From the Red Drop to the Z‐scheme of photosynthesis

Abstract

In 1943, the US-American biophysicist Robert Emerson (see Fig. 1) and his collaborator, the physicist C. Charlton M. Lewis, were investigating the efficiency of photosynthesis, determined as its quantum yield, i.e., the number of light quanta required for the release of one molecule of oxygen, and its relationship to the wavelength of light.1 In the course of this work, they observed, at low light intensities, an unexpected sharp decline of photosynthetic efficiency in the far red, that is, from 685 nm towards the infrared region of the spectrum [5, p. 166]. This was the phenomenon that later became known as the “Red Drop” of photosynthesis efficiency (see Fig. 2). The puzzling fact was that at wavelengths even above 685 nm chlorophyll absorption was still rather high, and no pigments were known to compete with chlorophyll in this region, whichmight have caused the drop in efficiency. Emerson and Lewis were completely at a loss as to how to explain this finding. And as in the years afterwards, Emerson became involved in an atrocious controversy on the maximum quantum yield of photosynthesis with the eminent – and extremely stubborn – cell physiologist Otto Warburg, over the next ten years he did not investigate this curious phenomenon any further.2 It was only in 1955 that Emerson came back to his former line of enquiry, and he hit upon an even stranger effect. It turned out that the Red Drop of photosynthetic efficiency disappeared if a supplementary light beam of shorter wavelengths was provided (see [7]). As a possible explanation, Emerson and his co-workers, Ruth Chalmers and Carl Cederstrand, suggested that “the significance of the supple-