“When we became aware of bacteria using infrared light to photosynthesize, we felt very curious about checking the photosynthetic potential with this light because this is one measure of whether life could thrive around hydrothermal vents,” said Rolando Cardenas, a physicist at Central University “Marta Abreu” de Las Villas
in Santa Clara, Cuba and a coauthor of the paper published in the May issue of Astrophysics and Space Science
The new findings suggest that photosynthetic life as we know it would struggle to flourish given the small amount of available light in hydrothermal vent environments. But organisms that could make use of lower-energy infrared light might find themselves with plenty to get by on in sunless circumstances.
In the oceans, hydrothermal vents form near underwater volcanoes where tectonic plates are moving apart at mid-ocean ridges. Hot magma that burbles up into the seabed superheats passing water that then spews out of the ocean floor, laden with minerals. The minerals precipitate out of the plume, building up chimney-like structures known as black smokers. Although these deep-sea hydrothermal vents do not sound like particularly hospitable places, the scalding billows are actually biological hot spots.
Various kinds of bacteria dine on the materials such as iron, hydrogen sulfide and ammonia belched out by the vents. These bacteria in turn support whole ecosystems around black smokers, most famously characterized by tube worms, but also home to strange snails, crabs and much more.
Eight years ago, researchers led by J. Thomas Beatty of the University of British Columbia discovered a hydrothermal vent bacterium whose livelihood requires more than just ensnaring vent-water chemicals. The bacterium, identified as belonging to the green sulfur family, needs light in order to obtain energy through a chemical reaction with sulfur. This green sulfur bacteria species, however, was found in waters some 2,400 meters (7,875 feet) deep in the Pacific Ocean, off of the coast of Mexico.
Photons of sunlight cannot beam down much past about 200 meters (660 feet) in the water column before being completely absorbed. Therefore, the bacterium must use the measly portion of geothermal light generated by hydrothermal vents to survive. This geothermal light is emitted when the erupting superheated waters rapidly cool in the surrounding, barely-above-freezing sea floor aquatic environment.
The bacterial species possesses an antenna-like structure that enables it to efficiently capture light. “It’s the only example of an organism found that is thought to live off geothermal light,” said Robert Blankenship, a professor of biology and chemistry at Washington University in St. Louis who was involved in the 2005 study. “The organism uses a giant antenna complex that allows it to live under extremely low-light conditions—it’s about the best candidate you could come up with for living off of a hydrothermal vent through the absorption of photons.”
Studying the hardy, sun-deprived life in remote areas such as hydrothermal vents is unfortunately a tricky and costly endeavor—the bacterium in question has not been re-isolated since. The new study by Cardenas and colleagues therefore turns to a mathematical model to assess the photosynthetic potential around the vents.