Scant Light at Ocean Floor Feeds Bacteria
The black bottom of the ocean floor might be the last place one would expect to find bacteria that depend on light for their growth, but an international team of researchers said this week that they had found such photosynthetic bacteria there, surviving on faint light from volcanic geysers.
Because many researchers believe life on Earth may have originated at such submarine geysers, the surprising finding could expand our understanding of how that life evolved and about the potential for life elsewhere in the universe.
Microbiologist J. Thomas Beatty of the University of British Columbia and his colleagues report the discovery in this week’s issue of the Proceedings of the National Academy of Sciences.
About 700 miles southwest of Acapulco, Mexico, scientists in the research submarine Alvin descended 7,844 feet, about 1 1/2 miles. Sunlight is completely absent by about 650 feet below the ocean surface. This location, called vent field nine north, is on the East Pacific Rise, where the spreading of the seafloor’s tectonic plates exposes volcanic activity below.
The researchers collected water samples inches from chimney-like vents called “black smokers” that spewed dark, mineral-laden water at temperatures near 600 degrees Fahrenheit into the 35 degree chill of the surrounding water. The scorching vents emit light, similar to the faint red glow that accompanies a heating electric stove element, though it is too faint to be detected without sensitive instruments.
During tests at the surface, exposure of the samples to light led to bacteria growth. “We said ‘Look! There’s green stuff growing in there!’ ” Beatty said. Further tests of the bacteria’s DNA and ability to absorb different wavelengths of light confirmed great similarity to other forms of green sulfur bacteria known to use only light as an energy source. Those bacteria had previously been found only in regions that received some sunlight.
Photosynthesis is the process by which plants and bacteria can convert light energy into biochemical energy needed for survival. Previous research had found bacteria near the vents that appeared to use photosynthesis to supplement other energy sources. The bacteria in the present study are “obligate,” meaning photosynthesis is their only source of energy.
