Astronomy and Astrophysics

"Photosynthesis and Astrobiology: Color Limits for Life Adapted to Others Stars and Atmospheres."
Kiang, N.Y., Antígona Segura, Giovanna Tinetti, Govindjee, Robert E. Blankenship, Martin Cohen, Janet Siefert, David Crisp, Victoria S. Meadows, Steven Mielke, David Mauzerall.

Photosynthesis on Earth produces the primary signatures of life that can be detected by satellites at the global scale: atmospheric oxygen in the presence of liquid water, and the surface spectra of photosynthetic pigments. Because of its ubiquity on this planet, photosynthesis is expected to be a successful process also on planets in other solar systems, and will offer "biosignatures" that future space telescopes will be able to detect at their fuzzy resolution. But should we expect alien photosynthesis to be mostly green and produce oxygen? First we must answer why photosynthesis is the way it is on Earth. The key question is not exactly why plants are green, but why chlorophyll a, which is at the heart of oxygenic photosynthesis, absorbs in the blue and performs its job absorbing photons in the red. And why does it dominate over the Earth, in comparison to other photosynthetic pigments? The answer, we found, lies in a combination of the mechanisms of light harvesting at the molecular scale, and environmental pressures from long-term atmospheric evolution at the global scale: light harvesting favors funneling of energy from shorter to longer wavelengths; oxygen from early marine cyanobacteria altered the oxidative state and the light transmittance properties of the atmosphere. The result: enabled by their forebears, green plants gained the competitive advantage to dominate on land. So, we propose rules for what wavelengths the dominant pigment will absorb, given adaptation to other stars and atmospheres. We identified some plausible colors of alien photosynthesis, both oxygenic and anoxygenic by simulating the light spectrum incident on the surface and under water for Earth-like planets with atmospheric compositions equilibrated with the radiation of F, K, G (our Sun), and M stars. There are still some things that we don't know about photosynthesis, particularly how the water molecule binds to the oxygen evolving complex, and what is the theoretical longest wavelength limit for oxygenic photosynthesis. These are areas for current research.

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Astronomy and Astrophysics

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