Deep in the ocean, invisible to the naked eye, trillions of tiny algae called phytoplankton drift through the water. These microscopic organisms are so small they could fit hundreds on a pinhead — yet together they produce roughly half of all the oxygen on Earth, matching the output of every forest, field, and meadow on land. Now, scientists at ETH Zurich in Switzerland have discovered something new about these ocean giants: each phytoplankton cell is wrapped in layers of chemicals, like the skins of an onion, that help bacteria find them.
The research team, led by Professor Roman Stocker from ETH Zurich's Institute of Environmental Engineering, used a powerful technique called Raman microspectroscopy to peek at the invisible world surrounding individual living phytoplankton cells for the first time. What they found surprised them. Previous scientists thought chemicals simply drifted away from the algae, spreading out evenly like ink in water. But Stocker's team discovered the reality is far more complex.
When they measured a reddish-brown pigment called fucoxanthin — which helps phytoplankton capture sunlight for photosynthesis — they found it didn't scatter evenly. Instead, it stayed packed tightly around the cell, dropping sharply in concentration within just 10 micrometers of the algae's surface. That's one-hundredth of a millimeter. The team discovered that a thin layer of mucus coating each phytoplankton cell is responsible for this effect. The mucus acts like a bubble, holding water-repellent substances close while still letting them spread through seawater.
Lead author Zachary Landry explained the result simply: "Much stronger chemical signals are created as a result. This makes it easier for the bacteria to locate tiny microalgal cells in the ocean." The area where algae and bacteria meet — called the phycosphere by scientists — is essentially a marketplace where carbon-containing nutrients get traded between species.
The findings, published in the journal Proceedings of the National Academy of Sciences, could change how scientists model the oceans' carbon cycle. That matters because the exchange of carbon between phytoplankton and bacteria affects how much carbon sinks to the ocean floor versus remaining in the water. Understanding these tiny interactions helps us grasp how the ocean breathes — and how it might keep breathing as the climate changes.