Plant Biology

Thermodynamic Limits On Oxygenic Photosynthesis Around M-dwarf Stars: Generalized Models And Strategies For Optimization

By Keith Cowing

Status Report

astro-ph.SR

September 25, 2023

Filed under astro-ph.SR, botany, exoplanet, Habitable Zone, M-dwarf, Photosynthesis, Planet biology

Thermodynamic Limits On Oxygenic Photosynthesis Around M-dwarf Stars: Generalized Models And Strategies For Optimization — a. The incident spectral fluxes, fp (λ; Ts, asp) for a range of stellar temperatures, Ts. The dotted lines indicate the approximate absorption windows for oxygenic (Photosynthetically Active Radiation, PAR) and anoxygenic photosynthesis. b. The absorption profile of major plant antenna complex LHCII (green, digitised from [Kondo et al., 2021] using WebPlotDigitizer [Rohatgi, 2022]) shown with a model Solar (Ts = 5800 K) flux. The red-absorbing Qy-band is composed of Chl b (∼ 650 nm) and Chl a (∼ 675 nm) band and are approximated as Gaussian line-shape functions (dotted lines) in subsequent models. The RC absorption maximum (due to redshifted Chl a ) is at ∼ 680 nm (black dotted line) and assumed to have a negligible absorption cross-section compared to the antenna. c. An LHCII trimer, viewed from ‘above’ the plane of the membrane in which is sits. Chl b and a form two interpenetrating sublattices, facilitating rapid (∼ 1 ps) energy relaxation from the former to the latter (structure taken from [Liu et al., 2004], PDB structure 1RWT). d. The PSII supercomplex as viewed from the same perspective as for the LHCII trimer. A dimer of RC complexes receive energy by a modular assembly of LHCs, including core and minor (monomeric) antenna complexes plus LHCII trimers. Not shown are an additional ∼ 10 LHCII trimer that are loosely coupled to the supercomplex and form a disordered antenna pool in the membrane. Interprotein energy transfer (solid arrows) is 1-2 orders of magnitude slower than intra-protein relaxation (Chl b → Chl a , dashed arrow). — astro-ph.EP

We explore the feasibility and potential characteristics of photosynthetic light-harvesting on exo-planets orbiting in the habitable zone of low mass stars (<1 M⊙).

As stellar temperature, Ts, decreases, the irradiance maximum red-shifts out of the 400nm≤λ<750 nm range of wavelengths that can be utilized by \emph{oxygenic} photosynthesis on Earth. However, limited irradiance in this region does not preclude oxygenic photosynthesis and Earth’s plants, algae and cyanobacteria all possess very efficient \emph{light-harvesting antennae} that facilitate photosynthesis in very low light. Here we construct general models of photosynthetic light-harvesting structures to determine how an oxygenic photosystem would perform in different irradiant spectral fluxes.

We illustrate that the process of light-harvesting, capturing energy over a large antenna and concentrating it into a small \emph{reaction centre}, must overcome a fundamental \emph{entropic barrier}. We show that a plant-like antenna cannot be adapted to the light from stars of Ts<3400 K, as increasing antenna size offers diminishing returns on light-harvesting. This can be overcome if one introduces a slight \emph{enthalpic gradient}, to the antenna. Interestingly, this strategy appears to have been adopted by Earth’s oxygenic cyanobacteria, and we conclude that \emph{bacterial} oxygenic photosynthesis is feasible around even the lowest mass M-dwarf stars.

Samir Chitnavis, Thomas J. Haworth, Edward Gillen, Conrad W. Mullineaux, Christopher D. P. Duffy

Comments: 5 Figures, submitted to Astrobiology and awaiting return of review
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR); Biological Physics (physics.bio-ph)
Cite as: arXiv:2309.12845 [astro-ph.EP] (or arXiv:2309.12845v1 [astro-ph.EP] for this version)
Submission history
From: Christopher Duffy
[v1] Fri, 22 Sep 2023 13:11:21 UTC (2,179 KB)
https://arxiv.org/abs/2309.12845
Astrobiology, Botany,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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