Phytoplankton flunk photosynthesis efficiency test

Phytoplankton that harvest sunlight in the world’s oceans make more heat than food, a new study finds.

The microscopic marine organisms, which serve as an important food source in the ocean, use photosynthesis to turn sunlight into cellular fuel. But nearly twice as much of the sunlight energy captured by phytoplankton in the ocean is released as heat than is used to make food, researchers report January 7 in Science. The finding suggests that phytoplankton don’t photosynthesize as efficiently as researchers had thought.

“The photosynthetic efficiency of global phytoplankton is very low, surprisingly low,” says study coauthor Paul Falkowski, an oceanographic biophysicist at Rutgers University in New Brunswick, N.J. “It was a complete shock to us.”

When phytoplankton harness sunlight, one by-product is fluorescence. Satellites tuned to detecting fluorescent red light have gathered data on the ocean’s photosynthesizing phytoplankton. But satellites can’t see through clouds, and some of the red light they pick up comes from particles in Earth’s atmosphere.

So Falkowski and his team developed an instrument highly sensitive to the red fluorescence of phytoplankton that could be deployed on oceangoing research vessels. The scientists gathered more than 150,000 phytoplankton fluorescent measurements during cruises in the Atlantic, Pacific, Arctic and Southern oceans from 2008 to 2014. From the data, the team calculated how much sunlight was directed toward making cellular fuel compared with how much was lost as heat or fluorescence.

While 35 percent of the absorbed light was used for making food to fuel the phytoplankton’s growth, nearly 60 percent of the light was converted to heat.

“That’s a large amount of light absorbed by phytoplankton in the ocean that’s just being reemitted as heat,” says Thomas Browning, a marine biogeochemist at the GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany, who was not involved on the study. He called it “a novel finding.”

In laboratory studies with nutrient conditions encouraging phytoplankton growth, the team observed the opposite result: Around 65 percent of absorbed light was used to make cellular fuel, while less than 35 percent was lost as heat. 

The team blames phytoplankton’s inefficient photosynthesis in the ocean on nutrient-poor waters, which cover 30 percent of the world’s oceans. Without sufficient nutrients, the photosynthesis structures in phytoplankton don’t work properly and struggle to efficiently convert sunlight into usable energy.  

The study illustrates the need for instruments that monitor phytoplankton changes in the future, Browning says. “We want to know how their activity and distribution are changing over time.”