Diurnal changes in the delayed fluorescence response of an ambient light-excited green alga

Delayed fluorescence (DF) is a characteristic feature of light-excited plant cells caused by the back-reaction of electrons through the electron transport chain. Targeting the response of light-adapted green algae to diurnal light changes, the present study presents novel results of DF measurements in the absence of an artificial excitation light source. Based on a linear relationship between the DF counts and light intensities from 0.15 to 0.65 W m(-2) during daybreak, we estimated an initial algal response to light intensities of 0.01 W m(-2). Dissolved oxygen concentrations began to increase at 1.0 W m(-2). A noon depression similar to that reported for prompt fluorescence occurred above 100 W m(-2). Our results from multiple day-night cycles emphasize that the DF response is a function of the chlorophyll concentration and of a rapid light adaptation. The DF counts alone cannot provide a reliable unambiguous measure of photosynthetic activity.