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

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