[CO2-Exchange and water relations in the aerophillic green-alga Apatococcus lobatus]

The relation between CO2-exchange in the pleurococcoid green alga Apatococcus lobatus and light, temperature and humidity was investigated, and the water-content in relation to vapour pressure was determined. The unwettable thalli achieve hydration equilibrium with the vapour pressure of the air, so that the measurement of physiological capabilities at defined water-potential is made possible. The relation between real CO2-uptake ("photosynthesis") and water-potential at 10 and 20° C was determined. Fifty percent of the maximum capacity is reached at a water-potential of 142 bar (90% r.h.) and 10% is still reached at 370 bar (76% r.h.). The limit of measurable CO2-uptake is at about 520 bar (68% r.h.). When the algae are being dried "photosynthesis" and dark-respiration do not decrease proportionally; up to 25% (water-potential 252 bar, 83% r.h.) of the initial values the CO2-uptake is favoured, since respiration decreases first an increase of the apparent CO2-uptake is observed. The change in the ratio apparent CO2-uptake/dark-respiration, that is, the change in the CO2-balance in relation to temperature and hydration is discussed. The imbibition with liquid water has an unfavourable influence on the CO2-balance, and the relation between apparent CO2-uptake and dark-respiration of vapour-saturated thalli is almost optimal. At 20° C and 0,025 cal cm(-2)min(-1) the most favourable CO2-balance corresponds to a water-potential of about 27-40 bar (97-98% r.h.) and to a saturation deficit of 35-43%. The CO2-exchange in these algae seems to be remarkably adapted to their aerophillic way of life.