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Researchers Sequence Rare Photosynthetic Microorganism

NEW YORK (GenomeWeb News) – Japanese, Australian, and American researchers have sequenced the genome of a rare photosynthetic microorganism that thrives on long-wave, far-red light, rather than the mundane red and blue light most of its contemporaries use.
The team, who reported their findings in the early online edition of the Proceedings of the National Academy of Sciences yesterday, sequenced the entire genome of Acaryochloris marina, an unusual cyanobacterium that employs a pigment called chlorophyll d — rather than the more common chlorophyll a — for photosynthesis.
“We now have genetic information on a unique organism that makes this type of pigment that no other organism does,” principal investigator Robert Blankenship, a biologist and chemist at Washington University in St. Louis, said in a statement.
A. marina is a type of cyanobacteria, or blue-green algae, found in marine environments in the South Pacific as well as terrestrial environments in Mexico and the Antarctic. The organism typically grows symbiotically with other organisms. For instance, in marine environments, A. marina often lives under marine animals called sea squirts, absorbing the far red light that has gone through the squirts’ tissues.
Its ability to do this stems from its unique photopigment, chlorophyll d. In general, plants or photosynthetic bacteria go through more than a dozen steps to create chlorophyll. Somewhere along the way, A. marina does this differently, substituting a formyl group for the vinyl group found in other chlorophyll molecules.
In an effort to characterize both A. marina and its unusual chlorophyll d photosystem, Blankenship and his team sequenced the entire A. marina genome, which is housed on one circular chromosome and nine plasmids, using Roche subsidiary 454 Life Sciences’ GS20 instrument.
The 8.3 million base pair genome — one of the largest bacterial genomes ever sequenced — seems to have expanded to provide the organism with new ways to adapt within the broader bacterial and algal community. This “fat and happy” genome, as Blankenship called it, appears to be related to the organism’s light-harvesting adaptation.
Acaryochloris marina lies down there using that far red light that no one else can use,” he said. “The organism has never been under very strong selective pressure to be lean and mean like other bacteria are. It’s kind of in a sweet spot. Living in this environment is what allowed it to have such dramatic genome expansion.”
Indeed, the cyanobacterium’s genome has several genes that seem to be related to this adaptation, including sensory kinases and response regulators. A. marina also contains several copies of genes facilitating DNA repair and recombination, which the researchers suspect could increase gene mobility and genome expansion.
But the largest group of ORFs — about 35 percent — code for hypothetical proteins of unknown function. “We don’t know what all the genes do by any means,” Blankenship said. “But we’ve just begun the analysis.”
Still, the researchers suggest that understanding — and perhaps transplanting A. marina’s photosynthetic system into other organisms — could ultimately increase the solar power available to other plants as well.
“When we find the chlorophyll d enzyme and then look into transferring it into other organisms,” Blankenship said, “we’ll be working to extend the range of potentially useful photosynthesis radiation.”

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