For sugar, tiny thieves kept algae captive

U. MELBOURNE (AUS) — Microscopic animals held algae hostage and stole their genes for energy production, thereby evolving into a new and more powerful species many millions of years ago.

The results, published today in Nature, reveal a “missing link” in evolution because the tiny animal thieves (protozoa) couldn’t completely hide all evidence of the captive algae, and have been effectively frozen in time and caught in the act by genetic sequencing.

The protozoa captured genes for photosynthesis—the process of harnessing light to produce energy that’s used by all plants and algae on earth—so the study also provides insight into the origin and repurposing of these genes and may be useful in algae biofuel production.


Study author Geoff McFadden, professor in the school of botany at the University of Melbourne, says that scientists had long suspected that quantum leaps of evolution occurred by one organism cannibalizing another, but we did not have much hard evidence.

But when they looked at two specific algae—Guillardia theta and Bigelowiella natans—the team realized the evolution was not quite complete. They could see that their cells had two nuclei (the “control centers” of cells that contain DNA). This is unusual because plant and animal cells only have one, so the genes were sequenced to find out more.

“We think that the genes for photosynthesis originally evolved only once about three billion years ago. So all plants, algae and blue green bacteria can produce their own energy from light because they have acquired these genes for photosynthesis,” McFadden says.

Like prisoners in Alcatraz, the captive algae appear to have been nurtured by their keepers and the precious sugars produced from photosynthesis became a vital part of the protozoan captors’ diet.

The captives lived inside the protozoan cell and, under the right conditions, the pair gradually became unified as a single organism—a process called endosymbiosis, literally living inside each other.

“We discovered that the captors were initially able to keep many separate clones of their slaves and occasionally pillage one or two for most of the essential genes. However, at some point in time, the number of captives reduced inside each jailer to just one individual,” says McFadden.

“So if they broke into the alga’s cell to steal the last essential genes, they would destroy it in the process and would not then be able to use the genes to run photosynthesis.

“So the two cells, one captive and one captor, had apparently reached an evolutionary stand-off situation where both are dependent on each other to survive.”

Researchers from Dalhousie University led the study, to which investigators from the University of Toronto, University of Washington, and other institutions in the Czech Republic, France, Germany, Japan, the UK, and US contributed.

Source: University of Melbourne