KINETICS OF BLUE‐LIGHT STIMULATION AND CIRCADIAN RHYTHMICITY OF LIGHT‐SATURATED PHOTOSYNTHESIS IN BROWN ALGAE: A SPECIES COMPARISON1

Abstract

ABSTRACTThe time courses of photosynthetic rates in red light, with and without additional blue light, were investigated and compared in 20 species of brown algae. Species could be separated into two groups on the basis of the rhythmicity of their photosynthesis in red light and the kinetics of their responses to blue‐light pulses. One group, which consisted of members of the Ectocarpales, Chordariales, and Dictyosiphonales, was characterized by strong and persistent circadian rhythmicity in red light. The photosynthetic responses of these species to blue‐light pulses started within 10–30 s of the beginning of blue‐light treatment and mostly contained at least two distinct kinetic components. An early component, which reached a maximum about 5–10 min after the blue‐light pulse, was always detectable. Later components were seen as separate peaks or shoulders after an additional 10–20 min. The decay of the response in this group of species was mostly slow, with half‐lives of between 0.5 and 1.5 h. In the second group of species, consisting of members of the Dictyotales, Laminariales, and Fucales, photosynthesis in red light was usually non‐rhythmic, although circadian rhythms with a weak amplitude or of transient occurrence were observed in some plants of some species. The increase in photosynthesis in response to a blue‐light pulse was not detectable until 70–330 s after the start of blue‐light treatment, and the response itself had only a single component, with a maximum after about 10 min and half‐life of 10–20 min. The lengths of the lag‐phases were positively correlated with the times taken to reach the peak in this group, although the lag‐phases and the half lives sometimes varied with time in individual plants. Two members of the Sphacelariales (Sphacelaria, Cladostephus) did not fit into either of the two groups because their photosynthesis was rhythmic, but their responses had long lag‐phases, a single component, and moderately long half‐lives. The differences in the kinetics of the photosynthetic response to blue‐light pulses, which have been described for the two main groups of species, are thought to indicate that there are two distinct mechanisms by which light‐saturated photosynthesis responds to blue light in brown algae. Since in some species the maximal photosynthesis after a blue‐light pulse and the rate of photosynthesis in continuous blue light also varied in a circadian pattern, the response to blue light itself may be under circadian control.