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December 12, 2008

Regents Professor Peter Reich.

Regents Professor Peter Reich helped discover a basic feature of how all organisms use energy.

Photo: Patrick O'Leary

From microbes to mammoths, metabolism is remarkably steady

By Deane Morrison

A blue whale may not have a lot in common with bacteria, nor do most of us admit to kinship with a potted plant. But a recent study by University researcher Peter Reich and his colleagues indicates that in terms of our metabolic rates, we're a lot more alike than was thought.

It turns out that to keep alive, the myriad forms of life spend energy at rates that vary, on average, by only 30-fold. That's a far cry from previous ideas, which predicted that metabolic rates for the tiniest organisms would be 4,000 to 65,000 times those of the behemoths.

From mice to elephants and from people to poplars, the work puts us all closer, physiologically, than our sizes suggest.

The phenomenon likely reflects the common origin of all life, "just as all plants and animals have to obey gravity, too," says Reich, a Regents Professor of forest ecology. Natural selection may have favored organisms designed to fit in a compact range of metabolic rates; if so, the range could be optimal for all life on Earth.

In the study, the researchers examined data on 3,006 species, ranging from microbes to trees and mammals. The biggest, an elephant, had a mass 100 billion billion—that's a "1" followed by 20 zeroes—times bigger than the smallest, a species of bacteria; even so, metabolic rates ranged only from 0.3 to 9 watts per kilogram body weight. As long as the rates were basal metabolic rates for a species—measured in resting or adult organisms, not ones that are rapidly growing or searching for food—the pattern held.

That finding was astounding enough, but Reich and his co-researchers dug deeper, searching for the roots of metabolic rates across the whole of creation. Those roots lie in living cells, every one a mini-powerhouse that runs within a limited range of speeds. But how to sort out their metabolic rates when organisms also contain inactive material such as wood or stored fat?

Fortunately, Reich and other colleagues had found a means to do so. Cells that burn energy contain enzymes and other molecules that are rich in nitrogen, whereas inert material is poor in nitrogen. Therefore, metabolic rates calculated on the basis of organisms' nitrogen content rather than weight will more accurately reflect cellular metabolic activity from species to species.

When the team did this, the range of metabolic rates narrowed to about four-fold. An exception was larger vertebrates with variable body temperatures such as fish, amphibians, and reptiles, whose rates fell below the range.

The similarity in metabolic rates comes down to a common thread in how cells operate, says Reich.

"Cells are like little car engines. There are limits to how fast you can rev them up," he explains. "Even if the difference is between bacteria and elephants, they may have much more the same 'turbines' to perform metabolism than we would have thought."

Cells can be made to run faster with more food or other stimuli, but, says Reich, all life on the planet basically runs on a particular set of biochemical machinery that strips energy from carbohydrates such as sugars. Given that, "it's hard to get astronomical yields from any organism."

The study appears in the Proceedings of the National Academy of Sciences. Reich's co-authors are from the University of California, Riverside, and from national laboratories and universities in Russia and South Africa.

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