What was the impact of the hydrothermal vent discovery on your field?

WIRSEN: You'd have to say that the discovery of deep-sea vents, from a microbiological point of view, was one of the major biological events in the 20th century. While the area of vents comprises only a very small portion of the ocean bottom, the discovery has lead to many other areas of research. We've been diving at vents with submersibles for many years now. Each time we go, we have certain questions we want to answer. We answer some of them, and come back with new questions.

These bacteria derive energy from the hydrogen sulfide-rich waters near the vent where they live.

The main question originally was "what is supporting these large communities of animals?" Hydrogen sulfide, poisonous to most forms of life, and other reduced chemicals were found in the water coming out of the vents. The hypothesis that these chemicals might be an energy source for the microbes around the vents was soon proven to be correct. The word "chemosynthesis" is often used when we talk about hydrothermal vents. The microbial process of chemosynthesis is more often called "chemolithoautotrophy" by microbiologists. "Chemo" means it is a chemical form of energy, as opposed to the light energy used in photosynthesis. "Litho" is from the Greek word "stone," meaning the energy is coming from an inorganic source, like hydrogen sulfide (H2S). "Autotrophy" means "self nourish" in which case the organisms are getting their carbon from inorganic carbon in the form of carbon dioxide, CO2, not from an organic molecule like glucose as most bacteria do.

Chemosynthesis was first described well over 100 years ago by a Russian-born microbiologist named Winogradsky. He was the first to propose that microorganisms, not just plants, use carbon dioxide as a sole carbon source. But when vents were discovered, the very important thing was not that many microbes there were chemolithoautrotrophs, but the fact that solar energy had been completely replaced by terrestrial energy or heat from the earth in support of these populations. This heat from the earth results in the production of these reduced chemicals— hydrogen sulfide and others. These compounds are rich in energy and certain microorganisms can take advantage of and use this energy.

These bacteria, the second largest microorganisms known, are unique to the Guaymas Basin vent in the Gulf of California.

So these vent bacteria, as the primary producers, play the same role green plants do in terrestrial habitats and on the ocean surface. If you go to your local salt marsh at low tide, you may smell that rotten egg smell— hydrogen sulfide. Well, many of the same microbial processes that occur in your salt marsh occur in the hydrothermal vent community. The difference is that in the marsh, the hydrogen sulfide is the end-product of a process that began with photosynthesis. Algae grow using light energy, they die off and then bacteria degrade the algae, producing hydrogen sulfide. If there were no algae, you would not have this hydrogen sulfide. So it's dependent initially on the sun.

At the vents, you have hydrogen sulfide coming out of the vent opening - but here it is not dependent on energy from the sun. Seawater on the bottom percolates down in through the rocks and it's acted on by the heat of the Earth. The chemicals in that seawater are converted, for example, from sulfate (SO4) to hydrogen sulfide (H2S). Then, it comes out of the vent and supplies the organisms with energy.
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Photos: WHOI