  Introduction: The search for life on distant exoplanets is expected to rely
on atmospheric biosignatures detection, such as oxygen of biological origin.
However, it is not demonstrated how much oxygenic photosynthesis, which on
Earth depends on visible light, could work under spectral conditions simulating
exoplanets orbiting the Habitable Zone of M-dwarf stars, which have low light
emission in the visible and high light emission in the far-red/near-infrared.
By utilizing cyanobacteria, the first organisms to evolve oxygenic
photosynthesis on our planet, and a starlight simulator capable of accurately
reproducing the emission spectrum of an M-dwarf in the range 350-900 nm, we
could answer this question. Methods: We performed experiments with the
cyanobacterium Chlorogloeopsis fritschii PCC6912, capable of Far-Red Light
Photoacclimation (FaRLiP), which allows the strain to harvest far-red in
addition to visible light for photosynthesis, and Synechocystis sp. PCC6803, a
species unable to perform this photoacclimation, comparing their responses when
exposed to three simulated light spectra: M-dwarf, solar and far-red. We
analysed growth and photosynthetic acclimation features in terms of pigment
composition and photosystems organization. Finally, we determined the oxygen
production of the strains directly exposed to the different spectra. Results:
Both cyanobacteria were shown to grow and photosynthesize similarly under
M-dwarf and solar light conditions: Synechocystis sp. by utilizing the few
photons in the visible, C. fritschii by harvesting both visible and far-red
light, activating the FaRLiP response.
