PHOTOSYNTHETIC ACTION SPECTRA AND LIGHT‐HARVESTING IN GRIFFITHSIA MONILIS (RHODOPHYTA)

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Abstract
Abstract— In cells of the red alga Griffithsia monilis the action spectrum of photosynthetic oxygen production at low light intensity shows that the phycobilins (including allophycocyanin) are the major light‐harvesting pigments. As the light intensity is increased carotenoids and chlorophyll a contribute proportionately more to the spectrum, since the phycobilin activity becomes light‐saturated. When action spectra are performed against a background light of various monochromatic wavelengths it can be shown that chlorophyll a increases in its light‐harvesting activity. Nevertheless light absorbed at a single wavelength (487 nm) by phycoerythrin (and possibly a carotenoid) still shows the highest photosynthetic activity. Fluorescence measurements at 77K indicate that a chlorophyll a fluorescence is small and that the amount of chlorophyll all (f 693) is very low. A model is proposed in which the phycobilins, in phycobilisomes, pass on absorbed light energy to either photosystem, whereas light absorbed by chlorophyll is passed on mainly to photosystem I.

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