Just a few decades ago, submersibles and remote sensing technologies allowed scientists to visit the farthest reaches of the ocean for the very first time. Of the many wonders they discovered, one of the most surprising was the existence of rich clusters of life flourishing in the darkness of the deep sea floor. The inner workings of these ecosystems have proved to be as unusual as their location, for they are powered not by the light of the sun but by the heat of the earth.
At the heart of these deep-sea communities is a process called chemosynthesis. Chemosynthesis is the use of energy released by inorganic chemical reactions to produce food. It is analogous to the more familiar process of photosynthesis. In photosynthesis, plants grow in sunlight, capturing solar energy to make organic matter. In chemosynthesis, bacteria grow in mineral-rich water, harnessing chemical energy to make organic material. Chemosynthesis can sustain life in absolute darkness.
The most extensive ecosystem based on chemosynthesis lives around undersea hot springs. At these hydrothermal vents, a chemical-rich soup bubbles out of the crust and into the bottom of the sea. Boiling hot, saturated with toxic chemicals and heavy metals, and more acidic than vinegar, vent waters are deadly to most marine animals.
This noxious brew is paradise to the bacteria that coats the rocks around the vent in thick orange and white mats. The bacteria absorb hydrogen sulfide streaming from the vents, and oxidize it to sulfur. They use the chemical energy released during oxidation to combine carbon, hydrogen, and oxygen into sugar molecules.
From this simple reaction, an entire ecosystem grows. Snails, clams, mussels, and a host of other grazing animals feed on the bacterial mats. Crabs and shrimp eat the grazers, and then are hunted by larger crabs, fish, and octopi.
The largest and most abundant vent creatures are tube worms and giant white clams—animals that thrive because they have developed a symbiotic, or mutually beneficial, relationship with the bacteria. Bacteria live within the hard-shelled animals where they are protected from predators. The tube worms and clams receive a built-in food supply because they absorb nutrients directly from the bacteria.
Tube worms, the signature inhabitants of hydrothermal vents, are absolutely dependant on their internal bacteria—as adults they have no mouth or digestive system, no means of getting food apart from their symbionts. Their blood rich tissues, colored red by hemoglobin, absorb dissolved gases from the vent water and from the seawater, and then carry them to the bacteria. The bacteria convert the chemicals to organic matter and share the excess with the tube worms.
This extraordinary relationship is highly satisfactory to both species. Millions of bacteria live safely within each tube worm. The tube worms in return are so well nourished that they are the fastest growing invertebrates on Earth, stretching up to 2 meters long in a single year.
Despite the total darkness, crushing water pressure, and temperatures that swing from above boiling to near freezing, life is good at hydrothermal vents thanks to chemosynthetic bacteria. Vent faunas have both large biomass and high diversity—over 300 species of animals have been found at vents, most living nowhere else on the planet.
But life based on chemosynthesis is also precarious. The hydrothermal vents—the source of life-sustaining chemicals—can be extinguished at any time by earthquakes, lava flows, or rock falls. Many vents close after a few months or years, and few seem to survive more than a couple of decades. Once the supply of chemicals stops, the bacteria die and the rest of the fauna either migrates or perishes.
Chemosynthetic communities are also found in marine settings other than hydrothermal vents. At so-called cold-seeps, where tectonic activity squeezes mineral water out of the ground and around sea bottom petroleum deposits, methane, ammonia, and hydrogen sulfide are released. Bacteria use these compounds to make organic molecules, which support a web of symbionts, carnivores, and scavengers.
Deep-sea chemosynthetic bacteria are attracting the attention of a wide range of scientists interested in their commercial potential. Biochemists intrigued by the ability of these tiny, delicate looking creatures to turn toxic chemicals into harmless compounds, hope to use such bacteria to clean up hazardous waste. Chemists seek to isolate the enzymes that allow chemosynthetic microbes to function under extremely high temperatures and put them to industrial use.
Other scientists are studying chemosynthetic organisms and communities to find clues in the search for extraterrestrial life. They speculate that chemical reactions could also support life on poorly lit, but geologically active planets and moons, such as Europa.
Paleontologists have recently proposed that the very first life on Earth was chemosynthetic bacteria. Conditions on the young planet at the time of the oldest fossils had much in common with the harsh conditions found at hydrothermal vents. Without chemosynthesis, our planet might well be little more than a lifeless rock.