Optimizing photosynthetic light-harvesting under stars: Generalized thermodynamic models

Since the law of physics and chemistry are universal, we must be able to use them to determine the feasibility and potential characteristics of photosynthetic life on exoplanets. Photosynthesis is the cornerstone of Earth’s complex biosphere and creates bio-signatures pivotal in the search for life beyond Earth. However, stars of lower temperature (𝑻𝒔) diminish the intersection between the irradiance at an exoplanet's surface and the wavelengths of light (400 nm ≤ 𝝀 < 700 nm) required for oxygenic photosynthesis, which includes Earth’s plants and cyanobacteria. This does not automatically preclude photosynthesis around such stars, since Earth's photosynthetic organisms have developed highly-specialized light-harvesting structures, known as antennae, to overcome extremely light-limiting conditions. Here, we construct a generalized thermodynamic model of a photosynthetic antenna and evaluate photosynthetic performance across various stellar spectra. We demonstrate that the light-harvesting process, entailing the capture of light across a broad area and channeling it to a small reaction centre, encounters an intrinsic entropic barrier that constrains its efficiency. Through structural and energetic optimizations of our model, we uncover strategies to alleviate the entropic barrier, akin to those seemingly adopted during the evolution of cyanobacteria. We propose that a cyanobacterial-like antenna is thermodynamically optimized even for the coolest M-dwarf stars (𝑻𝒔 ∼ 2300 K) while a plant-like antenna would face severe entropic constraints. By incorporating thermodynamics into biology, our models help linearize the search for photosynthesis based on the stellar spectra an exoplanet receives.