This alga may be seeding the world’s skies with clouds

An algae-killing virus may be helping seed the skies with clouds. That’s the implication of a new study, which finds that—after it dies—one of the ocean’s most abundant microorganisms provides the kernel on which water vapor can condense to form droplets, which in turn become clouds.

The finding is “something I hadn’t seen before,” says Christopher Fairall, an atmospheric physicist with the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, who was not involved with the work. It suggests that processes going on in the atmosphere above the ocean are more complex than previously believed.

Blooms of a single-celled alga called Emiliania huxleyi can cover thousands of square kilometers of ocean. The organism—along with other photosynthetic microbes—occupies the base of the food web in most of the world’s seas. Many such organisms sport shells that are made of calcium carbonate plates called coccoliths (Greek for “grain of stone”). And when these organisms die, their shells fall apart. The vast majority of these coccoliths fall to the ocean floor to accumulate with silt and other materials to ultimately become sedimentary rocks. But some are cast into the air by breaking waves or popping bubbles.

There, they’ve been found among the particles in sea spray, says Miri Trainic, an atmospheric chemist at the Weizmann Institute of Science in Rehovot, Israel. These particles scatter light and thus produce haze, but they also provide surfaces where water vapor can condense, she notes. The precise role that coccoliths and other biological bits play in cloud formation is currently unknown.

Organisms like E. huxleyi die naturally, but they—like people—can get wiped out in much larger numbers if they’re infected with viruses. To better understand whether and how the numbers of airborne coccoliths might fluctuate over time, Trainic and her colleagues took to the lab. In particular, they wanted to know how infecting the organisms with a virus—the aptly named E. huxleyi virus, or EhV, which commonly infects this tiny drifter—might change the numbers of free-floating coccoliths in their seawater tanks and in the air above.

At the start of their tests, each milliliter of water held about 20 million free-floating coccoliths. But 5 days after the team infected the algae with EhV, the number of tiny plates in the water more than tripled; in the air above the water, they had grown by nearly 10-fold. That means airborne coccoliths may play an important a role in cloud formation above the oceans, the researchers report today in iScience . In large numbers, the tiny particles provide a lot of surface area on which water vapor can condense to form droplets, they say.

The team’s experiments also suggest that once coccoliths begin to make their way into the atmosphere, their numbers can readily grow because they fall so slowly. The tiny bits are so light that they float like a feather and take, on average, about 100 seconds to fall 1 centimeter. A coccolith-size particle of salt spray, which forms when the water in a salty droplet evaporates, is much denser and thus falls about 25 times that speed, Trainic says. So, over time, bits of salt drop out of the air, leaving the proportions of coccoliths in the cloud of sea spray aerosols to gradually increase—and thus gain an increasing influence on cloud formation.

“These particles can provide quite a lot of surface area” for water vapor condensation, says Patricia Quinn, an atmospheric chemist with NOAA in Seattle, Washington. Still, she contends, the coccoliths may also be influencing atmospheric chemistry. For example, the calcium carbonate in coccoliths would likely react with dimethyl sulfide, a gas produced by E. huxleyi and other marine microorganisms, and remove it from the atmosphere. That process, in turn, might actually reduce the total number of cloud-forming aerosols over the ocean. So, increased numbers of coccoliths in sea air, because of viral infections of marine microorganisms, may actually reduce cloud formation, not boost it. The study, Quinn notes, “is a good first step. Now, researchers need to get out in the field and see what really happens.”

Trainic says she and her colleagues plan to do exactly that. Next stop: the waters off Norway, where blooms of E. huxleyi are fairly common.