When tiny sea algae get sick, they may sneeze the seeds of clouds.
Phytoplankton (Emiliania huxleyi) infected with a virus shed the small calcium carbonate plates that make up their shells much more quickly than healthy phytoplankton. Kicked up by thrashing waves into sea spray, those calcium bits may ultimately become part of the complex dance of cloud formation, researchers report August 15 in iScience. This is the first study to suggest the role that viruses, which often infect and kill phytoplankton in the ocean (SN: 7/9/16, p. 12), may play.
The finding adds to a growing body of work showing that cloud formation is regulated not just by physical processes, such as evaporation and heat exchange between ocean and atmosphere, but also by biological processes, says marine biologist Roberto Danovaro, who wasn’t involved in the new research.
Previous studies in the Southern Ocean have shown that the marine organisms increase the number of cloud-forming droplets lingering in the atmosphere over the ocean there by about 60 percent each year (SN Online: 7/17/15). Phytoplankton may add to cloudiness by contributing gases and particles that can become “seeds” around which water vapor in the atmosphere can condense to form clouds.
In a controlled laboratory study, atmospheric chemist Miri Trainic and colleagues watched the progression of a phytoplankton viral infection and monitored how it altered the shedding of the calcium carbonate plates as well as the composition of sea spray. “We were convinced that those interactions must change what’s in the seawater and what will be burst out into the air,” says Trainic, of the Weizmann Institute of Science in Rehovot, Israel.
The team filled a 10-liter container with four liters of seawater containing a population of E. huxleyi. Even healthy E. huxleyi shed some of the tiny plates that make up their shells, called coccoliths; during ocean blooms, those shed coccoliths appear in satellite images as vast, milky turquoise swirls.
But when infected by a virus — called E. huxleyi virus — the phytoplankton tend to burst and rapidly drop all of their coccoliths, the researchers found. One day after being infected, the phytoplanktons’ number of calcium carbonate cells abruptly dropped. Uninfected phytoplankton saw no change in the number of their calcified cells in that same time period.
Within three days, seawater surrounding the infected phytoplankton had three times as many platelets as water around the microorganisms’ healthy counterparts. To simulate sea spray, the researchers pumped air through the tank and measured the tiny particles released by breaking waves and bursting air bubbles.
Using polarized light microscopy to analyze particles in the spray collected on filters, the team found that the spray above the virally infected populations contained about two particles per cubic centimeter of air. That’s about an order of magnitude more of the coccolith particles than in the spray above the uninfected phytoplankton, the researchers say.
Once in the atmosphere, the flat, aerodynamic platelets tend to linger, increasing their opportunities to affect cloud formation in various ways — both helping and hindering. One cloud-boosting role the platelets may play is through chemical reactions in the atmosphere, forming calcium nitrate particles that can become giant cloud condensation nuclei.
But the particles may also hinder cloud formation, the researchers say, by removing other potential cloud seeds from the atmosphere. Dimethylsulfide gas emitted from phytoplankton, which transforms into sulfuric acid in the atmosphere, has also been hypothesized to help seed clouds. However, when there are particles in the atmosphere with relatively large surface area, such as the coccoliths, the acid may condense onto the particles instead.
The research demonstrates that viruses not only kill their E. huxleyi hosts, but cause them to burst, shooting out particles in the surface ocean like a bomb, says Danovaro, of the Università Politecnica Delle Marche in Ancona, Italy. “It’s a significant and novel piece of information,” he says, because it reveals how these viruses can help enrich the “soup” of microparticles in the surface ocean that ultimately can also enter the atmosphere.
There are still a lot of unknowns when it comes to how large a role those particles may actually play in cloud formation – or whether they may ultimately do more to help or hinder cloud seeding, says Patricia Quinn, an atmospheric chemist at the National Oceanic and Atmospheric Administration’s Pacific Marine Environmental Laboratory in Seattle. She notes that, for example, no one has yet measured the actual number of coccolith particles in the atmosphere over the ocean.
To understand what role the coccoliths may be playing in real-world ocean-climate interactions, scientists will need to have a better sense of how many of the particles are actually in sea spray. To that end, Trainic says, her team is hoping to do a full-scale field study of a natural phytoplankton bloom out in the ocean.