India, June 8 -- They drift unseen, but everything depends on them. Plankton - the ocean's lifeblood - regulate the climate, feed the seas, and shape life on Earth. On the French Riviera, scientists are racing to unlock their secrets before the hush of their decline echoes across the planet.

On a sun-drenched morning off the coast of Villefranche-sur-Mer, the Sagitta III cuts through the cobalt waters of the Mediterranean, past the quiet marinas and pine-fringed terraces of France's Cote d'Azur. The 40-foot scientific vessel - named after a fearsome zooplankton with hook-lined jaws - rumbles toward a lonely yellow buoy bobbing offshore.

In the distance, the resort town shimmers, a mirage of pastel villas and church towers clinging to th...