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Understanding photosynthesis by sequencing two algae genomes

The algae floating in the sea are microscopic plants of great consequence on a global level. They conduct a big chunk of the world's photosynthesis (turning sunlight into chemical energy); they control the carbon cycle (taking in carbon dioxide from the atmosphere and locking it away), and they form the base of the ocean's food pyramid, allowing other plants and animals to flourish.

To gain a better understanding of how algae do all this, a large team of scientists led by the U.S. Department of Energy Joint Genome Institute and the Monterey Bay Aquarium Research Insitute (MBARI) have unraveled the genetic code of two Micromonas algae: one from the South Pacific and one from the English Channel. The tiny plants, just two micrometers in diameter or roughly 1/50th the width of a human hair, boast genomes containing approximately 10,000 genes.

The first surprise in their findings presented in Science today: Geographically disparate colonies of the same algae have less in common genetically than, say, humans and chimpanzees. MBARI microbiologist Alexandra Worden ascribes that genetic disparity to the algae's ability to thrive in a wide variety of marine environments: Micromonas is one of the few species of algae that lives in all oceans from the poles to the tropics. But it also means that this genetic diversity between English Channel and South Pacific individuals "supports their classification as distinct species," the paper notes.

By comparing the Micromonas genome to the genomes of other species of the dozen or so algae already sequenced, scientists may be able to puzzle out the evolution of photosynthesis as well as its spread from tiny plankton to land plants. The Micromonas for example has a bigger genome than its cousin Ostreococcus—both so-called prasinophytes which are generally judged the earliest of the aquatic plants. Those genes seem to enable Micromonas to tolerate more toxics, such as heavy metals, as well as to build a tail that propels it through the water at 50 body lengths (i.e. 2 micrometers) per second. Pretty speedy for a tiny swimmer in salt water.

The results also suggest something that has never before been observed by scientists: Micromonas may reproduce sexually. "The prasinophytes are thus a lot more complex than previously believed," molecular biologist John Archibald of Dalhousie University notes in an accompanying commentary.  It's also possible that the tiny Micromonas are picking up genes from unrelated species—so-called horizontal gene transfer—a microbe level trick for survival in unfriendly seas.

Image: A.Z. Worden, T. Deerinck, M. Terada, J. Obiyashi and M. Ellisman (MBARI and NCMIR)

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