[Concerning the question of anaerobic growth in unicellular green algae].

1. With a strain of Chlorella the conditions have been established for a photoreduction to run for at least 3 days after activation of hydrogenase. These conditions are: 500 Lux, manganese deficiency in cultures with ammonia, or addition of DCMU (10(-5) mol) to normally grown algae (nitrate or ammonia). In the absence of DCMU nitrate accelerates the inactivation of hydrogenase. 2. During a period of pure photoreduction storage of carbohydrates is abundant; however, further syntheses leading to growth are cut off. 3. Another Chlorella strain is unable to grow anaerobically in dark or in light in a glucose medium free from CO2. After elimination of photosynthetic oxygen production (DCMU) cells stop growing even in the presence of CO2. Growth in light requires a minimum amount of CO2 or O2 to be available at least inside the cells. 4. Aerobic cultures in glucose media grow well in dark and light in the presence of DCMU. In this case illumination increases cell growth if the external oxygen concentration exceeds 0,4 Vol.-%. At very low levels of oxygen nitrate as compared with ammonia improves cell growth in light. 5. Both strains of Chlorella can start growing from anaerobiosis only if photosynthesis is active as an auxiliary source of oxygen. Specific biochemical pathways for syntheses which would allow anaerobic growth are not available in these algae.