Action Spectrum of the "Second Emerson Effect".

A peak is found at 670 mmu in the action spectrum of the "second Emerson effect" (22, 33), in the green alga Chlorella pyrenoidosa, and in the diatom Navicula minima; a shoulder appears in about the same location in the blue-green alga Anacystis nidulans and less clearly in the red alga Porphyridium cruentum. This peak (or shoulder) corresponds to an absorption band belonging to a form of chlorophyll a in vivo, which can be designated as "Chl a 670." Light absorption by this form can enhance the yield of photosynthesis caused by far red light (680 to 720 mmu), as effectively as does light absorption by chlorophyll b, chlorophyll c, fucoxanthol, phycocyanin, or phycoerythrin. The action spectrum of the second Emerson effect in Anacystis nidulans shows three peaks attributable to phycocyanin, at 570, 600, and 640 mmu. These correspond well to peaks on the curve, calculated by Emerson (17), which shows the fraction of total absorbed light absorbed by phycocyanin as function of wavelength. Intensity relation between the two participating beams has an important bearing on the second Emerson effect. In Anacystis, the "negative effect," described in (17), can be converted into a positive effect by change in this relation. In Anacystis, the saturation of photosynthesis occurs, at 700 mmu, in weaker light and on a lower level than at 680 mmu, and even more so, than at 600 mmu. This may explain, at least in part, the negative Emerson effect observed in this alga.