The Photosynthetic Efficiency of Phycocyanin in Chroococcus, and the Problem of Carotenoid Participation in Photosynthesis by Robert Emerson and Charlton

Chlorophyll in plant cells is regularly accompanied by other pigments. For many years there has been speculation as to whether the light absorbed by these so called accessory pigments is utilized for photosynthesis. Engelmann's skillful experiments (1883) on algae of various colors, with motile bacteria as an indicator of photosynthetic activity, have been the basis of a general belief that the light absorbed by certain accessory pigments (particularly fucoxanthin, phycoerythrin, and phycocyanin) is available for photosynthesis. This belief, although criticized by some who obtained conflicting evidence, has been strengthened by photosynthesis measurements with technique allegedly superior to that of Engelmann (cf. Harder, 1923). But a critical examination of both early and later work on this subject shows that the evidence is inconclusive, chiefly because the relative amounts of light absorbed in different wave lengths were not established. Comparison of the photosynthetic activity in different wave lengths can be decisive in establishing the part played by accessory pigments only if the absorbed energy in each wave length is known. Recent attention to the problem of the quantum yield of photosynthesis has resulted in development of techniques well suited to investigation of the activity of accessory pigments. Dutton and Manning (1941) have compared light absorption and photosynthetic activity in the diatom Nitzschia closterium in differer~t parts of the spectrum, and found good evidence that the energy absorbed by the carotenoids, especially fucoxanthin, is used in photosynthesis. Recent measurements of the quantum yield of photosynthesis for various species of algae (Emerson and Lewis, 1941, p. 803) gave strong indication that the phycocyanin of the blue-green alga Chroococcus is active in photosynthesis. These measurements were all made in sodium light, X = 589 m#. In the green algae, practically all the absorbed energy of this wave length is absorbed by chlorophyll, but in the case of Chroococcus the phycocyanin is responsible for a large part of the total energy absorbed. Since the quantum yield for Chroococcus photosynthesis was in good agreement with the yields for green algae