Sun-free photosynthesis?

Bacteria may live off thermal radiation from deep-sea hydrothermal vents

By | June 21, 2005

Photosynthetic bacteria may be able to live without solar light, instead using thermal radiation from hot fluid for energy, according to a study in this week's PNAS. Researchers led by J. Thomas Beatty of the University of British Columbia, Vancouver, have found obligately photosynthetic green sulfur bacteria at a deep-sea hydrothermal vent more than a mile below the ocean surface.

"They're seeing photosynthesis where there's no sunlight," said Carl Bauer of Indiana University, who was not involved in the study. "That's amazing."

Submarine hydrothermal vents, often called black smokers, host complex ecosystems that are largely fueled by chemicals dissolved in geothermally heated vent water. Chemotrophic bacteria near the vent orifice can get energy by breaking down inorganic compounds such as hydrogen sulfide, and the organic material these bacteria produce feeds other microbes and invertebrates.

Beatty and colleagues cultured water samples taken from various depths over the East Pacific Rise, an area with a variety of hydrothermal vents about 1,300 miles off the western coast of South America. In a culture medium specific for anaerobic photosynthetic sulfur bacteria, a green-pigmented microbe grew in just one water sample, which had been taken from within 50 centimeters of a black smoker plume.

The researchers isolated this bacterium, which they name GSB1, and determined that it absorbs light at wavelengths typical of light-harvesting bacteriochlorophyll and carotenoid pigments.

Using PCR, Beatty's team showed that GSB1 also contains a light-harvesting protein called the Fenna-Matthews-Olson protein, which is found only in green sulfur bacteria.

"This is actually very typical green sulfur bacteria," said Bauer. Based on its photopigments and morphology, it looks just like other known species of green sulfur bacteria, all of which are photosynthetic, Beatty told The Scientist.

The researchers can't rule out completely the possibility that the bacteria drifted down to the vent from elsewhere, "but they did do sampling in water nearby the vents...and they didn't recover any of these GSB1 organisms from that water," said Christopher Chyba of Stanford University, who is also the Carl Sagan chair for the study of life in the universe at the SETI Institute in Mountain View, Calif. "It's hard for me to see how they could do much better than that."

The authors instead suspect that GSB1 came from a microbial community right next to the black smoker vent orifice. There, temperature gradients are so extreme that organisms can live in cooler temperatures yet still receive radiation from nearby vent fluid that is very hot, said coauthor Cindy Van Dover of the College of William and Mary, Williamsburg, Va.

Previous work by Van Dover has shown that geothermal radiation from vent fluid is mostly infrared but extends into visible-light wavelengths. She and her colleagues measured photon flux at these vents and found "marginally sufficient light for photosynthesis."

Photon flux at 750 nm, which is what GSB1's bacteriochlorophylls absorb, is about the same as the solar photon flux available for a green sulfur bacterium found in the Black Sea, Van Dover told The Scientist. That bacterium has been estimated to take two to three years to divide, Beatty said. "It doesn't really correspond well with what we normally think of as growth," said Beatty. "It's more like just survival."

It is possible that GSB1 also uses light emitted from chemical reactions for photosynthesis, according to Van Dover. Her group has shown that deep-sea vents have more light in the visible spectrum than would be expected based solely on the water's temperature, and some of this light may come from chemiluminescence.

The discovery of GSB1 has implications for what type of life might exist elsewhere in the solar system, Chyba said, especially on Jupiter's moon Europa, which many scientists suspect has salty oceans underneath surface ice. No one knows if Europa has hydrothermal vents, Chyba said, but "this paper makes it possible to speculate that photosynthesis could evolve...down at a hydrothermal vent" on Europa, he said. "On Earth, it would probably be harder to make that case, since there's so much sunlight at the surface."

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