When 19th-century British biologist Thomas Henry Huxley first put seafloor mud under a microscope, he found tiny round and oval particles in abundance. Not knowing what they were, he dubbed them coccoliths, based on the Greek words for “seed” and “stone.” Only later would scientists learn that coccoliths are calcite plates that surround some species of phytoplankton like armor. Today, more than 100 years after Huxley, teams of researchers are still unraveling the role phytoplankton play in creating the air we breathe, the food we eat, the fuel we burn, even the ground we walk on.
SEM scan courtesy of Markus Geisen
By definition, plankton are waterborne animals or plants that cannot swim against an ambient current. For the most part they float, although some can maneuver as long as they “go with the flow.” Phytoplankton are single-celled plants. The largest can be divided into three groups: coccolithophorids, diatoms, and dinoflagellates. The exterior of the coccolithophorid at right, a Syracolithus quadriperforatus, is made of calcium carbonate, the stuff of chalk. By contrast, the shell of the diatom Cyclotella pseudostelligera is silica, the material that makes up sand, glass, and quartz. Coccolithophorids and dinoflagellates are largely marine plants, meaning they live in salt water. Diatoms exist in both fresh and brackish environments. The oldest diatom fossils are about 140 million years old, leading some scientists to speculate that they evolved along with the ascent of terrestrial grasses, which released silica into the sea after separating it from minerals.
Phytoplankton can be found floating in just about every ocean, lake, and river in the world. But scientists are looking mostly at saltwater species, which, taken together, may have influenced life on Earth more than any other group of organisms. Plankton are literally at the bottom of the food chain, a source of nourishment for virtually every animal in the sea. They are ancestors to terrestrial plants, which seem to have evolved from certain ocean phytoplankton hundreds of millions of years ago. They remove carbon dioxide from the atmosphere—producing oxygen through photosynthesis and, in another process, forming the calcite plates that Huxley found so interesting. Over the eons, layer after sedimentary layer of coccoliths created such landmarks as the White Cliffs of Dover. And the organic carbon in phytoplankton cells plays a major role in today’s global economy. “Over hundreds of millions of years, these organisms have rained down onto the seafloor,” says Paul Falkowski, a Rutgers University biological oceanographer. “A small fraction has been buried and become oil.”
Scanning electron microscope (SEM) images of the three main groups of large phytoplankton—diatoms, dinoflagellates, and coccolithophorids (those with coccolith shells)—show that they can be arrestingly beautiful. Coccoliths are not always round, flat plates, like hubcaps; many look like trumpets, cabbage leaves, daisies, or stars. Diatom exoskeletons, made of silicon dioxide, sometimes seem like tiny, ornate pillboxes, with one half fitting into the other half. Some look like mandalas. Dinoflagellates can look like anything from dimpled pollen grains to minuscule ship anchors. Whatever their specific form, the hard shells of these tiny creatures seem to have the same purpose: protection. It’s a plankton-eat-plankton world down there—a “watery arms race” says German biogeologist and electron microscopist Markus Geisen—with much of the eating done by zooplankton, a notch up the food chain.
Despite their diminutive size, phytoplankton can proliferate. When conditions are right, they grow into enormous “blooms” that are so large they can be seen from space. The coccoliths of one species, Emiliania huxleyi—named after Huxley—reflect sunlight, turning thousands of square miles of water white or aqua. Other widespread population buildups can be toxic—as in May when a bloom of dinoflagellates covered nearly 4,000 square miles off China with a so-called red tide that killed off millions of fish.
In the late 1980s, scientists on both sides of the Atlantic began large-scale, multidisciplinary studies that started with Emiliania huxleyi and expanded to include the global, long-term dynamics among coccolithophorids, dinoflagellates, and diatoms. Of particular concern to researchers is the impact of global warming and how it could affect phytoplankton populations. Any massive, long-term changes in their numbers could alter fish migration patterns, growth rates, and mortality, as well as change the amount of oxygen and carbon dioxide in the atmosphere. And that could, of course, affect us all.