CIRCADIAN GROWTH IN PORPHYRA UMBILICALIS (RHODOPHYTA): SPECTRAL SENSITIVITY OF THE CIRCADIAN SYSTEM

The circadian rhythm in growth of the red macroalga Porphyra umbilicalis (Linnaeus) J. Agardh was investigated under different spectral light conditions in laboratory‐grown thalli. A free‐running rhythm was observed in constant green or red light at irradiances of 2.5 to 20 μmol photons·m−2·s−1, whereas arhythmicity occurred in constant blue light at 6–20 μmol photons·m−2·s−1. The circadian oscillator controlling growth rhythmicity in Porphyra uses most of the visible sunlight spectrum and possibly multiple photoreceptors with a high sensitivity for blue light and a lower sensitivity for red light. This was inferred from three experimental results: (1) The free‐running period, τ, of the growth rhythm decreased with increasing irradiance, from approximately 25 h at 2.5 μmol photons·m−2·s−1 to 22 h at 20 μmol photons·m−2·s−1 in red or green light, (2) Dark pulses of 3 h duration, interrupting otherwise continuous green or red light, caused advances during the subjective day and delays during the subjective night; the circadian oscillator in Porphyra can discriminate darkness from green or red light, and (3) Low‐irradiance blue light pulses (2.5 μmol photons·m−2·s−1) shifted the growth rhythm in red light of higher irradiance (e.g. 10 μmol photons·m−2·s−1), and a strong, high amplitude, type 0 phase response curve was obtained that is usually observed with light pulses shifting a circadian rhythm in otherwise continuous darkness.