Park Microbe Harvests Light

For only the third time in the past 100 years, a novel bacterium has been found that transforms sunlight into chemical energy in the colorful microbial mats around the hot springs of Yellowstone National Park.

The park is known as a tourists' wonderland full of animals, strange rock formations, geysers and colorful hot springs, but it also is a scientific reservoir housing what may be the world's largest diversity of thermophilic (heat-loving) bacteria. Yellowstone habitats have been explored since the 1960s for new organisms that may have important applications in biotechnology, for cleaning up pollution (bioremediation) or in medicine.

A microbial mat, such as the one where scientists found a light-harvesting bacterium, lies in the foreground of Yellowstone National Park's Twin Butte Vista Spring. (Photos courtesy Dave Ward)

The research team led by Don Bryant, Ernest C. Pollard professor of biotechnology in the Department of Biochemistry and Molecular Biology at Penn State, and David M. Ward, professor of microbial studies in the Thermal Biology Institute and Department of Land Resources and Environmental Sciences at Montana State University, found the new bacterium living in the same hot springs as the most-famous Yellowstone microbe, Thermus aquaticus, which has revolutionized forensics and other fields by making the polymerase chain reaction (PCR) a routine procedure.

"Finding a previously unknown, chlorophyll-producing microbe is the discovery of a lifetime," said Bryant. "I wouldn't have been as excited if I had reached into that mat and pulled out a gold nugget the size of my fist!"

The discovery, the chlorophyll-producing bacterium Candidatus Chloracidobacterium (Cab.) thermophilum, also belongs to a new phylum, Acidobacteria. The discovery marks only the third time in the past three centuries that a new bacterial phylum has been added to the list of those with chlorophyll-producing members.

"This organism provides insights into the history of photosynthesis and increases our knowledge of how solar energy is captured in this microbial community," said Ward, who is also an adjunct professor in the Department of Microbiology. "Thus, this bacterium may have implications for alternative fuels."

Although chlorophyll-producing bacteria are so abundant that they perform half the photosynthesis on Earth, only five of the 25 major groups, or phyla, of bacteria previously were known to contain members with this ability.

Ward said the microbial mats where the bacterium was found consist of layers and look like a slice of lasagna. The mats are found in springs emitting water that became alkaline by interacting with volcanic ash and lava flows inside Yellowstone's caldera. The mats look yellowish orange in the summer and greener in the winter.

"The microbial mats give the hot springs in Yellowstone their remarkable yellow, orange, red, brown and green colors," said Bryant. "Microbiologists are intrigued by Octopus and Mushroom springs because their unusual habitats house a diversity of microorganisms, but many are difficult or impossible to grow in the lab. Metagenomics has given us a powerful new tool for finding these hidden organisms and exploring their physiology, metabolism and ecology."

Metagenomics is a means of studying organisms without having to culture them. Bulk samples are collected from the environment, then DNA is isolated from the cells and sequenced by so-called shotgun sequencing on a very large scale. Analysis of the DNA sequences reveals what types of genes and organisms are present in the environment.

The team focused on two genes: 16S ribosomal RNA, a crucial component of the machinery used by all living cells to manufacture proteins; and the gene for a protein called PscA, which is essential for converting light energy into chemical energy.16S ribosomal RNA is distinctive in each species.

The discovery of the new bacterium could have only come through genetic study, Ward said, because individual bacteria from the mat all look alike under a microscope. "It's like lots of people in the same disguise at the crime scene," he said.

"Finding two new genes with a computer is not enough to justify naming a new organism. You need to prove those genes come from the same genome," Bryant said.

This chunk of microbial mat was taken from Octopus Spring in Yellowstone National Park.

Because the two genes were close together in the genome, the team was successful in isolating a single fragment containing both. "We were lucky that a former graduate student in Ward's lab, Jessica Allewalt, had already grown a culture of mixed microbes from the mats," Bryant said, "Although she didn't realize at the time that the mixture contained Cab. thermophilum."

Cab. thermophilum grows near the surface of the mats together with cyanobacteria, or blue-green algae, where there is light and oxygen, at a temperature of about 122 to 151 °F. The organism was found in three hot springs -- Mushroom Spring, Octopus Spring and Green Finger Pool -- in the Lower Geyser Basin, not far from the Old Faithful Geyser.

Unexpectedly, the new bacterium has special light-harvesting antennae known as chlorosomes, which contain about 250,000 chlorophylls each. No member of this phylum nor any aerobic microbe was known to make chlorosomes before this discovery. The team found that Cab. thermophilum makes two types of chlorophyll that allow these bacteria to thrive in microbial mats and to compete for light with cyanobacteria.

This discovery is particularly important because members of the Acidobacteria have proven very hard to grow in laboratory cultures, which means their ecology and physiology are very poorly understood, Bryant said.

"Judging from their 16S rRNA sequences, the closest relatives of Cab. thermophilum are found around Mammoth Hot Springs in Yellowstone and hot springs in Tibet and Thailand. As we look more closely, we may find relatives of Cab. thermophilum in the microbial mats of thermal sites worldwide," he said.

A paper on research appears in the July 27 issue of the journal Science.

The research is funded by the National Science Foundation, the Department of Energy and the NASA Exobiology Program.

For more information, visit: www.montana.edu