Abstract
A circadian rhythm of O(2) evolution has been found in Euglena gracilis, Klebs strain Z. The rhythm persists for at least 5 days in constant dim light and temperature, but damps out in constant bright light. The phase of this rhythm can be shifted by a pulse of bright light and the period length is not changed over a 10 C span of growth temperature.The O(2) evolution rhythm is found in both logarithmic and stationary phase cultures, but CO(2) uptake is clearly rhythmic only in stationary phase cultures.The activity of glyceraldehyde-3-phosphate dehydrogenase was not rhythmic as previously reported (Walther and Edmunds [1973] Plant Physiol. 51: 250-258). Carbonic anhydrase activity was rhythmic when the cultures were maintained under a light-dark cycle with the highest enzyme activity coinciding with the fastest rate of O(2) evolution. However, the rhythm in carbonic anhydrase activity disappeared under constant conditions. Changes in the activities of these two enzymes are therefore not responsible for the rhythmic changes in photosynthetic capacity.