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Floating miles from shore, off the continental shelf, are some of the smallest and most productive organisms on this planet. The primary producers of the open ocean include diatoms, dinoflagellates and photosynthetic microbes. Together they form temporary oases—floating meadows that convert the Sun's energy into nourishment for nearly all life in the sea.
These microscopic marine organisms produce almost as much oxygen as all of Earth's terrestrial plants combined, while having less than one percent of their volume. They also play an important role in Earth's complex climatic systems by helping buffer against global warming and releasing sulfur into the air, which seeds cloud formation.
Samples taken near shore give the impression that diatoms and dinoflagellates are the ocean's dominant photosynthesizers. But existing in tandem with these larger plankton—and nearly alone in some parts of the open ocean—is an abundant and complex microbial community.
Marine microbes can exist as a self-contained system, or loop. Photosynthetic bacteria and protists produce food—perhaps half of the ocean's primary production. Microscopic grazers including ciliates and flagellates eat these tiny phytoplankton and bacteria, all of which release organic material into the water. Other microbes convert this dissolved organic matter into raw nutrients, which are again available to the photosynthesizers, completing the microbial loop. Wherever larger marine organisms exist, the microbial loop is also there, overlapping with the classic food web.
In 2001, the waters immediately below the sunlit layer were found to be dominated by archaea, microorganisms that were previously believed to exist only in rare, extreme environments, such as deep ocean volcanic vents. In the same year, scientists discovered bacteria with a unique photosynthetic metabolism that does not produce oxygen. How all of these organisms fit into the food web is part of the microbial puzzle that scientists are seeking to solve.
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| | The Heat is On
When surface waters off the coast of Southern California underwent prolonged warming, colder nutrient-rich water was trapped below. With reduced nutrients at the surface, phytoplankton density decreased, sending a cascade through the system. From the 1970s through the 1990s, zooplankton populations dropped 80 percent—providing less food for northern anchovies, rockfish and many other species. © Norbert Wu / www.norbertwu.com | |
An estimated 5.5 billion metric tons of carbon are released into the atmosphere each year as a result of fossil fuel use. Scientists estimate that the oceanic food web annually removes about two billion metric tons, acting as a buffer against the full effects of global warming. However, this may change as ocean temperatures rise, because even small temperature increases can change the way carbon flows through the food web.
In colder waters, diatoms tend to sink to the ocean floor, removing the carbon in their bodies from the system. But in warmer waters, bacteria decompose more diatoms before they can sink, releasing a larger amount of carbon near the surface. So if ocean temperatures rise, more carbon could be returned to the atmosphere—ever increasing the rate of global warming.
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