|Photomicrograph of Acaryochloris marina. Source: Phototrophic Prokaryotes Sequencing Project|
Researchers at Washington University in St. Louis and Arizona State University have sequenced the genome of the cyanobacterium Acaryochloris marina, the only well-studied photosynthetic species that contains chlorophyll d (Chl d) as the major photosynthetic pigment. Chlorophyll d absorbs “red edge,” near-infrared, long wavelength light.
The extension of Chl d absorption into the near-infrared, beyond the range of any other oxygenic photosynthetic organisms, could have immense agricultural consequences, noted the project team. If Chl d could be incorporated into higher plants, it has a potential capacity of increasing the energy conversion of sunlight by 5% compared to that of the chlorophyll a-containing organisms.
Robert Blankenship Ph.D., Lucille P. Markey Distinguished Professor in Arts & Sciences at Washington University, and principal investigator of the project, said with every gene of Acaryochloris marina now sequenced and annotated, the immediate goal is to find the enzyme that causes a chemical structure change in chlorophyll d, making it different from primarily chlorophyll a, and b, but also from about nine other forms of chlorophyll.
The synthesis of chlorophyll by an organism is complex, involving 17 different steps in all. Some place near the end of this process an enzyme transforms a vinyl group to a formyl group to make chlorophyll d. This transformation of chemical forms is not known in any other chlorophyll molecules.—Robert Blankenship
Blankenship said he and his collaborators have some candidate genes they will test. They hope to insert these genes into an organism that makes just chlorophyll a.
We now have genetic information on a unique organism that makes this type of pigment that no other organism does. We don’t know what all the genes do by any means. But we’ve just begun the analysis. When we find the chlorophyll d enzyme and then look into transferring it into other organisms, we’ll be working to extend the range of potentially useful photosynthesis radiation.—Robert Blankenship
Blankenship and his colleagues from both institutions are publishing a paper on their work in the 4 February online edition of the Proceedings of the National Academy of Sciences. The work was supported by the National Science Foundation and also involved collaborators from Australia and Japan. Three Washington University undergraduate students and one graduate student participated in the project, as well as other research personnel.
Acaryochloris marina was discovered 11 years ago, living in a symbiotic relationship with a colonial ascidian (sea squirt) in Australia’s Great Barrier Reef. The Acaryochloris marina lives beneath the sea squirt, which is a marine animal that lives attached to rocks just below the surface of the water. The cyanobacterium absorbs “red edge” light through the tissues the sea squirt.
Acaryochloris marina lies down there using that far red light that no one else can use. The organism has never been under very strong selection 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.—Robert Blankenship
As a result, its genome is massive for a cyanobacterium, comprising 8.3 million base pairs, and sophisticated. The genome is among the very largest of 55 cyanobacterial strains in the world sequenced thus far, and it is the first chlorophyll d–containing organism to be sequenced.
Other related organisms are being discovered. Researchers from the University of Oregon and the University of Montana found a strain related to Acaryochloris marina during a biotic inventory of the Salton Sea in California. In contrast to A. marina, however, which lives in clean oceanic waters, Acaryochloris 2A51 lives in a hypereutrophic high salinity environment.
Phototrophic Prokaryotes Sequencing Project: Cyanobacteria: Acaryochloris marina