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One Singular, Single-Celled Sensation

Meet one of your long-lost relatives—a distant patriarch from whom we all descend. And not just we humans. Michael Atkins suspects that lions, lobsters, mosquitoes, all fish, fowl, and fungi—every living thing besides plants that are composed of more than one cell—may all descend from the single-celled organism that he recently discovered living around hydrothermal vents on the seafloor.

Our common ancestor (the tiny guy magnified above) is called Ancyromonas, and Atkins has strong evidence that it is positioned on the evolutionary tree precisely at the momentous juncture where animals and fungi first branched out from unicellular life.

“If this organism turns out to be as important as we think it is, it holds the key to showing us what happened at a crucial evolutionary transition from unicellular to multicellular life,” he said.

Atkins found Ancyromonas in the course of research for his Ph.D., awarded in June from the MIT/WHOI Joint Program. In a way, Atkins was bound to discover something new and interesting because he undertook the first-ever survey of single-celled protists living near deep-sea hydrothermal vents.

Ever since the vents were first discovered in 1977, WHOI has led efforts to learn about the lush microbial life that thrives on sulfur-rich fluids emitted from magma-heated rocks beneath the seafloor. But those studies have focused on two of the three basic domains of life, bacteria and archaea. No one had ever explored single-celled members of the third domain: eukaryotes, which are distinguished by their nucleus-containing cells. Such deep-sea samples are notoriously difficult to collect and to culture in the laboratory.

“I had no idea what I would find,” Atkins said.
 
The organisms Atkins looked for are protists, single-celled eukaryotes equipped with flagella, or whip-like tails that they use to swim and to feed.

“These protists have huge ecological significance,” Atkins said. “They are integral to the marine food web. Bacteria and archaea are the most important organisms on the planet because they perform the essential biochemical reactions that convert sulfur to organic compounds, for example, or carbon dioxide to oxygen. But the organisms I work on exert important controls on the populations of bacteria and archaea by eating them.

“Protozoa consume bacteria and, in turn, are consumed by larger organisms, so they transfer energy and nutrients up the food chain,” he said. “They are also sloppy eaters, and they leave or excrete into the water dissolved, partially digested bits of bacteria—organic or inorganic materials that are used by other microbes.”

Atkins looked for protozoa in vent fluid samples collected by WHOI’s submersible Alvin. He expected to find new species that were adapted only to the unusual hot, high-pressure, high-sulfur conditions at the vents and would be found nowhere else on Earth. But to his great surprise, he mostly found the same species of protozoa that live in surface waters, throughout the ocean depths, along the coasts, and even in freshwater lakes.

“That realization led to a new hypothesis: that these organisms are ubiquitous because they have the ability to adapt to any conditions they encounter,” Atkins said.

He tested the theory in a series of laboratory experiments that exposed many types of deep-sea and shallow-water protozoa to the high-pressure and metal-rich conditions found at vents.

He was once again surprised to find no significant differences between deep-sea protozoa and those that live in shallow waters. All tolerated metal-rich fluids that would kill other life. He discovered that under high pressure, the protists “encyst”—that is, they draw in their flagella, enclose themselves within protective cell membranes, and become dormant. When Atkins reduced the pressure, the protists revived to become swimming, active participants in the food chain again.

“The findings offer an interesting theory to ponder,” he said. “Perhaps vent plumes can transfer encysted stocks of protozoa to higher levels in the water column, where the pressure is reduced and the cysts break down and the protists revive. This reversible encystment could be a mechanism to deliver protozoa back up to reseed populations that might have been decimated by adverse conditions at the surface. It would be hard and complex to prove, but if it’s true, it would have a large and important ecological significance.”

In the course of his research, Atkins used emerging molecular techniques to look for genetic differences among protozoa that are often impossible to distinguish by examining their physiology. That’s when he found Ancyromonas. Excited by the discovery, the National Science Foundation awarded Atkins a two-year postdoctoral fellowship to explore more fully its ancestral position in the tree of life. He will work with scientists at WHOI and the Marine Biological Laboratory in Woods Hole.

“It’s intriguing that this little guy could be the early ancestor of multicellular life in animals and fungi,” he said. “If we can pin that down, Ancyromonas will be very widely studied and will give us insights into where we came from and how we evolved to where we are now.”

Originally published: October 1, 2000