Investigating the Potential for Reducing Sodium Bicarbonate in the Nutrient Medium for Cultivation of Limnospira platensis at Elevated CO2 Concentration

Sodium bicarbonate (NaHCO3) in Zarrouk medium serves as both the main carbon source and an alkaline buffer, and it accounts for a large share of the cost of spirulina cultivation. This study assessed whether the NaHCO3 concentration in the medium can be reduced, down to complete elimination, for Limnospira platensis grown at elevated CO2 compared with atmospheric CO2. Two sequential experiments (12 days with CO2 enrichment to a 3% setpoint; up to day 8 at atmospheric CO2, by which time the cultures had died) were carried out in a hermetic gas chamber with 10 L column photobioreactors, using a strain adapted to 3% CO2. The experiments differed only in the chamber atmosphere (3% CO2 and atmospheric ~0.04% CO2), the CO2 concentration in the enriched chamber being raised to ≈3% after each sampling and declining to ≈0.5% by the next. In each, eight NaHCO3 concentrations (0–12 g/L, i.e., 0–100% of the reference level of 12 g/L, which is 71% of the standard 16.8 g/L in Zarrouk medium) were tested in triplicate at 3% CO2 and in duplicate at atmospheric CO2. Biomass, pH, carbonate and bicarbonate ions, nitrate and phosphate consumption, and trichome viability were monitored. At 3% CO2, the culture grew at all NaHCO3 concentrations, including medium without added NaHCO3, reaching 3.59–4.11 g/L of biomass by day 12 (7–8% less than in the 12 g/L control at 0 and 0.6 g/L). The pH remained within 8.40–9.40, and the proportion of live trichomes was 92.1–96.3%; trichome viability did not differ significantly between variants, whereas pH did. Under atmospheric CO2 (CO2-adapted inoculum transferred directly to air), the apparent pH rose to 12.75, bicarbonate was depleted, and the culture died within 2 days at 0–1.2 g/L NaHCO3 and by day 8 at 3–12 g/L, with no live trichomes remaining by day 6. These results indicate that, with CO2 enrichment of the chamber atmosphere (3% setpoint, ≈1.4% on average), added NaHCO3 is not required for growth over 12 days: the carbon fixed in the biomass was supplied from the gas phase, and the bicarbonate pool was formed in situ with alkalinity derived mainly from nitrate assimilation. This suggests a possible way to reduce medium costs in spirulina production, which has yet to be confirmed by a CO2 supply and cost analysis.