Carbon Dioxide Fixation and α-Linolenic Acid Production by the Hot-Spring Alga Cyanidium caldarium

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Focusing on CO₂ fixation and α-linolenic acid (ALA) production, photoautotrophic cultivation of the hot-spring alga Cyanidium caldarium was investigated using Allen’s medium under a 5% CO₂-enriched atmosphere. Although the growth rate was maximum at ca. 40°C, the optimum cultivation conditions (temperature, H⁺ concentration in the medium and light intensity) for ALA production were found to be 30°C, pH 2.0–4.0 and ca. 10,000 lux respectively. Simulation of a chemostat cultivation at these optimum conditions showed that the rates of CO₂ fixation and of ALA production could reach 0.75 kg- CO₂/(kg-cells·d) and 5.44 g-ALA/(kg-cells·d), respectively, in the exponential phase. This rate of CO₂ fixation corresponded to 0.030 kg-CO₂/(m³·h) at a cell concentration of 0.96 kg/m³. This activity of C. caldarium for CO₂ fixation was comparable to that of blue-green algae and was 10 to 100 times greater than that of green plants. Thus, C. caldarium was shown to be useful for the fixation of CO₂.