Abstract EANA2025-123

Oxygenic photosynthesis generates, on Earth, atmospheric and surface biosignatures, which are ideal targets for investigating the detectability of life beyond our planet. These are linked, respectively, to the oxygen release by oxygenic phototrophs and to the absorption of their photosynthetic pigments, which on Earth generate distinctive reflectance spectra. As prime exoplanet targets for astrobiology are the ones orbiting in the habitable zone of M-dwarf stars, this poses the question whether such stars, with emission enriched in far-red and infra-red wavelength, could sustain this remarkable metabolism, ultimately generating detectable biosignatures.

Indeed, due to the abundance of visible light (400-700 nm) emitted by the Sun, oxygenic photosynthesis has evolved a set of pigments that specifically harvest photons up to 680-700 nm to drive photochemistry. Far-red light, on the other hand, is not harvested by most common oxygenic phototrophs. It is sometimes posited that the 700 nm wavelength limit either represents an energetic constraint to supporting this metabolism, or that it is a legacy of evolution.

Interestingly, on our planet there are some environments that receive a spectrum poor in visible light and enriched in far-red light, which can be defined as “Exoplanet light analogues” due to the resemblance of their spectral radiation to that of exoplanets orbiting M-dwarf stars. These light niches on Earth are often inhabited by oxygenic phototrophs that display peculiar permanent or plastic alterations in the photosynthetic apparatus enabling them to photosynthesize in far-red light.

An example is the cyanobacterium Acaryochloris marina, a unique species that is characterized by the constitutive presence of up to 95% of chlorophyll d in their photosynthetic complexes, in the place of chlorophyll a. Chlorophyll d is a far-red absorbing pigment that, at such abundances, determines a severe shift of the in vivo absorption peak beyond 700 nm. 

The proposed talk will illustrate the response of Acaryochloris marina sp. str. Moss Beach, a strain isolated from a brown alga on the coast of California (1), to simulated exoplanetary conditions, exploring whether its peculiar photosynthetic apparatus is a good fit for photosynthesizing in M-dwarf like irradiance. We exposed cells to an Archean-like anoxic atmosphere enriched in CO2 and to the simulated spectrum of an M7 star. Real time monitoring of O2 evolution during the exposure allowed us to assess the suitability of the singular photosynthetic apparatus of A. marina Moss Beach in the simulated planetary conditions. These simulations, via tracking of O2 evolution rates and the assessment of cells’ spectral features, offers important hints on what kind of traces we can expect from similar metabolisms on exoplanets orbiting M-dwarfs, both atmospheric and surface biosignatures.