Fish? In the ocean?

January 28, 2009
MYSTERY SOLVED? Flounder in seawater (below) show concentrations of calcium carbonate much higher than in fresh water.  (Courtesy Rod W. Wilson, Exeter University)

MYSTERY SOLVED? Flounder in seawater (below) show concentrations of calcium carbonate much higher than in fresh water. (Courtesy Rod W. Wilson, Exeter University)

Scientists from the U.S., U.K., and Canada and recently discovered an entire ocean in the belly of a fish.

Rod Wilson is an animal physiologist at the University of Exeter who, with Martin Grosell (U. Miami), has spent the last several years elucidating how fish make calcium carbonate in their intestines. Fish drink seawater rich in calcium and magnesium. It concentrates in their digestive tracks and  these ions, which fish might otherwise only use for kidney stones, meet up with carbonate ions.

Coccolithopohres and foraminifera are the 800-lb. plankton of the ocean carbon cycle. Their shells drop through the water column, perhaps packaged in bulk by copepods, transporting inorganic carbon to the deep ocean. It tends to dissolve below the lysocline, where the carbonate concentration of the water dips below favorable pH and pressure conditions for keeping it in tact. The problem with this picture is that scientists have long detected aberrations in alkalinity much higher than the lysocline at which calcite shells tend to dissolve. It’s been an oceanography mystery for decades.

Fish make a different kind of carbonate crystal than the phytoplankton — aragonite, which has a marked higher amount of magnesium. As a result of this difference in composition, aragonite dissolves higher in the water column than calcite.

Wilson presented this example of good piscine renal hygiene while visiting the University of Miami. The study was met with interest by Frank Millero, the distinguished marine chemistry scientist, who pointed out that Wilson and Grosell might have stumbled on to a clue to very big problem: Why scientists observed the effects of dissolving carbonate so much higher in the water column than expected from calcite.

The three of them teamed up to see if fish might be responsible for the mystery. The trick would be to estimate how much carbonate fish leave behind and then multiply it by the number of fish in the ocean, to arrive at a sense of just how much calcium carbonate the fish are producing. Easy.

How many fish are in the ocean? The typical answer you hear these days — “Not as many as there used to be” — was too imprecise. No one had calculated how many fish might be in the ocean! “That was a big surprise,” says physiologist Grosell. The team brought on Simon Jennings (U. East Anglia) and Villy Christensen, who contributed modeling estimates, figuring a total fish biomass in the vicinity of 900 million to 2 billion tons. That yielded a conservative range of 3 to 15 percent of all surface ocean calcium carbonate — originating in fish guts.

The findings were published in Science Jan. 16. Grosell marvels at the interdisciplinary approaches it took to surmise, in essence, the power of fish in the sea: The study couldn’t have happened without a marine chemist, fish physiologists, and ecosystems modelers. Grosell: “In hindsight, it’s like how could we have missed this?”