Though at some level it’s a question of semantics, the debate also touches on the deeper question of what it means to be alive.
The intracellular parasites that Keeling studies, which also have reduced genomes and are unable to produce their own source of energy or survive without their hosts, are typically thought of as organisms. “But no one refers to mitochondria as an organism because it’s so integrated with its host,” he said.
The difference is that Keeling’s parasites steal energy, in the form of a molecule called adenosine triphosphate or ATP, from their hosts, but they possess the necessary genes to replicate DNA. An organelle, on the other hand, relies on proteins provided by the host to replicate its DNA. “We have arbitrarily decided that stealing ATP from a host constitutes an organism and stealing proteins does not,” said Keeling. “It’s truly just a matter of degrees.”
Reprinted with permission from Simons Science News, an editorially-independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the computational, physical and life sciences.