Photosynthesis on Alien Worlds: What might it look like, and can it be detected?

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In exoplanet surveys one of the key biosignatures to look for is the 'vegetation rededge', an enhanced reflectance at wavelengths >700nm, which on Earth signifiesoxygenic photosynthesis. Unfortunately, most known exoplanets orbit small stars withlimited emission in the 400-700nm range and tendency towards UV flaring. However,they might still support life. Earth's anoxygenic photoautotrophs exploit dim, nearinfraredlight (800-1000nm) and our biosphere is full of UV screening and repairmechanisms. Should we therefore be searching for signatures of strange UV-hardened,anoxygenic biospheres? This theoretical project will establish what these signaturesmight be. It will be supervised by theoretical biophysicist Chris Duffy (CD) and cosupervisedby bacterial photosynthesis expert Conrad Mullineaux (CM) andAstrophysicists Thomas Howarth (TH) and Edward Gillen (EG). Objective 1: With CDand CM the student will create a generalized systems model of photosynthesis, inwhich subsystems such as light-harvesting, photochemistry, repair, etc. form ametabolic network. This will predict the reflectance properties of the system based onthe spectrum of available light, and will be validated against known plants, algae, andbacteria. Objective 2: With CD, TH and EG the student will model the spectrum of lightavailable on typical exoplanets and adjust the photosynthetic model accordingly.Without worrying about biochemical details, we will determine whether a selfsustainingsystem is feasible. Objective 3: With CD, CM, TH and EG the student willhypothesize on biochemical realizations of the exo-photosynthesis model and simulatetheir potential reflectance signatures. This is constrained by diverse examples of Earthphotosynthesis and the universality of chemistry and physics. The outcome will be ageneralized framework for understanding photosynthesis, its relationship to light andits potential limits and possibilities. By exploring novel forms of photosynthesis inextreme environments, we can contribute to the search for extra-solar life byproposing new target bio-signatures.