Pond scum may seem like a mass of ungraceful goop, but look closer: That gunk could be in the midst of an elegant minuet. According to new research, tiny colonies of the common algae Volvox can whirl each other around for hours like ballroom dancers, driven by the rhythm of tiny, tail-like structures called flagella. The findings could shed light on how interaction between primitive organisms evolved.
Although some Volvox cells are loners, other species organize into translucent, spherical colonies that rotate as they swim. The colonies steer by coordinating flagella on thousands of individual cells. But Volvox is more than just a microscopic curiosity; it's a model for the study of how multicellular organisms evolved. By investigating the interactions within algal colonies, researchers hope to understand how single cells took the evolutionary leap to mass organization hundreds of millions of years ago.
Physicist Raymond Goldstein of the University of Cambridge in the United Kingdom and his doctoral student Knut Drescher took the question one step further. How, they wondered, do colonies interact with each other? They grew a colonial species, Volvox carteri f. nagariensis, in glass tanks, controlling for temperature so as not to create any currents. Then they began videotaping. Once after midnight, Drescher noticed something spectacular: Two colonies were hovering near the bottom of the tank, spinning like mad in a microscopic minuet (see video below). "I was so happy and wanted to tell everyone, but unfortunately the lab was completely empty and silent," Drescher says. He eventually decided to call the one person he knew would forgive the late-night interruption--his girlfriend.
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On the same film, the researchers observed another interaction, which they dubbed "the waltz." Here, colonies orbited around each other like planets near the top of the tank (see video below).
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According to the researchers' calculations, reported this week in Physical Review Letters, the colonies' flagella generate currents that interact with the top and bottom of the tank to create eddies of water. Those eddies draw the colonies together in dancelike whorls, a state that can last for hours.
"The algal dances are a result solely of the fluid flow that the algae create themselves," Drescher says. Such physical interactions could provide a basis for primitive social synergy between colonies, says Richard Michod, an evolutionary biologist at the University of Arizona in Tucson. "One can begin to ask questions about that social behavior," he says. "It's really interesting work."
The next step, Drescher says, is to grow Volvox in more natural conditions to see if the clusters provide an adaptive advantage during the sexual reproduction phase of the algae's life cycle.
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