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1. The effects of O(2), CO(2), and pressure were studied in two very different species of protozoa, a flagellate, Chilomonas paramecium, grown in acetate-ammonium solution and a ciliate, Tetrahymena geleii, grown in 2 per cent proteose-peptone solution. 2. Chilomonas and Tetrahymena live and reproduce in solutions exposed to a wide range of O(2) concentrations, but Chilomonas is killed at high O(2) tensions in which Tetrahymena grows best. The optimum O(2) concentration for Chilomonas is about 75 mm. pressure but it lives and reproduces in O(2) tensions as low as 0.5 mm. while Tetrahymena fails to grow in concentrations below 10 mm. O(2) pressure. 3. With a constant O(2) tension of 50 mm. pressure, it was found that there is no significant variation in growth in Chilomonas between 50 mm. and 740 mm. total pressure. In Tetrahymena, however, under the same conditions, an optimum total pressure was found at about 500 mm. and growth is comparatively poor at 50 mm. total pressure. 4. Tetrahymena does not live very long in CO(2) tensions over 122 mm., although Chilomonas grows as well at 400 mm. CO(2) as in air at atmospheric pressure (0.2 mm. CO(2)). Tetrahymena grows best in an environment minus CO(2), but the optimum for Chilomonas is 100 mm. CO(2) at which pressure an average of 668,600 +/- 30,000 organisms per ml. was produced (temperature, 25 +/- 1 degrees C.). 5. Chilomonads grown in high CO(2) concentrations (e.g., 122 mm.) produce larger starch granules and more starch than those grown in ordinary air at atmospheric pressure. 6. In solutions exposed to 75 mm. O(2) tension (optimum) and 122 mm. CO(2) plus 540 mm. N(2) pressure, chilomonads contain very little, if any, fat. This phenomenon seems to be due to the action of CO(2) on the mechanisms concerned with fat production. 7. In Tetrahymena exposed to pure O(2), there is very little fat compared to those grown in atmospheric air. This may be due to the greater oxidation of fat in the higher O(2) concentrations. 8. Further evidence is presented in support of the contention that Chilomonas utilizes CO(2) in the production of starch.

THE EFFECTS OF OXYGEN, CARBON DIOXIDE, AND PRESSURE ON GROWTH IN CHILOMONAS PARAMECIUM AND TETRAHYMENA GELEII FURGASON