Oxygen blew up ancient amoebas

Single-celled creatures' size spiked as oxygen levels rose

MINNEAPOLIS — Giant armor-clad amoebas that once swam Paleozoic seas may have owed their monstrous size to something in the water: oxygen. 

GOT OXYGEN? Fossil shells of Paleozoic amoebas, like the one shown here, grew to giant proportions as oxygen levels rose, suggesting that the gas fueled the creatures’ evolution. John Groves

A new look at the fossil record suggests that a spike in oxygen levels supersized many species of these fusulinids, a now-extinct type of single-celled microbe called foraminifera. About 300 million years ago, when the atmosphere contained almost enough oxygen to spontaneously combust, some of these critters grew to be 10 centimeters long. They would have been visible to the naked eye.

“Their average volume increased by at least factor of 100, maybe up to a factor of 1,000,” says Jonathan Payne, a paleobiologist at Stanford University who presented his team’s research October 12 at the Geological Society of America annual meeting.

Searching for evidence of oxygen’s influence, Payne and his colleagues recruited undergraduates and high school students to compare the intricate, multilayered shells left behind by more than 1,800 foraminifera that lived between 250 million and 325 million years ago. At first, the biggest species grew increasingly larger as atmospheric oxygen rose. After oxygen peaked at levels 66 percent higher than today and began to fall, the size of the creatures also declined.

The shapes of new, giant species fit with the idea that oxygen controlled their growth. Instead of ballooning like beach balls, the amoebas elongated. They grew no wider than about 2 millimeters — limited by how far oxygen could penetrate into cells after being absorbed, according to Payne’s calculations.

Some scientists believe that oxygen also boosted the size of insects (SN: 10/21/06, p. 270) and animals during the Paleozoic era. Fossils have revealed species of millipedes longer than a meter, amphibians longer than 2 meters and dragonflies with wingspans of more than 70 centimeters.

But tracing the evolution of size in larger organisms — and linking it to oxygen — has proven difficult because fewer of these species evolved into giants, says Phil Novack-Gottshall, a paleobiologist at Benedictine University in Lisle, Ill.

“You might have one or two species of giant centipedes from this time,” he says. “The advantage of foraminifera is that you have many giant species to work with.”

Payne also sees signs of oxygen influencing the size of modern amoebas. Among species living in waters off the coast of Australia, those that evolved to live close to the surface are bigger than those living at a depth of 500 meters, where oxygen concentrations are halved. “The trends we see in these modern species are quite similar to patterns we see in the fossil species,” says Payne.

These microscopic modern species are pretty puny compared to their ancestors. But some day, if a pulse of oxygen revs up their evolution, they might be giants once again.

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