Tiny, living stones of the sea

False-colored scanning electron microscope of a coccolithophore species that makes two kinds of limestone plates (shown in brown and light green) and is in the middle of transitioning from one to the other.

CREDIT: M. GEISEN ET AL / EUROPEAN JOURNAL OF PHYCOLOGY 2002

It was the summer of 1988 when early-career oceanographer Barney Balch spotted an unusually reflective patch of water in a satellite image of the Gulf of Maine. He and colleagues at the nearby Bigelow Laboratory for Ocean Sciences arranged a research cruise to investigate.

Ocean waters in this part of the temperate Atlantic typically run a deep sapphire hue. But this patch was much lighter, more typical of a tropical isle. “We got out in the middle of this thing and the water was basically turquoise in color from horizon to horizon,” Balch says. “You would have thought we had run aground in Bimini.”

The bright patch returned the next summer — a strikingly large, turquoise swirl spread across roughly 100,000 square kilometers. Water samples confirmed Balch’s suspicions of what had caused the bright patches: trillions of coccolithophores, tiny single-celled algae that sport reflective limestone plates made of the mineral calcite, as shown in the false-colored scanning electron micrograph above.

Those two blooms launched Balch, who still works at the Bigelow lab, on a career devoted to learning more about coccolithophores, one of the most abundant groups of phytoplankton in the world. As satellite imagery improved, researchers have identified and studied the blooms in ever more detail. And they have tracked several important ways the tiny organisms alter ocean chemistry, including the ability of the ocean to store carbon dioxide and thus keep it out of the atmosphere. So while each individual cell is no wider than a human hair, the sheer magnitude of these blooms gives coccolithophores an outsized impact on ocean chemistry and, in turn, global climate.

But as climate change makes the ocean more acidic and corrosive to limestone, some regions will become less hospitable to shell-forming organisms. Balch and colleagues are working to understand how ocean acidification may affect coccolithophore abundance and distribution around the world, and how these shifts could sway their role in regulating climate.

All photosynthetic organisms help moderate Earth’s thermostat to a certain extent by pulling the greenhouse gas carbon dioxide out of the atmosphere to make sugars. But coccolithophores help cool the planet in a variety of other distinctive ways related to their coccospheres, the chalky-white balls that surround their exteriors.

For one thing, these spheres shimmer so brightly that they can shield the ocean from the sun’s incoming radiation. “They are like little mirrors that are reflecting light back into space,” says Balch, who authored an overview of the shifting state of coccolithophores in the 2018 Annual Review of Marine Science. In the absence of the blooms, the open ocean is darker and more absorbent of the sun’s heat, he explains.

Laura Poppick, 25 June 2019, Knowable Magazine. Article.

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