Is A Constitutive Red-shift An Advantage For Oxygenic Photosynthesis Under M Dwarf Starlight? Insights From Acaryochloris Marina sp. str. Moss Beach

[FEMS Microbes] In the next decades, space telescope missions will search for life evidence on exoplanets, focusing on robust biosignatures associated with oxygenic photosynthesis, including atmospheric oxygen accumulation and the Vegetation Red-Edge in surface reflectance spectra.

Many habitable rocky exoplanets orbit M dwarf stars, whose spectral energy distribution may condition the rise and evolution of oxygenic photosynthesis. M dwarf stars emit predominantly far-red (700-750 nm) and near-infrared (750-1000 nm) light, and relatively little visible (400-700 nm) radiation, which on Earth predominantly drives photochemistry in most oxygenic phototrophs.

Previous experiments proved some oxygenic phototrophs can photosynthesize under simulated M dwarf light but less efficiently than under solar radiation simulated in the range 365-780 nm. Indeed, tested organisms present photosynthetic apparatus evolved to harvest Sun’s visible light, however, M dwarfs’ irradiation might select adaptations optimized for harvesting far-red/near-infrared light.

We measured sensitivities of a far-red/near-infrared-utilizing cyanobacterium, Acaryochloris marina sp. str. Moss Beach to simulated M dwarf spectrum and primeval anoxic, CO₂-rich atmosphere. This strain constitutively presents a high content of chlorophyll d, with in vivo absorption peak at 710 nm. Its permanently red-shifted photosynthetic apparatus required no acclimation to the stellar spectrum, maintaining strong growth and oxygen production, higher than that registered under simulated solar light.

Moreover, abundant chlorophyll d caused a shift in whole-cell reflectance: the red-edge was beyond 700 nm, resulting in a Chl d-near-infrared-edge. Overall, a potentially similar metabolism on exoplanets orbiting M dwarfs could successfully produce both a gaseous biosignature and a characteristic surface biosignature.

Is a constitutive red-shift an advantage for oxygenic photosynthesis under M dwarf starlight? Insights from Acaryochloris marina sp. str. Moss Beach, FEMS Microbes via PubMed

Astrobiology,