Issued by: Laura Hunt
414-229-6447
llhunt@uwm.edu
Date: Oct. 13, 2004
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Issued by: Laura Hunt Date: Oct. 13, 2004 |
MILWAUKEE – It’s almost as if the microscopic alga Thalassiosira
pseudonana wants to be examined.
The saltwater microbe, a common specimen in marine biology laboratories, belongs to a classification of organisms called “diatoms,” which form their cell walls out of glass, generating intricately woven “glass skeletons.”
Diatoms are an aquatic kind of protist, a simple organism that is neither plant nor animal but has some characteristics of both. They live in all types of marine and freshwater environments, including the Great Lakes.
“They look, at the genetic level, like a hybrid of plants and animals,” says John Berges, an assistant professor of biological sciences who recently helped complete the whole-genome annotation of the organism.
“But that isn’t really a fair way to describe them. Protists are an example of what life looked like before living things diverged into groups like plants and animals.”
Berges was a member of an international team of scientists from 26 institutions who “mapped” the genome for the U.S. Department of Energy’s Joint Genome Institute (JGI). Their findings were published in the Oct. 1 issue of Science magazine.
T. pseudonana is one of three dozen non-bacterial organisms for which scientists have a completely sequenced genome – a genetic blueprint which can pinpoint the “trigger” for individual biological traits. One is the enormous human genome.
What’s so special about T. pseudonana’s genome? The organism possesses a unique combination of abilities and attributes, says Berges, that have important implications for some of science’s highest-profile issues – like global climate change, biodiversity and nanotechnology.
For example, diatoms make their food through photosynthesis – and they do it on a grand scale, says Berges, a biological oceanographer who first became involved in the Department of Energy’s projects while a postdoctoral fellow at the DOE’s Brookhaven National Laboratory in 1994-96.
Photosynthesis accounts for the largest absorption of carbon dioxide on Earth, and half of it occurs in the sea and other aquatic environments. “As a group, diatoms are a major ecological player in affecting the global levels of carbon dioxide,” he says.
It’s no wonder that major funding for the annotation project came from the U.S. Department of Energy.
“This critical information enables us to better understand the vital role that diatoms and other phytoplankton play in mediating global warming," says Dan Rokhsar, who heads computational genomics at the JGI. Carbon dioxide is a greenhouse gas and a byproduct of many human activities, especially fossil fuel use like car emissions.
“Their role in global carbon cycling is predicted to be comparable to that of all terrestrial rain forests combined,” according to the article in Science. “Over geological time, diatoms may have influenced global climate by changing the flux of atmospheric carbon dioxide into the oceans.”
With the help of modeling and supercomputing, Berges says the team painstakingly compared each of the diatom’s genes (more than 11,000) to genes that already have been sequenced in higher forms of life, identifying those that are unique to diatoms.
“One of the many interesting things that has come out of this annotation project,” he adds, “is that diatoms are able to show us how life on our planet diversified in evolutionary history.”
The diatom’s genome provides a rare picture of how ancient life forms changed genetically as they evolved. Diatoms were formed when two organisms – a single-celled animal and a bacterium that could photosynthesize – traded some of their genetic material, creating a new organism with a hybrid genome.
“When we compare the genomes of a plant and a diatom, we can actually look back in time to see how each one evolved along different paths,” he says.
Then there are those peculiar cell walls.
Diatoms have been making silica-based cell walls since between 120 and 180 million years ago, says Berges. Since their hard parts fossilize, they have left behind a long geological record. Because different diatoms prefer different environmental conditions, examining their skeletons in lake sediments is one of the most important ways of learning what climate was like in the distant past.
Exactly why diatoms adopted their hard cell walls is not known. After all, glass doesn’t seem advantageous to an organism trying to stay afloat in the water and get enough sunlight to photosynthesize.
But the cell walls probably provide protection against viruses or animal predators, says Berges. The genome revealed other parts of the story, too. Modern plants don’t tend to accumulate fats like animals do, but the scientists uncovered genes in the diatom associated with storing and metabolizing fat. Fats are much less dense than other cell components, and help the diatom to remain buoyant.
What is still unclear, however, are the genetics behind how the cell walls
are formed,
says Berges. The project pointed to an unusual way the organism
metabolizes silicon. Learning more fully how diatoms are wired to create such
intricate microscopic structures can offer clues to fabricating our own at
the molecular level, he says.
“If scientists can figure out how they do it,” he says, “there is a huge potential in such knowledge in the fields of biotechnology and nanotechnology.”
For more on the project, log onto www.jgi.doe.gov/.
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