The Inorganic Carbon Requirements of Chlamydomonas Segnis (Chlorophyceae) for Cell Development in Synchronous Cultures¹

The time in the cell cycle when CO 2 provision was required for cell development and division was determined in synchronous cultures of Chlamydomonas segnis Ettl bubbled with air (0.03% CO 2 ) or air enriched with 5% CO 2 under continuous light at 25°C and pH 7. Provision of CO 2 (% in air v/v) during the G 1 ‐phase was found to be essential for the completion of the cell cycle. There was no demand for CO 2 supply throughout the S‐phase and mitosis. Using cultures adapted to CO 2 concentrations ranging from 0.03 to 5% in air, the apparent CO 2 concentration (K m ) required for the cells to develop during the G ‐1 ‐phase and to attain one half the maximal rates of photo‐synthetic O 2 evolution was calculated as 0.05%. This value increased to 0.1 and 0.5% during the S‐phase. For total protein and carbohydrate accumulation, which would reflect inorganic carbon (CO 2 + HCO 3 − ) assimilation, the K m (% CO 2 ) were ca. 0.1 and 0.14 throughout the cell cycle, respectively. The CO 2 concentration at which the cells exhibited the shortest generation time (6.7 h) was 0.1%. These results showed that during development, cells photosynthesizing (evolving O 2 ) at maximal rates but accumulating protein and carbohydrate at one half the maximal rates or less would complete their vegetative life cycle in the shortest time.