In addition to finding Cab. thermophilum in Octopus Spring, the microbiologists also retrieved it from nearby Mushroom Spring and from Green Finger Pool in the Lower Geyser Basin not far from the famous Old Faithful Geyser. Ronald Weiner of the National Science Foundation (NSF) characterized the discovery of Cab. thermophilum as “an excellent example of how metagenomic information reveals how little we know about life on Earth.” The NSF had helped to fund Bryant’s and Ward’s Yellowstone project, along with the U.S. Department of Energy, NASA’s Exobiology Program, and the Thermal Biology Institute of Montana State University.
Before Bryant and Ward fished Cab. thermophilum out of Octopus Spring, most acidobacteria had been found in poor or polluted soils that were also highly acidic, often with a pH below 3—hence the phylum’s name. But Cab. thermophilum inhabits an alkaline, or basic, environment. Says Bryant, “When we look at the 16S ribosomal RNA sequences of other thermophilic bacteria, it appears that close relatives of Cab. thermophilum also exist in Tibet and Thailand,” in hot springs that are chemically similar to those in Yellowstone. In fact, Bryant speculates, additional relatives may turn up in microbial mats of hot springs worldwide.
The light gatherers
The greens and blues in Octopus Spring’s microbial mats come from chlorophyll. Cab. thermophilum carries its chlorophyll in antenna-like structures known as chlorosomes, each of which holds an estimated 250,000 chlorophyll molecules.
Transmission electron micrograph of thin-sectioned Cab. thermophilium cells. Chlorosomes can be seen as dark stained, oval structures pressed to the inner surface of the cell membrane.Courtesy Amaya Garcia
“Think of a chlorosome as a sac of chlorophylls,” says Bryant. “In Cab. thermophilum, the chlorophylls don’t use a protein scaffold, as in other photosynthesizing organisms. This bacterium has evolved a unique way of presenting its chlorophyll to the sun, and it does so on a scale vastly larger than other photosynthetic organisms. This means Cab. thermophilum can photosynthesize in lower-light situations—we estimate about a thousand times lower than the light required by a blade of grass.” He adds, “Cab. thermophilum is a distinctly different phototroph. The proteins that actually do the solar energy conversion process are quite different from any previously described.”
Cab. thermophilum makes three types of chlorophyll, something that allows it to live productively and in close proximity with its microbial neighbors. “By having chemically distinctive chlorophylls,” says Bryant, “Cab. thermophilum can absorb different light wavelengths, so it can thrive in the same place in the mats as the cyanobacteria, and even underneath the cyanobacteria. These different microorganisms do not compete directly with each other for light,” a situation that allows the community as a whole to be more productive.
The orange, reddish, and brownish colors in the microbial mat come from carotenoids. These molecules function as auxiliary light-harvesting pigments, and they also provide protection from the intense sunlight in this high-altitude setting, and from toxic oxygen compounds. “In addition to having plenty of green chlorophyll, Cab. thermophilum is a major carotenoid producer,” Bryant says.
Fluorescence micrograph of
Cab. thermophilium cellsCourtesy Amaya Garcia
“During the day, when sunlight reaches the mats, the cyanobacteria start producing oxygen—so much of it that the mat actually begins to give off bubbles of oxygen in the first few hours after sun-up.” When photosynthesis is in high gear, the environment in the mat has about five times the oxygen present in the atmosphere. “This supersaturation with oxygen requires that these microbes protect themselves,” says Bryant. “This is a really toxic situation—you might call it too much of a good thing.”
Bryant’s colleague David Ward has been studying the ecology of the microbial mats at Octopus Spring for more than thirty years, since he was a graduate student under Tom Brock of the University of Wisconsin. (It was Brock who discovered Yellowstone’s most famous microbe, Thermus aquaticus, from which the heat-stable enzyme known as Taq polymerase was later isolated. Taq polymerase has revolutionized science by making the polymerase chain reaction, or PCR, a routine procedure, widely used today for sequencing and analyzing genes, diagnosing hereditary diseases, identifying genetic fingerprints in forensics and paternity testing, and many other applications—including finding new photosynthetic microbes.) As a professor of Microbial Studies in the Thermal Biology Institute and the Department of Land Resources and Environmental Sciences at Montana State, Ward works to understand the functioning of the mats and the interrelationships between the resident microbes.
“During the day, at least seven different types of organisms in the mat conduct photosynthesis,” Bryant says. “They convert carbon and oxygen into sugar polymers, such as glycogen—effectively carbo-loading. Then at night, they feed on the stored carbohydrates made during the day.” Some photosynthesizing bacteria switch to fermentation after oxygen has been consumed, producing organic acids, foods that are released into the mat to be used by other community members. Cab. thermophilum probably relies on such products supplied by the cyanobacteria, which ferment at night when they’re not photosynthesizing. Returning the favor, Cab. thermophilum removes these organic acids from the mat environment that could accumulate and inhibit the growth of the cyanobacteria, which don’t do well under acidic conditions.