This study examined how carbon dioxide (CO₂) and oxygen (O₂) partial pressures affect the growth of Chlorella, a photosynthetic alga, using chemostat continuous culture systems. The researchers found that large, abrupt increases in either gas (>0.2 atm steps) inhibited growth, but gradual increases allowed cultures to adapt. Adapted Chlorella cultures tolerated CO₂ partial pressures up to 0.6 atm and O₂ levels up to 0.8 atm without growth inhibition. Starch production remained largely unaffected across tested pressure ranges. When measuring photosynthetic efficiency in cultures with 0.2 atm CO₂ and 0.8 atm O₂, the maximum growth yield from light was 0.0172 g/kJ with 36.5% photosynthetic efficiency. Maintenance energy and growth yields were identical at O₂ partial pressures of 0.2 and 0.8 atm. These results contrasted with previous studies on resting cells, demonstrating that growing Chlorella cells can adapt to high gas partial pressures through phenotypic changes occurring within three to four generations.

Key findings

• Chlorella cultures adapted to high CO₂ (up to 0.6 atm) and O₂ (up to 0.8 atm) partial pressures when increases were gradual but became inhibited by large stepwise increases
• Photosynthetic efficiency (36.5%), growth yield, and maintenance energy remained constant regardless of O₂ partial pressure from 0.2 to 0.8 atm
• Starch production was unaffected by increases in CO₂ or O₂ partial pressures in adapted cultures
• Gradual adaptation over 3-4 cell generations allowed tolerance to previously inhibitory gas levels, suggesting phenotypic rather than genetic changes