The Single-Celled Organisms That Farm Bacteria

By Alan Boyle on Sunday, November 24, 2013
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“Cultivators of the earth are the most valuable citizens.” —Thomas Jefferson

In A Nutshell

The Scottish anthropologist Arthur Keith said “the discovery of agriculture was the first big step toward a civilized life.” The revolution that gave birth to civilization is very human, so it’s not the sort of thing to be expected in other creatures. Yet scientists have found amoebae (single-celled organisms) that farm bacteria for food. They carry the bacteria, seed them, and harvest them. The farmers also guard their crops from amoebae that don’t do it themselves.

The Whole Bushel

Amoebae have been used for research for many years in laboratories. One species, Dictyostelium discoideum, was isolated in 1933. Since then, it’s gotten used to living in test tubes and petri dishes, so a couple of biologists from Houston’s Rice University decided to have another look at wild specimens of Dicty (as it is nicknamed). What they found was surprising, and it took a while for their peers to be convinced.

The amoebae were carrying around bacteria. Not only that, they were keeping bacteria aside so they always had a reserve to use to grow new food. These were very much like “seeds.” While carrying food around with you makes some sense, it means they only eat half of what’s available. That’s a disadvantage when around non-farmers, who are happy to steal all the yummy bacteria from everywhere.

To combat this, the farmer amoebae also carry around inedible bacteria that are toxic to non-farmers. The amoeba reproduce by collecting together and forming a stork. A small orb at the top of the stork releases spores, which grow into new amoebae. Chemicals produced by the “guard” bacteria allowed the farmers to produce greater numbers of spores than the competition. The presence of the bacteria also seems to help the farmers grow in other ways, but the scientists have yet to figure it out.

Further research found an even more surprising factor—the bacteria involved were originally inedible. The bacteria actually evolved to allow themselves to be eaten, with a mutation that knocked 10 percent of their genome out of action. While that may seem a flawed plan in general, it meant that their kin were being carried around and seeded in new places. Overall, having some members devoured was good for the species.

Joan Strassmann, one of the main biologists behind the research, said the key to finding such an unusual trait (in an organism that has been studied for decades) is to be open-minded. “When you see things that don’t quite fit the standard story, a good scientist will not try to jam them into the story, but instead recognize something different is going on and try to figure it out,” she noted.

Show Me The Proof

Amoebas farm bacteria and carry guards to protect their crops in the wild
PNAS: A bacterial symbiont is converted from an inedible producer of beneficial molecules into food