An Evidence Indicating That a Weak Orange Light Absorbed by Phycobilisomes Causes Inactivation of PSII in Cells of the Red Alga Porphyridium cruentum Grown under a Weak Red Light Preferentially Exciting Chl a

Abstract

Changes in the PSII fluorescence upon shift of light quality were studied with the red alga Porphyridium cruentum IAM R-l and supplementary with P. cruentum ATCC 50161, the cyanophytes Synechocystis spp. PCC6714 and PCC6803 and Synechococcus sp. NIBB1071. When Porphyridium cruentum grown under a weak red light (PSI light) preferentially absorbed by Chi a was illuminated with a weak orange light (PSII light) mainly absorbed by phycobilisomes (PBS), a change of PSII fluorescence at room temperature was induced. The ratio of Fvm (Fm — Fo) to Fm was reduced rapidly accompanying the increase in Fo (Ti/2, ca. 3 min). The effects of DCMU and 2,5dibromo-3-methyl-6-isopropyl-/7-benzoquinone indicated that the fluorescence change is induced when plastoquinone pool is highly reduced. The fluorescence change after a short PSII light illumination was reversible; it rapidly recovered in the dark (T1/2, ca. 3 min). The reversibility was gradually reduced and disappeared after 40 h under PSII light accompanying decrease in PSII activity per PBS down to almost 50%. Since the pattern of the fluorescence change resembles that observable when PSII is photoinactiva ted, PSII light probably induces the photoinactivation of PSII, possibly reversibly at first and irreversibly after prolonged illumination. Such a rapid fluorescence change was insignificant in Synechocystis sp. either PCC6714 or PCC6803. Only a slow and small decrease in Fvm/Fm level appeared after prolonged PSII light illumination (the reduction of PSII activity per PBS was around 20%). In Porphyridium, shift from PSII light to PSI light caused a rapid and chloramphenicol-sensitive Fvm/Fm elevation during the first 10 h while the increase in PSII activity per PBS was only 10% of that before the light shift. Then, a gradual elevation followed up to the level at the steady state under PSI light. A similar rapid increase in Fvm/Fm was observed with Synechocystis PCC6714, in which the synthesis of PSII is not regulated, suggesting that a rapid increase in Fvm/Fm