Hydrothermal vents, or hot vents as they are often called, were first predicted to exist in the 1970s. Scientists predicted that hot springs were likely to exist at the rifts in the ocean floor where tectonic plates were moving apart to form new ocean bed; the hot magma welling up to fill in the gaps, so to speak.

The first site with hydrothermal vents was discovered in 1977, on the Galapagos Rift off the coast of Ecuador, at a depth of 2500 meters. This discovery was expected. What came as a total surprise, however, was the abundance of life that surrounded the hydrothermal vents. The makeup of the communities centered on these vents was also extremely unexpected, consisting of sea life that had never before been seen - giant tubeworms, huge clams, mussels and crabs. And all of this abundance in extremely stark contrast to the normally close to barren sea floor.

Since this first discovery numerous other sites have been found in oceans around the globe. These sites have been, and still are, the subject of intensive study, new additions to the tree of life being found on a nearly weekly basis. And the wonder hasn't left the field of benthic studies yet. The discovery of an extensive site on December 5, 2000, in the Atlantic Ocean, led Margaret Leinen, National Science Foundation assistant director for geosciences to say the following:

The first non-photosynthesis based ecosystem

Studies have shown that most of the hydrothermal vent ecosystems exist through an intricate symbiotic system, wherein some of the larger organisms (like the tubeworms) are host to hydrogen sulfide-oxidizing bacteria. These bacteria form the base of the ecosystem's food chain, much like plant life forms the base in all other known ecosystems (up to now, at least). Plants do this through photosynthesis, the process whereby they convert CO2 (carbon dioxide) and H2O (water) into O2 (oxygen) and C6H12O6 (sugar, or energy). Up until the discovery of the life forms surrounding these vents, this was thought to be the only way for organisms to 'create' energy1.

The bacteria that provide the energy for the hydrothermal vent ecosystems use (mostly) H2S (hydrogen sulfide, the gas that smells like rotten eggs) to produce their sugars. This type of sulfur-oxidizing bacteria, and other forms of bacteria that gain energy from the metabolism of inorganic compounds, belong to the category of organisms called chemoautotrophs. These bacteria oxidize2 compounds like hydrogen sulfide and store energy in the form of ATP (adenosine triphosphate), which is the universal 'energy' molecule in all organisms. These bacteria then use this energy to convert carbon dioxide into simple sugars and other molecules, just like plants. This process has been named chemosynthesis, as opposed to the photosynthesis done by plants.

Since the first discovery of chemosynthesis at these hydrothermal vents, studies have shown bacteria - often extremophiles - in other niches of nature to utilize chemosynthesis for their energy needs.

Symbiosis and plain old eat and be eaten

While some of the life forms directly utilize the energy produced by the chemoautotrophs, by eating them (as some of the species of crab have been seen to do), others maintain a more friendly relationship with these bacteria. The tubeworm is probably the prime example of this (or maybe just the most intensively examined one). The tubeworms have a peculiar body, in that they have no digestive tract whatsoever. No gut, no mouth, and no anus, which on first inspection is strange, as they grow to be over a meter in length. If they can't eat, how do they survive and grow?

A graduate student at Harvard University, Colleen Cavanaugh, came up with the answer. Inside the body of the tubeworm is an organ called a trophosome. The trophosome is highly vascularized and contains specialized cells packed full of chemoautotrophic sulfur bacteria. By means that are not entirely understood, the tubeworm provides all the chemicals necessary for the bacteria to make food, including sulfur, oxygen, and carbon dioxide, and the bacteria manufacture sugars or some other form of energy-rich molecules that provide nutrition to the tubeworm. The blood-red hemoglobin that fills the tubeworm's cardiovascular system is almost certainly important in the transport of sulfur and oxygen. How this is accomplished and how nutrition is provided to the tubeworm from the bacteria is less well understood.

Other vent organisms use similar symbiotic mechanisms to obtain their nutrition. As stated, others simply feed on the sulfur bacteria directly. The larger organisms, such as some types of crab and fish, are most likely feeders living on other living or dead vent organisms. Thus, a food chain is established, consisting of primary producers (chemoautotrophic sulfur bacteria), the secondary producers (tubeworms, mussels, clams, shrimp), and predators (fish) or detritivores (crabs).

Implications for other disciplines

One of the more profound impacts of all these wonderful discoveries is that science has been made to see that there are indeed more roads that lead to Rome. Rome being, in this case, life itself. It was always thought that the basis for life could only exist where some essential ingredients were in supply, two of those being water and sunlight. Now, with the discovery of hydrothermal vents, one example is known where life spurned one ingredient once thought essential.

One of the fields this has made an impact on, is extra-terrestrial science. The people at NASA and their colleagues are now cautiously considering the possibility of life on, for example, a number of moons of Jupiter, like Io, Ganymede and Callisto. The last two apparently have liquid oceans (and, it follows, are heated internally in some way), while the first, Io, is one of the most volcanically active bodies in our solar system. If life can so thoroughly surprise us on our own planet, maybe it's possible it'll put an effort into surprising the hell out of us somewhere outside our atmosphere?


1 Out on a limb here. Not entirely sure this was the first instance of such a discovery
2 That is, they remove electrons from the compounds

September 21, 2001