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Genomes of simple algae more complex than scientists thought

by Margaret Mroziewicz Apr 1 / 13

Photosynthesis is vital for life on our planet, and for decades scientists have been trying to understand how and why it evolved. Recent technological advances in DNA sequencing have made this possible and have allowed scientists to decipher some of the mysteries of this process.

Left: The cryptophyte alga Guillardia theta. Image credit: Dr. David Hill, University of Melbourne, Australia. Right: The chlorarachniophyte alga Bigelowiella natans. Image credit: Dr. Geoff McFadden.

For the first time ever, a team of scientists from 27 labs in 10 different countries sequenced the entire genome of two common algae—from the cryptophyte and chlorarachniophyte lineages —which photosynthesize and live in oceans. The researchers were interested in better understanding the evolutionary history of photosynthesis in these organisms and were surprised to discover just how complicated their genomes were. Their findings were reported in Nature.

“We know that photosynthesis first evolved in simple organisms like cyanobacteria, which then started living inside the cells of more complicated organisms, which in turn were swallowed up by still other organisms,” says Senior Fellow John Archibald (Dalhousie), who led the team. “It’s analogous to a set of Russian nesting dolls. But over time, as an organism spends more time living within another organism, its DNA gets shared and swapped with its host—soon enough the lines between the two organisms become blurred. Knowing this, we were still very surprised that the ‘master’ genomes of the cryptophyte and chlorarachniophyte algae were a complex mosaic of the different organisms that have given them the ability to photosynthesize.”

The algae that the team studied are also unique to biology because in contrast to most plants and algae, they still contain the nucleus (called a nucleomorph) of the previously ingested organism. Normally, the nucleomorph disappears once its essential genes have been transferred to the master nucleus of the host cell.

The researchers, which also included Fellow Claudio Slamovits (Dalhousie), Director Patrick Keeling (UBC), and Advisor Michael Gray (Dalhousie), were interested in understanding why the two algae still had their nucleomorph and found that it was because the algae had lost their ability to move DNA from the nucleomorph to their master nucleus. As a result, the nucleomorph never disappeared.

“Some of the most abundant organisms that evolved photosynthesis in this manner are incredibly important in driving the biogeochemistry of Earth,” explains Dr. Archibald. “By studying them, we are not only getting insights into the basic biological principles that have governed the evolution of life, but we are also learning about how the climate and atmosphere came to be the way it is.”

Since this study is the first main glimpse into the gene complement of two important algal lineages, many biologists, geneticists and evolutionary biologists will be using the findings from the paper as a resource for their own work. For example, scientists interested in developing algae biofuels will most likely mine the genomes to look for genes that have important enzymatic properties.

This work was supported by the Office of Science of the US Department of Energy, the Gordon and Betty Moore Foundation, and the Natural Sciences and Engineering Research Council of Canada.