Earlier this spring, science writer Connie Barlow sent me an e-mail about an awe-inspiring scientific discovery. Scientists in Uruguay had stumbled across the first example of a photosynthetic organism living inside a vertebrate’s cells! Just as the nervous system of an embryonic salamander began to develop, a tiny algal species bloomed inside the amphibian’s cell membranes like little green flames next to its countless mitochondria. Usually, the immune systems of vertebrates ban such close interspecific relationships so this discovery seemed to challenge – once again – the dogma about the integrity of species. Here was a living green flame to “solarize” an otherwise common salamander, revealing what looks like the early steps in a co-evolutionary dance for two far-flung species.
The main characters in our Uruguayan drama: the spotted salamander, Ambystoma maculatum; the single-celled alga, Oophila ambystomatis. The stage: the tiny spaces in the amphibian’s cells adjacent to their multitudes of mitochondria. That space is key to understanding the possible outcomes of the play. Mitochondria are tiny organelles where cellular respiration occurs. These cytoplasmic powerhouses convert simple sugars to an energy-rich compound called ATP. Thus, the viridescence in the salamander’s cells makes sense. The flare is a bloom of algae, probably drawn to a release of nitrogen-rich waste from the embryo’s cells, that provides – in turn – oxygen and carbohydrates needed by the mitochondria to “burn” glucose into ATP.
Such intimacy between an autotroph (sugar-producer) and a heterotroph (sugar-eater) had been noted in invertebrates such as corals and sea slugs, but never before inside a vertebrate’s cells. So what does this discovery of little green flames and solar salamanders teach us about modern-day biodiversity?
The lesson is a simple one. Our cells disclose a 3.5-billion-year-old framework wrestled from fortunate, but unplanned, reciprocity in the madcap chemistry of ancient times. Darwin himself encapsulated the awesomeness of this lesson by stating, humbly, “There is grandeur in this view of life.” Furthermore, evolution moves forward by taking surprising twists and turns as Earth’s 30 million species dance together on our whirling blue-green mote.
At this point, we must question the meaning of that otherwise ineffable term in biology, species. Biologists usually employ a seemingly straightforward definition: a species is a population of similar organisms that can interbreed successfully. One of the problems with this definition is that it equates species with a single behavior, i.e., reproduction. But think about all other activities in which the individuals of a given species engage during their lifetimes: for example, all the wondrous variety of race, culture, and creed for our own species, Homo sapiens. One species of human, one species of dandelion, one species of monarch butterfly, one species of northern red oak, and so forth, adding up to 30 million different kinds of life forms on Earth, each with a range of genetic diversity ready to go off like a cocked pistol to interact with its brethren species in the economy of nature. So how can we reckon the merits of a species just by its reproductive success? As it turns out, the word, species, may be as indefinable as life itself. Like the 1964 admission of Supreme Court Justice Potter Stewart about pornography, we may know a species when we see it, but that’s about the limit of our concreteness.
With this discovery in Uruguay, we see that species are dynamic entities that may or may not cohabit with others in the most intimate ways. This discovery forces us to answer, philosophically, “What is an individual?” In other words, is the salamander an individual or not? If it’s made up of salamander cells and algal cells, is it then two organisms? If they’re two organisms at first, but later their interrelationship becomes essential, then are they considered a single organism? By extrapolation, are we humans seven billion individuals or seven billion communities since our mitochondria, originally, were separate and distinct organisms? As a transgenic animal, am I an “I” or a “we”?
As researchers Timothy Allen and Thomas Hoekstra wrote in their 1992 book, Toward a Unified Ecology, “nature is continuous.” What will become of this viridescent salamander is anyone’s guess. Any perceived progression in evolution is always viewed with the twenty-twenty certainty of hindsight. But you can bet that this kind of interspecific “hanky panky” is not exclusive to this relationship, but may instead be a tenet for life itself as it struggles to survive any way it can. Green flames and solar salamanders are but two awe-inspiring colorful combinations in life’s palette. They also represent two reasons why I’m so exhilarated by life’s messy exuberance, part of Darwin’s “entangled bank” of discovery.