Coral bleaching, increasing storms, the loss of polar bears: many impacts of climate change are already vivid in our minds. We naturally worry about the things we can see. Huge waves and the loss of big fish and colorful corals get our attention.

But what about things we can’t see, like the tiny creatures called plankton? They are also poised for dramatic changes.

A recent dive in the sapphire waters of the Caribbean offers a close encounter with plankton. While most of my dive buddies hurry to reach the bottom, I linger as I usually do, pondering the “blue” and looking out for the visible and the invisible.

Suddenly, clouds of tiny filaments come sharply into focus. It’s blue-green algae–Trichodesmium–a type of phytoplankton that plays an important role in these nutrient-poor waters. They essentially break gaseous nitrogen’s tough triple bond and convert it into a form other phytoplankton can feed on.

What would these waters look like without them?