Extrapolating the Rules for Life: Determining Alternative Photosynthetic Biosignatures Evolved With Other Stars and Atmospheres

Résumé : Photosynthesis - broadly, the utilization of light by life to drive biochemical processes - is so successful a process that it provides the foundation for virtually all life on Earth. Its presence is visible at the global scale, in our abundant atmospheric oxygen and in the wide distribution of chlorophyll over the Earth's surface. Since stellar radiation is such a ubiquitous energy source for a planet, we expect photosynthesis to be successful also on other habitable planets. However, to detect alien photosynthetic life, should we look for the same gaseous and pigment biosignatures as on Earth? We can obtain some clues as to what sort of extrasolar photosynthesis might dominate on another planet from the diversity of photosynthetic organisms on Earth and their path of evolution with our changing atmosphere: the constraints are both environmental resources (stellar radiation spectrum and intensity, carbon source, nutrients, and electron donors)and molecular mechanisms (light harvesting, redox requirements), and how the one is bounded by the other. This talk addresses: What are the likely rules for adaptation of photosynthesis to other stars and atmospheres? What has been the range of biosphere productivity from the Archaen to modern times on Earth? How much can the spectral characteristics of pigments vary? What do we need to know to determine is the long wavelength limit of photosynthesis, and of oxygenic photosynthesis in particular? An attempt to answer these questions is described through current work in progress on the thermodynamic efficiency of photon energy use in Acaryochloris marina, a unique cyanobacterium that uses chlorophyll d instead of cholorophyll a to perform oxygenic photosynthesis at wavelengths as long as 715-720 nm in the far-red/near infrared. ------------------------------------------------------------------------ Séminaire de Géophysique spatiale et planétaire ------------------------------------------------------------------------