acidification may affect such basic processes as metabolism, enzyme
activity, and protein function. Because it will change the makeup of
microbial communities, it will alter the availability of key nutrients, like
iron and nitrogen. For similar reasons, it will change the amount of light
that passes through the water, and for somewhat different reasons, it will
alter the way sound propagates. (In general, acidification is expected to
make the seas noisier.) It seems likely to promote the growth of toxic
algae. It will impact photosynthesis—many plant species are apt to benefit
from elevated CO 2 levels—and it will alter the compounds formed by
dissolved metals, in some cases in ways that could be poisonous.
Of the myriad possible impacts, probably the most significant involves
the group of creatures known as calcifiers. (The term calcifier applies to
any organism that builds a shell or external skeleton or, in the case of
plants, a kind of internal scaffolding out of the mineral calcium
carbonate.) Marine calcifiers are a fantastically varied lot. Echinoderms
like starfish and sea urchins are calcifiers, as are mollusks like clams and
oysters. So, too, are barnacles, which are crustaceans. Many species of
coral are calcifiers; this is how they construct the towering structures
that become reefs. Lots of kinds of seaweed are calcifiers; these often feel
rigid or brittle to the touch. Coralline algae—minute organisms that grow
in colonies that look like a smear of pink paint—are also calcifiers.
Brachiopods are calcifiers, and so are coccolithophores, foraminifera, and
many types of pteropods—the list goes on and on. It’s been estimated that
calcification evolved at least two dozen separate times over the course of
life’s history, and it’s quite possible that the number is higher than that.
From a human perspective, calcification looks a bit like construction
work and also a bit like alchemy. To build their shells or exoskeletons or
calcitic plates, calcifiers must join calcium ions (Ca2+) and carbonate ions
(CO 3 2−) to form calcium carbonate (CaCO 3 ). But at the concentrations that
they’re found in ordinary seawater, calcium and carbonate ions won’t
combine. At the site of calcification, organisms must therefore alter the
chemistry of the water to, in effect, impose a chemistry of their own.