On the Minimum Quantum Requirement of Photosynthesis

An analysis of the shape of photosynthetic light curves is presented and the existence of the initial non-linear part is shown as a consequence of the operation of the non-cooperative (Kok’s) mechanism of oxygen evolution or the effect of dark respiration. The effect of nonlinearity on the quantum efficiency (yield) and quantum requirement is reconsidered. The essential conclusions are: 1) The non-linearity of the light curves cannot be compensated using suspensions of algae or chloroplasts with high (>1.0) optical density or absorbance. 2) The values of the maxima of the quantum efficiency curves or the values of the minima of the quantum requirement curves cannot be used for estimation of the exact value of the maximum quantum efficiency and the minimum quantum requirement. The estimation of the maximum quantum efficiency or the minimum quantum requirement should be performed only after extrapolation of the linear part at higher light intensities of the quantum requirement curves to “0” light intensity

Quantum Requirement of Photosynthesis in the Primarily Chlorophyll d Containing Prokaryote Acaryochloris marina

The recently isolated and characterized unicellular photosynthetic prokaryote Acaryochloris marina (Miyashita et al., 1996) contains chlorophylls a, d , and traces of a chlorophyll c-like pigment as well as phycocyanin. a type of allophycocyanin, zeaxanthin and cx-carotene, chlorophyll d being the predominant chlorophyll component. Quantum requirement measurements of the photosynthetic oxygen evolution resulted in about 12 quanta for excitation of chlorophylls a and d and 18 for phycocyanin. The data also revealed that these pigments are involved in energy absorption for photosynthetic oxygen evolution. Energy is transferred efficiently and equally well between the chlorophylls. Light absorbed by phycocyanin which is organized in phycobiliprotein aggregates (M arquardt et al., 1997), results in a less efficient energy transfer to the reaction center chlorophylls