Amoeba: Secrets of the micro-farm

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Amoeba: Secrets of the micro-farm
A glob morphs into a sombrero-like shape, then into finger-like, finally into the globe-on-stem shape

The single-celled amoeba Dictyostelium discoideum has no brain, but its complicated social cycle enables farming.

Amoeba, single-cell, shape-shifters that eat bacteria and live in the dirt, don’t get much respect. When they run out of food, they gang up and move their sorry selves to greener pastures.

Pastures with edible bacteria, that is.

If ever a creature needed re-branding, this is it.

Could labeling amoeba as farmers boost their brand? In the human realm, farming gave rise to cities, writing, metallurgy and the computer in front of your face.

Amoeba don’t use the Internet. And although they do have a cell nucleus, nobody claims they have an ounce of smarts.

But now we know that some amoeba move “seeds” of bacteria to a new location and plant them as a food source. In other words, they farm.

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Here’s how dicty divides, in images made 10 seconds apart.

Ants grow fungus. Termites and some saltwater snails do ditto. Damselfish grow algae. But until now, nobody has identified any life form that “farms” bacteria, and nobody has identified any single-celled farmers, says Debra Brock, a graduate student in ecology and evolutionary biology at Rice University.

Adds Brock, whose report on farming amoeba appears in Nature tomorrow, “Certainly there has never been an amoeba that’s known to farm.”

Bring on the rebranding!

Working with the well-studied amoeba Dictyostelium discoideum (“dicty” to you and me) Brock noticed that the fruiting bodies — reproductive structures that distribute the amoeba in new habitat — seemed to contain bacteria. That was odd, Brock admits. “To get anybody to believe me, I had to prove that the little spots were bacteria, and not an infection.”

When she spotted the sorus (mass of spores) on growth medium, colonies of bacteria grew on some of the plates — showing that about one dicty in three transports bacteria. The bacteria didn’t seem to be a harmful infection, since amoebas with and without bacteria grew similarly, she says.

She fed the shape-shifters antibiotic to kill their bacterial cargo, but when the amoebas resumed eating bacteria, some bacteria showed up in the sorus. Since this only happened with amoebas that had originally carried bacteria, Brock concluded that this was normal, healthy behavior for those amoeba, although she’s can’t yet say whether the bacteria are inside or alongside the amoeba spores.

Dozens on gold translucent globes on the ends of thin, string-like stems

Photo: Scott Solomon
Fruiting bodies of the amoeba Dictyostelium discoideum contain bacteria and spores of amoebas. Each sorus is attached to a single slug, comprised of about 100,000 individual amoebas.

Wild about amoeba

The project began when Brock was studying wild amoeba rather than a strain that had been living in labs since the 1930s, and she noticed that some clones consistently carried bacteria.

Brock says dictys are “social amoeba” because “they have a structured society, and can exist in two states.” Individual amoebas in the soil eat bacteria, divide and eat some more. So long as edible bacteria are available, “they are perfectly happy to do this,” says Brock. “But if they use up all the food, they start talking to each other with chemical signals: ‘Wow! There’s not enough food!’ And then approximately 100,000 come together to form a slug.”

Development in a social amoeba

Flat translucent globe with tentacles coming out from itA translucent slug-like organism on left, globular organism with slug emerging from its top on rightTranslucent slug crawling

Image credits (Above, L-R): Bruno in Columbus (1, 2, 3), and below: David Brown & Joan E. Strassmann (4). Click any image to enlarge

Social: Aggregation of many single cells morphs into mound, then finger, slug, hat, fruiting body, and spores. Vegetative: cycle with cell division but nothing fancy.

Thousands of dicty amoebas are merging to form a slug that can wander to find food. Three photos show part of the amoeba’s social cycle, which is shown in its entirety in the last panel. Last panel shows the social and vegetative cycles of Dictyostelium discoideum.

The slug serves as a truck to haul amoeba to new territory, Brock says. “During the multi-cellular part of the life cycle, they are starving, and they want to go somewhere else.”

These amoeba transport bacteria to a new location and plant them as a food source.

The slug eventually shoots up a stalk containing amoeba spores, and among the farmers, bacteria. When the sorus opens, the bacteria can plant themselves as amoeba food.

Reminds us of Johnny Appleseed

The Darwinian decision

Why does the same species of dicty use two survival strategies? Why do some farm while others don’t? “It’s a smart evolutionary strategy,” says Brock. “It’s bet-hedging. If you happen to land in a patch without bacteria, farmers have a great advantage because they bring their food with them, which allows them to grow and divide and bear a huge number of progeny while the poor non-farmers have nothing to eat.”

But while the farmers quit eating before they remove all bacteria from their old location, non-farmers can eat all those bacteria, so non-farmers do benefit if the new home already contains edible bacteria.

Apparently, both strategies work, because both have survived the evolutionary gauntlet. Brock is exploring whether a “farmer gene” causes some amoeba to hoard bacteria…

It’s enough to give a person a new respect for protozoans, which offers a firm basis for rebranding. “From quite a long time ago, we’ve thought we are so special,” says Brock, “but you can’t imagine the number of genes the amoeba has that are just like human genes. It’s scary; it takes you down a notch or two.”

— David J. Tenenbaum

