Changes in the capacity of algae to fluoresce during steady-state photosynthesis

A suspension of Chlorella was illuminated long enough for the induction period to be complete and thus for stead-state photosynthesis to be achieved. Then the exciting light was chopped by a slotter about 4 times per sec and the capacity of the algae to fluoresce was followed by means of a low-intensity measuring beam during the dark period. The decay of the fluorescence capacity was approximately exponential and was characterized by a time constant of about 20 msec at room temperature. The temperature dependence of this time constant fits an Arrhenius equation with an activation energy of 0.34 eV. We concluded from these results that fluorescence provides a direct measure of photosynthetic energy migration during steady-state photosynthesis and also that the kinetically limiting step in this energy migration involves the electron transport chain enzymes.