Cracking the Whip Has Many Surprising Benefits
When single-celled organisms such as sperm crack their whip-like appendages called flagella, the beating sets them in motion. But in certain colonies of green algae, flagella also boost nutrient uptake, according to surprising new research.
In the early online edition of the Proceedings of the National Academy of Sciences, researchers from the University of Arizona and Brown University explain how flagella allow these algae to get the energy they need to multiply and create colonies – the critical secret that allowed them to evolve into multicellular organisms.
“This is the first evidence that flagella not only help organisms move, but can help them feed at a rate that allowed them to evolve to a larger size,” said Thomas Powers, an assistant professor of engineering at Brown who studies microorganisms in motion. “This is a critical piece of information, since understanding how one-celled life forms evolve into many-celled ones is a fundamental question in biology.”
The team studied a group of green algae known as the volvocines, organisms so common they can be found in puddles of rain. Biologists study the group, which runs the gamut from single-celled organisms to teeming colonies, to understand how cells differentiate and multiply. But how did the volvocines jump from solo cells to Volvox, a colony of as many as 50,000 cells?
It’s a puzzler of a question, given the size of a Volvox colony and the laws of physics. Bigger organisms need more energy – a lot more energy – to survive. And Volvox is the largest colony that the volvocines make, a giant ball of flagella-waving body guards protecting a small cluster of reproductive cells. When the radius of the spherical colony increases by a factor of two, the area of the sphere increases by a factor of four. So it follows that the energy demands for Volvox would quadruple, too, as it grows.
Yet microscopic organisms such as volvocines get nutrients through diffusion, a process by which bits of food bump into the cell and pass through the cell membrane. Doubling the radius of the colony doubles – not quadruples – the colony’s food intake rate. So a large organism such as a Volvox colony shouldn’t survive because it would demand more energy than passive feeding could supply, a conundrum that researchers refer to as the “bottleneck problem.”
The research team had a hunch that flagella somehow played a role in bringing in nutrients needed for Volvox to grow and survive. Raymond Goldstein, a professor of physics and applied mathematics at the University of Arizona, gathered together a group of scientists with expertise in physics, mathematics, engineering and biology to work on the problem.
The team created a mathematical model that allowed them to calculate how the flagella created a flow of water around the colony and verified this prediction with experimental measurements. Then they used the model to show that the coordinated beating of the flagella concentrated the nutrients just ahead of the moving colony. The colony plows into this nutrient-rich region and leaves a plume of waste in its wake.
So a Volvox colony doesn’t just passively feed, it actively increases the concentration of nutrients around it using its flagella. Put another way, these tiny protein whips not only acts as legs, but also as arms, gathering in food the colony needs to grow and thrive.
Powers, brought in to help with biomechanical theory, said the surprise in the finding is that the nutrient current created by Volvox was proportional to the surface area of the colony. In other words, Volvox met its rapidly increasing demand for nutrients through flagellar beating, allowing the organism to make the multicellular leap.
“Previous models would have predicted that the nutrient demands of Volvox would outstrip the supply,” Powers said. “But we showed that metabolic supply can, in fact, keep up with metabolic demand. The colony beat the bottleneck problem. Its increasing size is actually an advantage, allowing it to create a faster flow of nutrients.”
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