In today’s Academic Minute, Dr. Nancy Kiang of Columbia University’s Earth Institute explains a recent discovery that hints at the potential color of extraterrestrial plant life.
Nancy Kiang is a research scientist at Goddard Institute for Space Studies, an affiliate of Columbia University’s Earth Institute, where she conducts research on the interaction between the biosphere and the atmosphere, focusing on life on land. She also relates this work to research in astrobiology, particularly with regard to how photosynthetic activity produces signs of life at the global scale and how these may exhibit adaptations to alternative environments on extrasolar planets, resulting in other "biosignatures" that might be detected by space telescopes.
Dr. Nancy Kiang – The Color of Extraterrestrial Plants
As kids, we learn that we owe the air we breathe to the greenery around us. Chlorophyll allows plants (and algae and cyanobacteria) to perform the magic trick of photosynthesis. It absorbs energy from visible light to split water molecules—and splitting H2O is what releases oxygen into our atmosphere. Chlorophyll looks green to us because it absorbs just a little less green light than blue or red.
A century ago, astronomers looked for green reflectance patterns to try and detect life on Mars. Now, we know that the more remarkable color of plants is not that visible green, but an invisible wavelength range: the near-infrared. Plants strongly reflect near-infrared photons, because they’re lower in energy, so plants don’t use them. Satellites can see this bright reflectance in the near-infrared to tell us where vegetation is on our planet.
But will plants on other planets necessarily be like on Earth? Conventional chlorophyll is so ubiquitous on our planet, that for a long time, scientists thought that only visible light had enough energy to split water and produce oxygen from photosynthesis. Recently, though, Japanese researchers discovered a strange cyanobacterium named Acaryochloris marina, which lives mostly on far-red and near-infrared photons, but it still makes oxygen. It uses a different variety of chlorophyll, called chlorophyll d. Recent research that my colleagues and I have done shows that Acaroychloris doesn't struggle at all on these low-energy photons; it’s just as efficient, or more so, than the organisms we’re familiar with.
Acaryochloris helps us to figure out rules to predict what photosynthesis might evolve to look like on another planet. So if we see it, we’ll be able to say, there’s photosynthesis on that planet, there’s other life out there! Like, planets orbiting red dwarf stars may not get much visible light, but they'll get a lot of near-infrared light, so maybe alien plants will evolve to be more like Acaryochloris. This has implications for the future evolution of life on Earth, too. The genetic tree of Acaryochloris suggests it evolved relatively recently, probably because it adapted a light niche resulting from leftover photons from the regular chlorophyll organisms. And, because it absorbs both visible and near-infrared photons, it can make use of a fifth more of the solar spectrum. Could chlorophyll d organisms some day outcompete today’s plants and take over the world? Maybe we could use chlorophyll d to improve crop productivity, or to harness more solar energy. Thus, looking for life in outer space can make us find out things that can improve life here back home. Now that’s pretty far-out.