Ocean's Most Abundant Microbe Lives Two Lives, Stores Carbon

The tiny ocean microbe that gives us every fifth breath just revealed a secret double life that could change how we understand Earth's carbon cycle.

MIT researchers discovered that Prochlorococcus, the ocean's most abundant photosynthetic organism, doesn't just drift alone through sunlit waters as scientists thought for decades. These microscopic powerhouses can also form sticky communities called biofilms that cling to particles and sink toward the deep sea, potentially locking away far more carbon than anyone realized.

"We can now think of Prochlorococcus as existing in two very different but connected worlds," says Maya Anjur-Dietrich, the postdoc who led the study at MIT's Department of Civil and Environmental Engineering. When floating free, these cells drift hundreds of cell lengths apart through nutrient poor water, but on particles, they experience a completely different physical and chemical world.

The discovery started with clumps of cells mysteriously appearing at the bottom of lab culture tubes. When Anjur-Dietrich tested strains from every major branch of the Prochlorococcus family tree, she found something shocking: every single strain could form biofilms.

In typical cultures, about 6% of cells stuck to tube walls. But when researchers repeatedly transferred only the sticky cells to fresh media over many generations, eventually 85% of the population became attached.

Here's where it gets really interesting: genetically identical cells can exist in either state, switching back and forth without any DNA mutations. The same genetic instructions get read differently, like musicians playing the same sheet music loudly or softly, quickly or slowly.

The Bright Side

Life in a biofilm transformed how the cells behave in ways that matter for our planet. Attached cells produced more chlorophyll and grew more slowly than free floating cells, but they released almost twice as much organic carbon into surrounding water.

That extra carbon release could mean Prochlorococcus plays a much bigger role in Earth's biological pump, the natural process that moves carbon from the atmosphere into the deep ocean where it gets stored for centuries. Scientists previously thought these surface dwelling cells contributed very little to carbon sequestration because they lived and died quickly near the top.

The team found DNA evidence of particle attached Prochlorococcus throughout ocean samples from the global Tara Oceans Project database, from sunlit surfaces down to 1,000 meters deep. The sticky lifestyle exists everywhere in nature, not just in lab tubes.

Why would a microbe need two lifestyles? Diversity probably ensures survival in the ocean's unpredictable conditions, Anjur-Dietrich explains. Free floating cells get better sunlight access and spread farther, while attached cells gain nutrients from neighboring microbes and protection from predators, though they risk sinking out of the sunlit zone.

The discovery opens new questions about how much carbon these tiny organisms actually move into ocean depths, and it reminds us that even Earth's most studied microbes still have surprises to share.