Acclimation responses to M-dwarfs light and CO₂ enriched atmospheres in Synechococcus sp. PCC7335

This thesis is part of the Italian Project of Astrobiology “Life in space”, which investigates the origin, presence and persistence of life on exoplanets. The thesis aimed at studying the acclimation responses of cyanobacteria to simulated environmental conditions (e.g., continuous irradiation, low light intensity, atmosphere depleted of oxygen) that could be present on exoplanets orbiting the Habitable Zone (the planet-star distance that allows liquid water to be present on the surface of the planet) of M-dwarfs stars, stars characterized by low emission of visible light and high emission of far-red and infrared light. Moreover, it aimed at assessing the capability of these organisms to perform oxygenic photosynthesis under such a peculiar starlight spectrum (different from the Sun) and generate under said conditions detectable biosignatures, sign of extant life detectable from remote. The purpose of this thesis was hence twofold: the study of the acclimation of cyanobacteria under exoplanetary conditions of light and atmosphere together with the consequent estimation of remotely detectable biosignatures linked to their photosynthetic activity. The organisms selected were cyanobacteria, the very first oxygen-evolving organisms and befitting candidates due to their high metabolic plasticity, which allows them to be highly tolerant to harsh environmental conditions. Moreover, some strains are known to perform the Far Red-Light Photoacclimation response or FaRLiP which enable them to pursue oxygenic photosynthesis under both low light and far-red light, by synthesizing Chl d and f which absorb in the far-red. One of them is Synechococcus sp. PCC7335, that, in addition, also performs Complementary Chromatic Acclimation (CCA), which allows the strain to modulate its light-harvesting pigments ratio in response to different light spectra. The strain was exposed to a simulated M-dwarf light spectrum under low light continuous irradiation for 7 days, comparing its acclimation responses to those of control conditions (solar light and far-red light). The tested strain behaved differently from others already tested in the same set-up. An in vivo absorption in the far-red was detected in the far-red control, due to the FaRLiP response. However, only CCA and no FaRLiP response was detected in M-dwarf, suggesting the strain could already harvest for photosynthesis efficiently the small portion of visible light available without the need to synthetize new pigments. Given the different acclimation, the photosynthetic activity and linked biosignatures also showed differences from those already studied. This thesis thus highlights a variability not previously known in the acclimation of different FaRLiP strains to the M-dwarf starlight, which will affect the detectability of biosignatures from remote. Results are finally important to implement databases of oxygenic photosynthetic organism’s responses to different environmental conditions, that can be used by astronomers to compare data from remote sensing measurements of possible biosignatures on distant exoplanets.