This is sort of cool. It's long been known that the ocean functions as a giant carbon sink, absorbing CO2 from the air. As man-made greenhouse emissions have grown, those carbon-sucking oceans have become more acidic as a result, with ugly consequences for coral reefs and other marine ecosystems. And, even more alarmingly, the seas are now losing their ability to absorb carbon in the first place, which means that more of it will stay in the atmosphere, which means that the planet could start to warm even more rapidly.
Anyway, until recently it was thought that plankton were chiefly responsible for producing the calcium carbonates that help maintain the delicate pH balance of the ocean. But along comes a new study in Science, written by a slew of marine scientists at the University of Miami., which finds that fish may play an even more crucial role here, by drinking up sea water and excreting their own, fast-dissolving brand of calcium carbonate as "gut rock." This might be yet another more reason to fret about overfishing: Healthy fish populations can help slow the pace of acidification—and hence, global warming. Depleted populations... well, that might not be so delightful. (See update.)
Of course, it's never quite as simple as all that. As one of the authors, Martin Grosell of University of Miami's Rosenstiel School, explains to Mongabay.com, heavy fishing can often reduce the average size of individual fish in the ecosystem (as explained here), and smaller fish, in turn, produce more calcium carbonate pound for pound. Plus there are also a whole host of other factors involved—as water temperature rises, fish start producing more calcium carbonate—all of which need further study. So nothing definitive yet. Still, fish guts. Kinda important. Who knew?
Update: Okay, apologies, I appear to have part of this badly confused. Correction courtesy of Cameron MacDonald:
I wanted to point out that you have this backwards. Marine carbon chemistry is complex and counterintuitive. Net production of marine carbonates is a source of CO2 making surrounding waters MORE not less acidic. You are removing carbon from the system as solid carbonates, but the pH is not controlled by the amount of carbon, but the EQUILIBRIUM between the different carbon species. Removing carbonate ions (a base) causes the pH to drop. I have seen scientists get confused by this in Journals before. The upshot is that increased carbonate production by fish will accelerate warming and acidification, in the same way that dissolution of coral reefs will work in the opposite direction...