Abstract EANA2024-112

Prime astrobiological targets beyond the Solar System are the exoplanets orbiting in the Habitable Zone (HZ) of M-dwarf stars, as they are extremely common in our galaxy and could theoretically allow life evolution due to their long lives. However, differently from the Sun, they are far less luminous and emit only a minority of the light in the Visible (VIS, 400–700 nm), mostly red, and most of it in the far red (FR, 700 – 750 nm) and in the infrared (IR, 750 – 1000 nm). This peculiar light spectrum has led researchers to wonder if oxygenic photosynthesis, the most prominent biological process that shaped life evolution on our planet, could work on exoplanets orbiting those stars. Indeed, most of the oxygenic photosynthetic organisms function by harvesting VIS photons from the Sun to drive the primary production of organic compounds and release oxygen, a fundamental molecule made available to all other organisms. Oxygenic photosynthesis generates atmospheric and surface biosignatures, ideal targets for investigating the detectability of life beyond Earth. This is linked, respectively, to the oxygen release activity of oxygenic photosynthetic organisms and to the absorption of their photosynthetic pigments, which on Earth generate a distinctive reflectance spectrum.

To investigate the potential of oxygenic photosynthesis in exoplanets and possible generated biosignatures, different  approaches can be followed: i) performing laboratory experiments, exposing oxygenic photosynthetic organisms to M-dwarfs simulated spectra; ii) exploring the photosynthetic biodiversity of cyanobacteria, algae and plants living in terrestrial niches that can be considered exoplanets’ light analogues, being characterized by very low VIS and FR enriched spectra; iii) performing numerical simulations using data from laboratory experiments and from organisms living in light analogues.

Even if all oxygenic photosynthesizers perform photosynthesis using photosynthetic complexes with very conserved reaction centres, the associated light-harvesting antenna systems have instead different organizations and compositions depending on the taxa. These complexes have different light absorption capabilities based on the specific interactions between pigments and proteins constituting the antennae. Moreover, peculiar morpho-physiological adaptations allow the use of very dim VIS light and FR photons, both in prokaryotes and eukaryotes. Some cyanobacteria can utilize constitutively FR light through chlorophyll d or acclimate to FR through mechanisms like Far-Red Light Photoacclimation (FaRLiP) [1,2]. Some eukaryotic algae can harvest FR light by changing the organization of antenna complexes without synthesizing FR-absorbing pigments [3]. Even on plants, recent studies demonstrated that shade adapted understory species have evolved peculiar strategies to photosynthesize under very dim FR enriched light spectra [4]. The proposed talk will explore the acclimation or adaptation strategies to FR-enriched and simulated M-dwarf spectra of different oxygenic photosynthetic organisms, showing the implications for assessing the plausibility of oxygenic photosynthesis on exoplanets orbiting M-dwarfs.

 

[1] Miyashita, H. et al. Nature 383, 402 (1996).

[2] Gan, F. et al. Science 345, 1312–1317 (2014).

[3] Wolf, B. M. & Blankenship, R. E. Photosynth. Res. 142, 349–359 (2019).

[4] Colpo A. et al., Plant Science 336 111833 (2023).