The maximum quantum yield controversy. Otto Warburg and the ‘Midwest-Gang’

Plant scientists, particularly those not deeply involved in photosynthesis, may find the title of this illuminating, fascinating, entertaining and well-written book obscure and be tempted to pass it by: that would really be a pity. So what is it about? Well – essentially the historical scientific politics of the light reactions of photosynthesis! Not an aspect most practitioners of the ‘art’ get involved with. The scientific background to this story of recurring international experimentation and disagreement, against a background of war, is rather straightforward. Assimilation of carbon dioxide, nitrate, sulphate, etc, to form the great range of compounds required for an oxygenic organism's biochemistry uses light energy, quanta in the form of photons, to dissociate water, freeing electrons and so generating reductant, which is used in the assimilation: as a by-product oxygen is released. The ‘Maximum quantum yield’ (QY) of the title is the maximum number of events (in the book it is release of one molecule of O2) resulting from the minimum number of photons (quanta) absorbed. This is standard textbook information, with QY taken as about 0·1 O2 per photon or, put the other way (the quantum requirement = 1/QY), about 10 photons required per O2. However, it was not easy to acquire this knowledge. There was a long development of understanding of photosynthesis, with emphasis on the light reactions starting from considerably before World War II to long after it. Progress was particularly fraught: the ‘controversy’ of the title. And there were major international scientists involved who drove the science and the controversy. On one side was the renowned German biochemist Otto Warburg, recipient of the Nobel Prize for Medicine or Physiology in 1931 for work on oxidation (respiratory) processes in cells. The methods he developed (particularly the Warburg manometer) in the period 1912–1920 were applied to photosynthesis, addressing fundamental questions of the nature of the processes. Einstein had shown in 1912 that to understand the mechanisms of photochemical phenomena, the maximum yield (minimum requirement) was the essential feature. So, it is perhaps not surprising that Warburg addressed the question, measuring QY in Chlorella cultures. Around 1919–1923 he and Negelein published results that were interpreted as a quantum requirement of 4–5 photons per O2 released. This fitted with assumptions about mechanisms and was accepted for many years. Such a large QY implied considerable efficiency in energy use and later led to the usual feverish speculation about the power of science to deliver, via plants, unlimited solar energy for human exploitation. Of course, others were also measuring QY: in 1938 a research group at the University of Wisconsin, USA, published values of 16–20 photons required. Also before the war, others had data questioning (and roughly doubling) Warburg's values but did not publish it (which may be understood – but it is a warning not to delay). The principal US challenger to Warburg was, however, Robert Emerson, who did his PhD with Warburg in 1927 and apparently accepted the Warburg number. Only much later and, particularly, when at the University of Illinois, Urbana-Champaign (the ‘mid-West’ of the title) did he re-examine the QY values, coming to about 12 photons per O2, and a consensus supporting this value developed in the US. However, Warburg did not agree (the book suggests a master–pupil conflict could be a contributing factor to the conflict) and continued experimenting and publishing supporting evidence, much with the US scientist Burk, which further complicated matters. The controversy dragged on, despite attempts to arrange collaborative experiments and scientific meetings to thrash out the chaff and get to the kernel. What were the right conditions? Was the water suitable for Chlorella in the US? Mud flew – who could or could not measure light properly? The science so gripped the participants that even the war could only delay attempts to get to that elusive kernel. Driven as always by egos and the desire to be ‘right’, the controversy faded only when other data emerged to support the ‘mid-West gang’, and led to the well-tested model of photosynthetic light reactions accepted now. In undergraduate courses in plant sciences it is probably rare to consider history and the struggle towards a consensus that the mass of ‘facts’ relies upon. Perhaps such study would destroy the myth of ‘objectivity’ that science likes to present (it might frighten the paymasters, who would seek another priesthood to rely on). So there is detachment of the human side of scientific endeavour from the achievements. In reality there is continual conflict – competition rules! However, better understanding of the human history in delivering scientific ‘facts’ would not hurt – indeed would enhance – all aspects of the wider scientific process. That is why this book is so fascinating and informative. Govindjee was Emerson's PhD student (the last) and met many of the participants in the story, before carving his own unique niche in photosynthesis. He and Karin Nickelsen have provided a valuable and detailed record of how QY was conquered, full of human stories and of potential value to today's scientists in the cut-and-thrust of a what is often anything but objective evaluation. The illustrations and the selections from letters show real people, so that it is not a cold narrative but full of life. The book has photographs of the protagonists and many citations from scientific and private correspondence, does not focus on the photosynthetic mechanism, and is well presented. The lack of an index detracts from its use as a reference text. Some aspects of the analysis leave questions. One is how Warburg's view of the events is presented and how objective the appraisal of him, as a person and as a scientist, is. Of course, he lost the argument and (as the book suggests) could not accept 10 or 12 instead of 4 or 5 photons. His early vision and achievements were undoubted and come over clearly in the analysis. However, other features of character and his responses did detract from these, and led to questions, raised in the later part of the book, about probity and objectivity. Ultimately disagreements in science spill over into animosity – that very animal feature of human behaviour. Studies of photosynthesis have not been isolated (nor are) from such wider aspects of endeavour. This book deserves to be read, enjoyed and digested from many view points. It is for general readers, but also specialists, and deserves a wide audience.