"We all breathe the same air," U.S. president John F. Kennedy said 50 years ago in a speech that marked a turning point in his administration's Cold War policy. But Mr. Kennedy might have added that we don't all breathe it the same way.
Now, in a series of studies published this week in the journal Science, three separate teams of researchers illuminate the ingenious adaptations that allow various creatures to maximize their oxygen intake in extreme environments such as at high altitudes or in the ocean's depths. The results reveal how the quest for oxygen has shaped animal evolution and allowed vertebrates to exploit a wide range of environments, and could help researchers develop such medical advances as artificial blood substitutes.
"These are insights into how oxygen delivery has become optimized under very different selection pressures," said Kevin Campbell, a professor of environmental and evolutionary physiology at the University of Manitoba.
Oxygen is essential for all animals because it plays a key role in the chemical reaction that allows their cells to convert nutrients from food to energy. The emergence of multicellular life on Earth more than two billion years ago may be tied to the development of oxygen-based metabolism.
Dr. Campbell is a co-author of one of the studies. It looked at how diving mammals from whales to beavers conserve and use oxygen while submerged. Although the animals are diverse, the study revealed a common adaptation related to the structure of the protein myoglobin, which is found in muscle tissue, and is the pigment that makes meat red. Myoglobin binds to oxygen, and so acts as an oxygen storehouse for muscle cells.
The researchers found that all diving mammals have myoglobin with a structure that has been slightly altered by natural selection in a way that increases its overall electrical charge. The presence of that charge causes the myoglobin proteins to repel each other. This allows more myoglobin to be safely packed into a smaller volume since it reduces the chance that they will stick together and crystallize. More myoglobin means the diving mammals can bring more oxygen with them when they plunge below the surface. When coupled with body size, the innovation allows sperm whales, for example, to hold their breath for up to two hours.
"That's just out there," Dr. Campbell said. "These animals are operating at the limits of what's physiologically possible."
Although the modifications to myoglobin are not identical in all diving mammals, they play the same role. This suggests that evolution has converged, allowing very different animals to find separate paths to the same solution. The study also sheds light on the lifestyles of vanished species because it allows researchers to make reasonable estimates for how long an extinct creature could stay under water based simply on its body size and evolutionary lineage.
Another curious detail is that the work confirms that elephants, which have similarly altered myoglobin, must have evolved from an amphibious ancestor.
In another study, researchers looked at oxygen delivery in the circulatory systems of rainbow trout. In trout, as in the vast majority of fish, the oxygen-carrying protein hemoglobin is unlike that of other vertebrates because it can be induced to surrender oxygen when blood PH is lowered. The fish use this effect to move oxygen into an organ known as the swim bladder, which provides buoyancy, but the study revealed that it enables fish to withstand the stress of low oxygen levels in water – a huge survival advantage.
"Fish hemoglobin can deliver oxygen at least 20 times more efficiently than human hemoglobin can," said lead author Jodie Rummer, who is based at James Cook University in Queensland, Australia, but did much of her trout research at the University of British Columbia.
Dr. Rummer noted that the success of fish, which account for half of all living vertebrates, may have been sealed by their ability to adapt to an oxygen crisis in the world's oceans hundreds of millions of years ago. Marine vertebrates that could not adapt in the same way may have been driven to shore, laying the ground for the eventual emergence of humans and all other land vertebrates.
A third study revealed additional insights by looking at differences in oxygen binding between the hemoglobin of deer mice who live in the highlands of the Rocky Mountains and that of their lowland cousins.
Collectively, the studies may eventually help doctors improve oxygen delivery in humans with respiratory ailments, said Enrico Rezende, a senior lecturer in zoology at the University of Roehampton in London. "My guess is that, with protein engineering, it would eventually be possible to manipulate hemoglobin and myoglobin … to enhance their performance," he said.
"Oxygen is key to life," Dr. Rummer said. "The more that we understand all of the different adaptations that a wide array of living organisms utilize, the more we can understand how our own bodies do well or do poorly" in different circumstances.