Circadian rhythm and fast responses to blue light of photosynthesis in Ectocarpus (Phaeophyta, Ectocarpales) : I. Characterization of the rhythm and the blue-light response.

The photosynthetic oxygen production of Ectocarpus siliculosus (Dillwyn) Lyngb. under continuous high irradiances of red light displayed a circadian rhythm with maxima at about noon. Pulses of blue light induced rapid transient increases in the rate of photosynthesis. The increases started about 15 s after the beginning of blue light and reached their maxima after 3-4 min. This was followed by a gradual decrease. A second peak or shoulder about 20 min later indicated that at least two reactions were involved in the blue-light enhancement of photosynthesis. The magnitude of the response to blue light depended on the phase of the rhythm at which blue light was given. It was high when the red-light photosynthesis was at its troughs, and low at its peaks. Fluence-response curves indicated that the sensitivity to blue light at the peaks of the rhythm was identical to that at the troughs. In addition, blue light shifted the phase of the photosynthetic rhythm, where the essential trigger was the light-off signal. Red light had no specific influence on the circadian rhythm. After darkness, photosynthetic rates were different from those under steady-state conditions. Two different transient bursts in the rate of O2 evolution could be distinguished, an early non-rhythmical one that was probably caused by accumulation of inorganic carbon inside the cells, and a second later one that appeared at the peak activity phases of the ciradian rhythm or after blue light. Its origins are unclear.