Gas exchange measurements

The cyanobacteria photosynthetic rate was measured for 24 h when the organisms, acclimated for 21 days to the different spectra, were directly exposed to the respective growth light conditions inside the ASC. Before these measurements, cell cultures were grown in flasks in the cabinets as for the other experiments, then they were used after checking that their physiological responses were those expected. For each strain and light condition, samples were pooled together, due to the impossibility to evaluate more than one sample at a time in the ASC. Then, 70 mL from the pool were directly poured into previously sterilized glass Petri dishes and immediately closed inside the ASC for 24 h under the different light sources at 30 μmols m⁻² s⁻¹ into an initial atmospheric composition of air + 5% CO₂ at 1 atm, 30°C. A concentration of 5% CO₂ is used to sustain the photosynthetic activity of the strains throughout the experiments; otherwise, cyanobacteria in the closed chambers will suddenly deplete terrestrial CO₂ concentrations (Battistuzzi et al., 2020). When the ASC were positioned inside the temperature-controlled cabinet, the latter was kept at a slightly higher temperature than that of the ASC itself. This helps to avoid condensation phenomena on the top of the ASC glass windows and improves the accuracy of the ASC temperature control loops. Raw data of O₂ levels registered from the O₂ sensors were elaborated through Matlab (MathWorks) to obtain the O₂ production expressed in micromoles by applying the ideal gas law (Battistuzzi et al., 2020).