Phytoplankton’s response to climate change has its ups and downs

Armor-plated marine microbes surprised scientists a few years ago by recovering their shell-building prowess in levels of ocean acidification expected under future climate change. But those gains were short-lived, new research shows.

For four years, marine ecologist Lothar Schlüter and colleagues steeped Emiliania huxleyi phytoplankton in seawater acidified by carbon dioxide. After an initial drop in shell calcification — a process that helps sequester CO₂ from the atmosphere — the microbes mostly restored their calcification activities within a year, the researchers had reported.

But as the experiment continued, the phytoplankton began making less and less shell material. By the end of the experiment, the phytoplankton in the acidified water were calcifying less than a population that hadn’t been exposed to such harsh conditions, the researchers report July 8 in Science Advances.

In the future, the shell-making phytoplankton “may calcify even less than we assume today based on short-term experiments,” says study coauthor Thorsten Reusch, a marine ecologist who works with Schlüter at the GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany. “One year just isn’t long enough to tell us something about how evolutionary adaptation will play out.”

While phytoplankton in the ocean may ultimately follow a different evolutionary path than those under lab conditions, the work shows that the evolutionary response to climate change is more complex than previously thought, Reusch says. There is a silver lining, though: When returned to present-day seawater conditions, the phytoplankton bounced back to their original calcification rates. So even if ocean acidification continues, the phytoplankton could quickly restart calcifying if conditions ever improved. “This isn’t a case of ‘use it or lose it,’” Reusch says.

Photosynthetic plankton produce about half of Earth’s oxygen and their sinking carcasses transport carbon from the ocean surface to the seafloor — both key steps in the temperature-regulating carbon cycle. The weight of E. huxleyi’s circular, shieldlike shells serves as ballast during the descent, accelerating the carbon drawdown.

The shell-making process requires E. huxleyi to lower its own acidity by pushing protons out through its cell wall. But as the ocean becomes more acidic, that proton pushing will require more energy to overcome an increasing acidity difference between the inside and outside of the cell. Many scientists worry that that energy cost could cause calcifying phytoplankton such as E. huxleyi to ultimately give up their shells. That would slow the CO₂ drawdown and worsen climate change, the scientists fear.

Schlüter, Reusch and colleagues started their tests with a single cell of E. huxleyi collected off the coast of Norway in 2009. Populations grown from this cell lived in containers of acidified seawater about the size of soda cans. Around 2,100 generations later, at the end of the study, the acidity-acclimated phytoplankton population calcified about four-fifths as much shell material as a population that had been kept in regular seawater before being plopped into acidified water.

That calcification decline could be an evolutionary trade-off, Reusch says. The shells probably protect E. huxleyi from predators and pathogens. But in more acidic waters, the energy costs of building shells may outweigh their benefits. The researchers plan to conduct the same experiment again, this time introducing predators to see if the added hazard makes the phytoplankton hold on to their shells.

“There are a lot of surprises in store for us in terms of the kinds of evolutionary responses these organisms can have,” says Tatiana Rynearson, an oceanographer at the University of Rhode Island’s Narragansett campus who was not involved in the study. “Evolution continues.”