Scientists studying the origin of life in the universe often focus on a few critical elements, particularly carbon, hydrogen, and oxygen. But two new papers highlight the importance of phosphorus for biology: an assessment of where things stand with a recent claim about possible life in the clouds of Venus, and a look at how reduced phosphorus compounds produced by lightning might have been critical for life early in our own planet’s history.
First a little biochemistry: Phosphine is a reduced phosphorus compound with one phosphorus atom and three hydrogen atoms. Phosphorus is also found in its reduced form in the phosphide mineral schreibersite, in which the phosphorus atom binds to three metal atoms (either iron or nickel). In its reduced form, phosphorus is much more reactive and useful for life than is phosphate, where the phosphorus atom binds to four oxygen atoms. Phosphorus is also the element that is most enriched in biological molecules as compared to non-biological molecules, so it’s not a bad place to start when you’re hunting for life.
In the second of the new papers, Benjamin Hess from Yale University and colleagues highlight the contribution of lightning as a source of reduced phosphorus compounds such as schreibersite. It has long been recognized that meteorites supplied much of the reduced phosphorus needed for the origin of life on Earth. But Hess thinks the contribution of lightning has been underestimated. For one thing, lightning was much more common early in our planet’s history. The authors calculate that it could have produced up to 10,000 kilograms of reduced phosphorus compounds per year—which may have been enough to jump-start life, especially because we don’t know how much of the reduced phosphorus from meteorites actually survives (in that form) the impact on Earth.
I find the Hess study intriguing, because if lightning really is so important, then we’re back to the “Darwin’s little pond” scenario for the origin of life that Miller and Urey tried to simulate in their famous experiment in the 1950s.
As for the possibility of life in the Venusian atmosphere, how has that claim held up in the seven months since the original paper was published? Well, it’s still controversial. Some scientists claim that the reported observations actually misidentified sulfur dioxide. But another recent paper claims to have found phosphine in old data from the Pioneer-Venus mission. The recent controversy prompted the original authors to re-examine their claim and correct the amount of phosphine down to 1-4 parts per billion globally, but otherwise they stood by their detection.
If we assume the measured phosphine concentrations at Venus are real, either we have to (A) propose an unknown way for this element to be continuously produced, which is difficult in an oxidizing atmosphere, or (B) to attribute it to biology, which also is difficult to fathom given the scarcity of water and hyper-acidity in the Venusian atmosphere. In fact, any life forms would have to rely on biochemical adaptations that are unknown on Earth.
In other words, there is no clear answer after seven months. But in a recent paper I offer a few suggestions for how to move forward. The first goal should be to confirm the phosphine detection by looking in the infrared spectrum, and also to search for related phosphorus compounds, which should be present if phosphine is there. Then I suggest conducting lab experiments to test how well selected acid-loving microbes can adapt over several generations to the extremely high sulfuric acid concentrations found on Venus. Could they also survive without the trace metals found on a planetary surface, which would be missing in the Venusian clouds? Finally, can we think—at least theoretically—about possible methods of biochemical adaptation in such an alien environment?
However the Venus phosphine debate turns out, the results of these investigations would be interesting and relevant to searching for life on exoplanets. And either way, phosphorus will continue to be critical to our thinking about life on Earth and beyond.