How tiny phytoplankton travel long distances up the ocean

It’s one of the most massive migrations on Earth: a huge biomass of tiny plankton traveling from the deep sea to the surface. However, not all of these organisms have limbs to propel themselves up. So how some of them manage to undergo such a long journey has been a mystery.

Now, a team of researchers has shown that one type of phytoplankton has a clever solution: swelling to six times its original size. The process reduces its density and allows it to float aloft like a helium balloon, bioengineer Manu Prakash and his colleagues at Stanford University report Oct. 17. Current Biology.

“This is unique,” says Andre Visser, an oceanographer at the Technical University of Denmark in Kongens Lyngby. “They’ve actually made a case for a new way where these cells can stay alive or stay near the surface.”

The team collected water samples about 100 miles off the coast of Hawaii, looking for and observing the behavior of Pyrocystis noctiluca. These 1-millimeter-long single-celled phytoplankton, best known for their bioluminescence, make a once-in-a-lifetime journey from about 125 meters deep to about 50 meters, where there is more sunlight they need to photosynthesize. Such journeys for phytoplankton can take days, unlike small animals or zooplankton, which usually make the journey on a daily basis.

In the lab, the team used special microscopes that put the phytoplankton on a kind of “hydrodynamic treadmill” to recreate the motion of the cell traveling up the water column. “This is a bit like a virtual reality machine for single cells,” says Prakash.

P. noctiluca it is denser than seawater and must sink. But early in its life cycle, it swells, reducing its density and traveling up the water column, the team found. At the end of its seven-day life cycle, the cell then begins to divide into two daughter cells as it sinks. When the division is complete, the two newborn cells swell by filling with seawater – reaching six times their original size in about 10 minutes. And so the cycle begins again.

The researchers hypothesize that the cell becomes less dense and more buoyant as aquaporin proteins in the cell filter out the dense salt from the seawater. “That way, you can have much less dense material flooding the cell, making it capable of being less dense than the surrounding seawater,” says Stanford bioengineer Adam Larson.

Calcium in seawater may play a role in inducing and bringing about this transformation, suggest experiments using seawater with and without calcium.

Inflation doesn’t just help phytoplankton grow. “Getting big actually has big consequences for other parts of their lives,” notes Visser. “Larger cells tend to have lower predation risk. There are fewer things that can eat them.” It also helps with nutrient uptake and photosynthesis: A larger surface area allows the cell to capture more sunlight.