A diatom benefits from its ciliated hitchhikers

Phytoplankton are important players in aquatic food webs and in the global carbon cycle. Through photosynthesis, they suck up carbon dioxide and collectively generate about as much oxygen as all of Earth’s terrestrial plants.

And like plants, the organisms need more than just sunlight to survive and absorb nutrients such as iron, nitrate, and phosphate from their surroundings. But phytoplankton don’t swim well—the word plankton comes from the Greek word for “drifter.” And some, like the diatom shown in the left image, don’t swim at all. Once those organisms deplete their immediate surroundings, they have to wait for nutrients to diffuse back in. Diffusion alone, however, can be too slow to provide an adequate supply.

Now Jack Costello of Providence College in Rhode Island, Eva Kanso of the University of Southern California in Los Angeles, and their collaborators have identified another nutrient-replenishment mechanism: the flapping cilia of smaller cells that adhere to the larger ones. Such cell groupings are common, but it wasn’t clear a priori that they conveyed any advantage to the host cell.

The researchers investigated the symbiotic relationship between the single-celled phytoplankton shown in the images: a large nonmotile diatom, Coscinodiscus wailesii, and a small motile ciliate, Pseudovorticella coscinodisci, that’s known to stick to it. Using particle-image velocimetry, they characterized the fluid flows generated by P. coscinodisci‘s beating cilia (right image). Of the fresh, potentially nutrient-rich fluid drawn in by the flows, more than half made it to the diatom’s surface. Additionally, the torques and forces generated by the cilia’s currents were strong enough to move the entire group of cells, which could also replenish the fluid—and the nutrients—at the diatom’s surface.

Based on the flow data, the researchers concluded that diatoms decorated with beating cilia receive 4–10 times as much nutrients as isolated ones get. That increase is at least as much as a diatom could get by sinking—the immotile cell’s only way of refreshing its own nutrient environment. But a sinking diatom runs the risk of going so deep that sunlight can no longer reach it, at which point it ceases to photosynthesize. Ciliated adherents present no such danger. (E. A. Kanso et al., Proc. Natl. Acad. Sci. USA 118, e2018193118, 2021 .)