Effect of Temperature on the Growth Dynamics and Cell Viability of Chlorella vulgaris Under Elevated CO2 Conditions

Chlorella vulgaris is a promising platform for biomass production and CO2 fixation, while temperature strongly affects cultivation performance. Here, the effect of temperature on growth and cell viability was investigated in 10 L column bubble-type photobioreactors under an elevated, intermittently restored CO2 atmosphere. Cultures were grown for 18 days at nominal temperatures of 19, 23, 27, and 31 °C using two independent photobioreactors per temperature. Continuous monitoring confirmed mean culture temperatures of 19.12 ± 0.42, 23.09 ± 0.09, 27.19 ± 0.20, and 30.96 ± 0.44 °C, respectively. Using an OD750-to-biomass coefficient previously established in our laboratory for this C. vulgaris culture by paired OD750 and gravimetric dry-weight measurements (0.4 g L−1 per OD unit), final OD750-derived biomass estimates were 5.43, 5.82, 5.90, and 4.05 g L−1 at 19, 23, 27, and 31 °C, respectively, and corresponding average OD-derived productivities were 0.282, 0.303, 0.308, and 0.205 g L−1 d−1. Reactor-level analysis detected no significant productivity differences among 19, 23, and 27 °C, whereas the OD-derived productivity at 31 °C was significantly lower. Non-viable cells appeared earlier at 31 °C; at day 18, the group-level viable-cell fractions were 93.3% at 19 °C and 91.7% at 31 °C. Because the OD750 conversion coefficient was not independently validated at each temperature and the day-18 count-derived cell densities at 19 and 31 °C were similar, the magnitude of the apparent biomass difference should be interpreted as an OD750-derived response rather than as a validated dry-biomass difference. The data therefore support a temperature-dependent cultivation response at 31 °C but do not resolve its physiological or gravimetric basis.