The Full Palette of Photosynthesis
Plants and other photosynthetic organisms use special molecules for absorbing light. These pigments have a distinctive color, or spectrum, that is known to leave an imprint on the light reflected off our planet’s surface. A new database catalogues the diverse palette of light-absorbing biological molecules on Earth in order to better predict what the photosynthetic signature might look like on other planets.
Life on our planet has adapted to the light from our Sun. Plant photosynthetic pigments, for example, use the Sun’s abundant visible light – most strongly absorbing in the blue and red part of the spectrum, which is why they appear green in reflected light.
Plants also strongly reflect infrared light. The abrupt change in plant reflectance between infrared and red wavelengths is called the vegetation red edge (VRE). The VRE is so pronounced that it can be detected remotely by satellites. Some scientists have conjectured that an alien civilization might know Earth is inhabited by recognizing the VRE.
"The VRE is unique — there are no minerals that look like it," says Nancy Kiang from NASA’s Goddard Institute for Space Studies.
But plants aren’t the only organisms that use sunlight on Earth. Kiang studies various photosynthetic bacteria that have an entirely different set of pigments for absorbing light. She compares the spectra from these pigments to try to understand what mechanisms drive the evolution of the light harvesting ability.
"Every photosynthetic pigment has to accommodate both environmental and molecular constraints," Kiang says.
What this tells us is that photosynthesis is not a one-size-fits-all solution, and the pigments used on a different planet will likely be adapted to local conditions. To help imagine what these alien pigments might be, Kiang has started the Biological Pigments Database. This contributes a biological component to the NASA Astrobiology Institute Virtual Planetary Laboratory’s Spectral Database, which brings together spectral data stellar radiation, molecular line lists for atmospheric radiative transfer, and now biological pigments.
"I want [the pigment database] to be a community resource that can help in modeling the potential biosignatures from other planets," Kiang says.
On our turf
The basic purpose of a photosynthetic pigment is to turn light energy into chemical energy, but not all pigments work in the same way.
Different pigments can be distinguished by the set of wavelengths at which they absorb light. This spectrum can be thought of as the solution to two constraints: the energy available and the energy needed. In other words, a pigment must be well-suited to the particular light conditions in which its host organism lives, and it must supply enough energy to drive the chemical reactions that its host organism relies on.
The most familiar pigment is chlorophyll. There’s actually a handful of different kinds of chlorophyll, but the most essential one is chlorophyll a, used by plants, algae, and cyanobacteria. Chlorophyll a absorbs most strongly in the violet-blue and orange-red part of the spectrum, which is a natural choice for plants growing in direct sunlight.
Plants and algae use the energy absorbed by chlorophyll a to split water molecules. This splitting allows electrons from the water to be "donated" towards the reduction of carbon dioxide into carbohydrates (i.e. sugars). The byproduct of these reactions is oxygen, which is why the process is called oxygenic photosynthesis.
But this is not the only light-harvesting mechanism "under the sun."
Many bacteria don’t have the luxury of direct sunlight. They live in moderately deep water or under a mat of pond scum. In 1996, scientists discovered that a certain cyanobacterium species carries a unique pigment named chlorophyll d. This molecule absorbs far-red wavelengths, which makes it a good adaption to conditions under water where the only light is that left over from chlorophyll a organisms.
Other bacteria use sunlight for different chemical purposes. Splitting water – as plants do – requires a lot of input energy, so some organisms "make a living" off less demanding reactions. For example, green sulfur bacteria use hydrogen sulfide as an "electron donor" in place of water. This process and others like it are referred to as anoxygenic photosynthesis because no oxygen is produced. The associated pigments are called bacteriochlorophylls.
From plants to planets
The Biological Pigment Database contains spectral information for chlorophylls and bacteriochlorophylls, as well as for other accessory pigments that absorb light energy and transfer it to the main chlorophyll pigments responsible for photosynthesis. In addition, it includes biological sunscreen compounds that photosynthetic organisms produce to protect against excessive or harmful radiation, and carotenoids that serve the roles of both anti-oxidants and light harvesting pigments.
Currently, the database has absorption spectra for about 50 pigments, including extracts in solvents and pigments bound in intact membranes. Absorption is important for biophysicists studying how organisms process sunlight, but Kiang is also trying to add more data on reflectance spectra from various intact biological samples, since reflectance is what astronomers observe in their telescopes.
"The database can be used by scientists who are searching for spectroscopic signatures of life on exoplanets," says Jonathan Lindsey of North Carolina State University, who has contributed spectra to the database.
These signatures can be modeled by inserting different pigment data into a planetary model. For example, one could imagine a planet full of hydrogen sulfide and covered by green sulfur bacteria, or a similar type of organism. Using the relevant pigment spectrum from the database, researchers can predict what the light might look like reflecting off the surface of such a planet.
We don’t know what pigments an alien photosynthesizer might use, but Kiang thinks the molecules may not be all that unfamiliar to us. She says biophysicists have studied the criteria for a good photosynthetic molecule, and their analysis seems to suggest that these pigments may converge towards specific designs.
For example, chlorophylls are built around ring molecules, called porphyrins, whose properties make them very good for energy transfer. It’s conceivable that alien organisms will incorporate porphyrins in their own pigments.
But Lindsey says we have to be careful about this type of speculation. Porphyrins and other details about Earth’s photosynthetic chemistry may merely be "a consequence of numerous contingencies and frozen accidents — i.e., workable but perhaps non-ideal solutions that became locked-in early on and have never changed in the course of evolution."
"It may be entertaining and even enlightening to speculate on the nature of alien pigments," Lindsey says. "But any meaningful discussion must take into account physics, chemistry, biology, and, as David Mauzerall [a pioneer in the study of photobiology] would say, the ‘fine-toothed comb of evolution.’"