[The metabolic interactions between Paramecium bursaria Ehrbg. and Chlorella spec. in the Paramecium bursaria-symbiosis. II. Symbiosis-specific properties of the physiology and the cytology of the symbiotic unit and their regulation (author's transl)].

The endosymbiotic association of Paramecium bursaria Ehrbg. with Chlorella spec. (green Paramecium) was studied both physiologically and cytologically. Comparison of the properties of the symbiotic unit with those of the symbiotic partners which had been isolated from it revealed the following features and differences: 1. Up to 6000 lux the photosynthetic capacity of the symbiotic unit is higher than that of the isolated symbiotic algae grown independently in mass culture under defined conditions. Alga-free Paramecium bursaria (colourless Paramecium) show a very low rate of CO2-fixation. 2. The green Paramecium has a higher compensationpoint of photosynthesis (4000-5000 lux) than the isolated alga (200-400 lux). 3. Green paramecia consume less oxygen in darkness than colourless organisms but more than the isolated algae. 4. The uptake of carbohydrates from the culture medium by green parpmecia is lower than the uptake by alga-free P. bursaria but higher than the one of the isolated algae. 5. Symbiotic algae within the intact symbiotic unit show tightly packed photosynthetic membranes and an intense disposition of starch. In the presence of 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (DCMU) or in darkness the arrangement of thylakoids is less compact and the deposition of starch is reduced. The growth and the number of the symbiotic algae in situ is regulated by a complex mechanism to which the intracellular level of carbohydrates belongs. The results are discussed in connection with ecological aspects of the Paramecium bursaria-endosymbiosis.