Study Finds Hidden Chemical Signals Fueling Ocean Carbon Cycle

Scientists have identified previously undetected molecules released by marine phytoplankton that fuel ocean microbes and influence Earth's carbon cycle, offering new insights into how the ocean stores and moves carbon.

Researchers at the Woods Hole Oceanographic Institution and Columbia University have pinned down a group of small molecules that phytoplankton release into seawater. Bacteria feed on them, and that feeding is part of how carbon moves through the ocean. The work, published in PNAS, answers a question that's sat unresolved for years — scientists already knew carbon passed from phytoplankton to microbes; they just didn't know which compounds were doing the carrying.

The molecules were hard to find for a simple reason: they're tiny, they blend into the background chemistry of salt water, and bacteria eat them almost as soon as they're released.

Phytoplankton are microscopic organisms. They take in carbon dioxide and turn it into organic carbon through photosynthesis, much like land plants do. Across the ocean, that process moves tens of billions of tons of carbon a year and produces a large share of the oxygen we breathe.

To find the missing compounds, the team grew six phytoplankton species covering the major groups found worldwide, keeping them under steady lab conditions with plenty of light and nutrients. Yuting Zhu, a co-lead author who worked on the study at WHOI and is now at Old Dominion University, said the group used a chemical-tagging method built at WHOI to track the small molecules each species gave off.

They found these compounds made up as much as 23% of the dissolved organic carbon released by phytoplankton — enough to power a real share of microbial life in the ocean.

Different phytoplankton species released different mixes of compounds, some carrying nitrogen, phosphorus, or sulfur along with the carbon. Columbia's Sonya Dyhrman compared it to a kind of economy: whatever chemical mix a species puts out shapes which bacteria can survive nearby.

When the researchers ran their lab data through global ocean models, they found these molecules could supply up to 5% of the daily carbon needs of SAR11, one of the most abundant bacterial groups in the sea.

The study was funded through the Center for Chemical Currencies of a Microbial Planet, backed by the National Science Foundation. Next, the team wants to see how rising temperatures, changing nutrient levels, and ocean acidification affect which molecules phytoplankton produce — and how the microbes that depend on them respond.