Based On Body Size, Bacteria And Elephants Have Similar Metabolism, Ecologists Find

Life scientists have long maintained that, based on body size, small organisms are more metabolically active than large organisms. But a new study led by Bai-Lian Li, professor of ecology at UC Riverside, shows that this is true only for organisms that are closely related evolutionarily and have body masses differing by no more than 6-7 orders of magnitude – about the difference in body mass between an elephant and a shrew.

For a pair of organisms that don't meet these conditions, that is, organisms that are not closely related evolutionarily and whose body mass difference exceeds the 6-7 orders of magnitude range, the researchers find that the small organism consumes about the same amount of energy per unit mass as the large organism: 1-10 watts per kilogram of body mass in the resting state of the organisms.

In other words, while metabolic activity per unit body mass varies within a group of organisms, such as mammals, it tends not to vary much when two diverse groups of organisms that differ greatly from each other in size are compared – such as bacteria and mammals.

“Our findings indicate there is a universal rate of energy supply per unit mass which can maintain life in organisms regardless of size,” Li said. “Living matter appears to be able to function at its own optimum rhythm, overriding various limitations imposed by the evolutionary increase in body size.”

Results from the study appear in the Oct. 22 issue of the Proceedings of the Royal Society: Biological Sciences.

The study is the first to compare bacterial metabolism with that of larger organisms, and the first to cover very diverse groups of organisms – from unicellular and multicellular organisms to plant leaves, insects and mammals.

Li explained that all living organisms have to transport energy obtained from food they eat to support the working of their internal organs, such as the brain or heart. The larger the organism, the further away are the organs from the body surface. “This makes energy supply more and more difficult for larger organisms,” he said. “For example, bacteria have to transport the obtained food over less than one micron, which is their body length, while the distance between an elephant's trunk and an organ such as its brain or heart is about ten million times longer. Despite this physical limitation of large size, elephants, we found, appear to be capable of supplying their tissues at a rate similar to that of tiny bacteria.”

So far, life scientists have held the view that the properties of living organisms are shaped by the changing external physical environment to which the organisms must continuously adapt. The new study posits, however, that living organisms are able to overcome the physical limitations imposed on them by their own physical properties and their external environment in order to maintain optimal, biochemical characteristics, such as the mass-specific metabolic rate the researchers studied.

The researchers’ analysis also shows that the rate of energy consumption per unit body mass declines with growing body size in groups of evolutionarily close organisms, such as mammals. For example, one gram of an elephant’s body uses up 25 times less energy than does one gram of a shrew’s body, accounting for why shrews have to eat more often than elephants. On the other hand, a bacterium, which is not closely related to an elephant in an evolutionary sense, consumes approximately the same energy per unit body mass as the elephant.

The researchers analyzed a total of 80 bacteria species and conducted work at UCR and St. Petersburg, Russia. Anastassia M. Makarieva of UCR and the Russian Academy of Sciences; and Victor G. Gorshkov of the Russian Academy of Sciences collaborated on the study. The U.S. National Science Foundation and Russian Basic Science Fund provided support.