Field Work The Invisible Forest Under The Sea Half of the planet’s oxygen comes from tiny plant-like organisms under the ocean’s surface. by Lauren J. Young, on August 16, 2018 <img src="https://www.sciencefriday.com/wp-content/uploads/2018/08/DSC05675-copy-min.jpg" /> Out on the open ocean, you're surrounded by a vista that is a shocking shade of bright blue, explains Anne Thompson, a biological oceanographer. Credit: Allison Lee The surface waters of the open ocean just about 100 miles north of Hawaii can be an unforgiving place for life. The seemingly endless stretch of electric blue water can be like a desert—zapped of the nitrogen, phosphorus, iron, and other essential nutrients needed to support the growth of larger organisms. “When we go out there, if you look around, all you see is blue water, and it’s really hard to imagine that there’s anything in there,” says Anne Thompson, a biological oceanographer and research assistant professor at Portland State University who coasted out on several expeditions between 2016 and 2017. But she knows down 200 meters deep, in that harsh environment, there’s actually a team of organisms hard at work—tiny microorganisms that produce the oxygen that fills your lungs every five or so breaths. Anne Thompson sets up a filter rig for water sampling. Credit: Mark Farley Thompson studies the billions of tiny plant-like organisms that make up the ocean’s “invisible forest”—a considerably thin layer of ocean filled with microscopic photosynthesizers called phytoplankton that suck up carbon dioxide from the atmosphere and bubble out 50 percent of the planet’s oxygen. Like a terrestrial forest, energy transfers between the sun, the plants, the animals and organisms, and so on. The phytoplankton are found floating in the top layers of the ocean above about 200 meters, or as far as sunlight can filter down. Not only do they help drive the cycling of carbon and produce a significant bulk of the world’s oxygen, phytoplankton also serve as a buffet of nutrients—the foundation of the food web feeding larger creatures in the sea. And like terrestrial forests, the invisible forest’s phytoplankton is made up of many species of photosynthetic microorganisms, from larger bodied silica-covered diatoms to the tiniest cyanobacteria. “This is what’s happening in the open ocean—but instead of trees, we have these single-celled photosynthetic organisms that you just can’t see if you look,” Thompson says. The forest of photosynthetic microorganisms in the open ocean remains unseen to the naked eye, but like any ecosystem, “it’s beautiful, it’s huge, and it’s really wild,” she says. Related Video How To Grow Coral The Tiniest Photosynthetic Organism On Earth Researchers used to think phytoplankton couldn’t be found in large numbers in the open ocean of the tropics and subtropics, where large systems of circulating currents, or ocean gyres, form pockets of the nutrient poor water. Then in 1979, scientists at Woods Hole Oceanographic Institution discovered the tiny cyanobacteria Synechococcus that would unlock a new piece to the invisible forest. “It was like discovering grass on land when all you had seen before was trees and bushes,” says Sallie (Penny) Chisholm, ecologist and marine biologist at Massachusetts Institute of Technology who has been studying marine microorganisms since the 1970s. This pasture of primitive photosynthetic cells was underlying bigger cells, spreading across the nutrient poor regions in large numbers, she explains. Synechococcus was just the beginning. Less than a decade later, while studying Synechococcus, Chisholm and a colleague happened upon an even tinier (only 0.6 micrometer) cousin called Prochlorococcus—a picoplankton that has become the a prized model organism of the research community in the three decades since its discovery. “They’re tiny, but they’re powerful.” Together, Prochlorococcus and Synechococcus fill an important role in the invisible forest, says Chisholm, but Prochlorococcus has taken on a legacy of its own within the microbe world. Over the years Prochlorococcus has received much attention and enthusiasm from the research community. (Chisholm and fellow researchers threw a ProchlorococcusFest on its 20th anniversary in 2008.) Lisa Moore and Penny Chisholm celebrate the 20th anniversary of the discovery of Prochlorococcus with a song sung to the tune of “La Cucaracha.” Originally written by Jim and Shelley Tripp, 2/23/2008. The vibrant green picoplankton is the smallest, most numerically abundant photosynthesizing cell known on the planet—of the half of the oxygen emitted from the invisible forest, the estimated 3 billion billion billion Prochlorococcus cells churn out 10 percent of the total. Packed inside these tiny cyanobacteria are stories about the invisible forest. “I think of them as little reporters,” says Chisholm. And these little reporters are hardy organisms. “They’re tiny, but they’re powerful,” says Anne Thompson, who was first in...