“Penny Chisholm has had a 35-year love affair—with a microbe. For her, it’s been the perfect partner—elusive during courting, a source of intellectual fulfillment, and still full of mystery decades after their introduction during an ocean cruise.
To look at, the object of her passion is just a green mote, floating in vast numbers in the world’s oceans. But Chisholm has found hidden complexity within Prochlorococcus, a cyanobacterium that is the smallest, most abundant photosynthesizing cell in the ocean—responsible for 5% of global photosynthesis, by some estimates. Its many different versions, or ecotypes, thrive from the sunlit sea surface to a depth of 200 meters, where light is minimal. Collectively the “species” boasts an estimated 80,000 genes—four times what humans have, and plenty to deal with whatever the world’s oceans throw at it. “It’s a beautiful little life machine and like a superorganism,” Chisholm says. “It’s got a story to tell us.””