The Peak Absorbance Wavelength of Photosynthetic Pigments Around Other Stars From Spectral Optimization

Abstract

In the search for life on other planets, the presence of photosynthetic surface vegetation may be detectable from the colors of light it reflects. On the modern Earth, this spectral reflectance is characterized by a steep increase in reflectance between the red and near‐infrared wavelengths, a signature known as the “red edge”. This edge-like signature occurs at wavelengths of peak photon absorbance, which are the result of adaptations of the phototroph to their spectral environment. On planets orbiting different stellar types, red edge analogs may occur at other colors than red. Thus, knowing the wavelengths at which photosynthetic organisms preferentially absorb and reflect photons is necessary to detect red edge analogs on other planets. Using a numerical model that predicts the absorbance spectrum of extant photosynthetic pigments on Earth from Marosvölgyi and van Gorkom (2010), we calculate the absorbance spectrum for pigments on an Earth-like planet around F through late M type stars that are adapted for maximal energy production. In this model, cellular energy production is maximized when pigments are tuned to absorb at the wavelength that maximizes energy input from incident photons while minimizing energy losses due to thermal emission and building cellular photosynthetic apparatus. We find that peak photon absorption for photosynthetic organisms around F type stars tends to be in the blue while for G, K, and early M type stars, red or just beyond is preferred. Around the coolest M type stars, these organisms may preferentially absorb in the near-infrared, possibly past one micron. These predictions are consistent with previous, qualitative estimates of pigment absorptance. Our predicted absorbance spectra for photosynthetic surface organisms depend on both the stellar type and planetary atmospheric composition, especially atmospheric water vapor concentrations, which alter the availability of surface photons and thus the predicted pigment absorption. By constraining the absorbance spectra of alien, photosynthetic organisms, future observations may be better equipped to detect the weak spectral signal of red edge analogs.

Optimizing photosynthetic light-harvesting under stars: Generalized thermodynamic models

“…This optimization neatly predicted the spectrally-distinct absorption peaks between chlorophyll pigments of plants and bacteriochlorophyll pigments of purple bacteria. When applied to stellar spectra, they found an optimal antenna will progressively red-shift with decreasing 𝑇 𝑠 , again arriving at the NIR (𝜆 ∼ 1000 nm) for M-dwarfs (Lehmer et al, 2021). Arp et al (2020) argued that LHCs are optimized to be robust against ŕuctuations in irradiance (łexternal noisež) and pigment-to-pigment energy transfer (łinternal noisež).…”

Photosynthetic Fluorescence from Earthlike Planets around Sunlike and Cool Stars

“…However, according to Takizawa et al (2017), even around M dwarfs, the evolution of photosynthesis in water may drive a preference for using visible light rather than the NIR, even after organisms colonize land surfaces. Moreover, the light absorption properties of land vegetation could be optimized after long-term adaptive evolution, depending on the stellar irradiations, as estimated by Lehmer et al (2021). Anoxygenic photosynthesis, as performed by organisms such as purple bacteria, is thought to precede the emergence of oxygenic photosynthesis, whose global effect has been characterized by the great oxidation event (∼2.3 billion years ago (Ga)).…”

Growth and Photosynthetic Efficiency of Microalgae and Plants with Different Levels of Complexity Exposed to a Simulated M-Dwarf Starlight

“…The possibility of oxygenic photosynthesis in the HZ of M-dwarfs has been so far vastly discussed using models and a theoretical approach [ 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 ], but, except experimental work on cyanobacteria [ 19 , 20 ], investigations into more complex photosynthetic organisms’ responses to simulated M-dwarfs spectra are missing.…”