Growth of Limnospira platensis Under Elevated CO2 with the Addition of Sodium Selenite

Cultivation of L. platensis under elevated CO2 ensures CO2 biofixation, while adding Na2SeO3 to the culture medium, produces valuable Se-enriched biomass. This study evaluated the effect of Na2SeO3 supplementation (20–640 mg/L) on the growth of L. platensis under 3 vol.% CO2. The cultivation was carried out in 10 L column-type photobioreactors under continuous (24 h·d−1) illumination with intensity of 220 µmol·m−2·s−1, a constant temperature of 27 °C and aeration rate of 1 L/min. Two consecutive cultivation cycles (8 days each) were conducted: the first with the original strain and the second with an inoculum adapted to 20 mg/L (taken from the first cycle). The growth rate, cell viability, pH, and the concentrations of nitrates, phosphates, carbonates, bicarbonates, NH4+ ion were investigated. It was established that under intensive cultivation at elevated CO2, L. platensis is very sensitive to Na2SeO3 supplements: concentrations > 160 mg/L led to culture death within the first 2–4 days, while the acute toxicity threshold lies in the range of 80–100 mg/L. The highest biomass growth rate in the 1st and 2nd cycles was achieved in the control (0 mg/L of Na2SeO3): 329 and 239 mg·L−1·d−1 dry weight, respectively. At 20 mg/L, the growth rate drops to 289 and 208 mg·L−1·d−1 in the 1st and 2nd cycles, respectively. The percentage of live trichomes at the end of the 1st and 2nd cycles at 20 mg/L was 85 and 67%, respectively, indicating low adaptation capacity even to this concentration of Na2SeO3. The addition of Na2SeO3 did not lead to significant pH deviations from the control. Nitrates and phosphates consumption slowed down at Na2SeO3 concentrations of 40–60 mg/L. Bicarbonate and carbonate ions did not change significantly across all samples due to the maintenance of elevated CO2 concentration. In a separate experiment under atmospheric CO2 (0.04%), selenite toxicity was less pronounced than under 3% CO2, and growth declined after day 4 due to carbon limitation rather than selenite toxicity.