as
important—to many organisms probably more important—and this is in
terms of a property of seawater known, rather cumbersomely, as the
“saturation state with respect to calcium carbonate,” or, alternatively,
the “saturation state with respect to aragonite.” (Calcium carbonate
comes in two different forms, depending on its crystal structure;
aragonite, which is the form corals manufacture, is the more soluble
variety.) The saturation state is determined by a complicated chemical
formula; essentially, it’s a measure of the concentration of calcium and
carbonate ions floating around. When CO 2 dissolves in water, it forms
carbonic acid—H 2 CO 3 —which effectively “eats” carbonate ions, thus
lowering the saturation state.
When Langdon showed up at Biosphere 2, the prevailing view among
marine biologists was that corals did not much care about the saturation
state as long as it remained above one. (Below one, water is
“undersaturated,” and calcium carbonate dissolves.) Based on what he
was seeing, Langdon became convinced that corals did care about the
saturation state; indeed, they cared about it deeply. To test his
hypothesis, Langdon employed a straightforward, if time-consuming,
procedure. Conditions in the “ocean” would be varied, and small colonies