MICROBIAL MAT: In thick mats of microbes that cover the surface of certain Yellowstone hot springs lives Chloracidobacterium thermophilium, a heat-loving, photosynthetic microbe. Image: DAVID WARD, MONTANA STATE UNIVERSITY
The microbial mats that thrive in the hot springs of Yellowstone National Park are a rainbow of red, orange, yellow, green and brown. Within these thick growths live millions of microbes, busily thriving on the energy provided by the volcanic activity, their peers and, of course, the sun. Five of the 25 phyla of bacteria—such as the ubiquitous, oxygen-tolerant Cyanobacteria that contribute the largest share of photosynthesis on Earth—produce chlorophyll, the magic molecule that helps organisms convert sunlight into food. And now there is a sixth: Chloracidobacterium thermophilum.
Prospecting for genetic data in the bacterial hot spots, molecular biologist Don Bryant of Pennsylvania State University in University Park and microbiologist David Ward of Montana State University in Bozeman found new versions of common genes: one for manufacturing proteins that varies from species to species and one for converting sunlight into chemical energy. They were looking for evidence of a green sulfur bacterium that they suspected was present; what they found was a new "wildly different kind of phototrophic bacterium" in the 150-degree Fahrenheit (66-degree Celsius) water, Ward says.
"We found evidence for the organism by metagenomics," Bryant adds. "We validated that suggestion by sequencing a large fragment of DNA from the genome of the organism [and] by generating a highly enriched culture of the organism."
Cab. thermophilum had already been thriving unbeknownst to researchers in Ward's lab in another culture dedicated to the cyanobacteria that dominate these Yellowstone mats. "As soon as we knew that, it was pretty simple to eliminate the cyanobacteria using a herbicide—atrazine," Bryant says. By then keeping the remaining monoculture in darkness (to show that it needed light to grow) and testing its tolerance for oxygen, the researchers could conclusively determine that it was a new, photosynthetic microbe.
In addition to being one of only two types of bacteria that perform photosynthesis in the presence of oxygen, it is also the only oxygen-tolerant bacterium that constructs chlorosomes. These molecular antennae contain hundreds of thousands of chlorophyll molecules and "allows organisms that have them to live at vanishingly low light intensities," Bryant says.
Despite its phylum name—Acidobacterium—this microbe thrives in alkaline environments, unlike its relatives who persist in acidic soil and other harsh environment, raising the possibility that microbial photosynthesis is also occurring underfoot in the dirt, the researchers say. Its closest relatives, however, live in other hot springs in Yellowstone as well as in springs in Thailand and Tibet.
In addition to extending the broad span of bacterium in the book of life, the new microorganism sheds light on the evolution of photosynthesis, suggesting that Cab. Thermophilum diverged from its photosynthetic peers "a very long time ago," Bryant says. It might also improve microbial mats grown for biofuels, Ward suggests, by broadening the spectrum of light utilized.