FACTORS INFLUENCING THE EFFICIENCY OF PHOTOSYNTHESIS

WARBURG AND Negelein (1923) reported that Chlorella, when grown under suitable culture conditions, could assimilate carbon dioxide at the rate of 0.25 molecule per absorbed quantum of visible light, a yield close to the supposed theoretical limit. Before this quantum yield becomes an acceptable basis for theoretical discussions of the assimilatory mechanism, three questions require consideration. First, is the value reproducible in other laboratories? Second, can still higher values be obtained? And third, do the measured values truly represent the quantum yield of the assimilatory mechanism? Although Manning, Stauffer, Duggar and Daniels (1938), using methods different from Warburg and Negelein's, found considerably lower yields, the first question has been answered in the affirmative by Rieke (1939), who obtained yields close to 0.25 by following the technique of Warburg and Negelein in detail. The second and third questions are dealt with in the present paper. Part I shows that the efficiency of photosynthesis is critically dependent on a number of factors, and that favorable combinations of these factors lead to quantum yields as high as 0.33. Part II presents experimental evidence that two assumptions upon which ordinary manometric measurements of photosynthesis depend do not hold for the conditions of the quantum yield measurements, so that the published values should be regarded as characteristic of the method by which they are obtained, rather than of the assimilatory mechanism itself. Our experimental technique and method of calculation of results are explained in Part III. The general procedure was the same as that of Warburg and Negelein. Liquid cultures of Chlorella were grown to a suitable density, harvested, washed, and subjected to manometric measurements of photosynthesis in sodium light (A 589 m,u) of known energy content. The pressure change due to photo1 Received for publication September 14, 1939. synthesis was computed from observations made during alternating ten-minute periods of light and darkness, and from this pressure change the photosynthesis was computed, exactly in the manner of Warburg and Negelein. As will be shown in Part II, the values obtained in this way are probably not true efficiencies but may be anticipated to be proportional to the true values. In Part I, where we discuss the influence of various factors on efficiency, we mean only the efficiency measured by this particular system of computation. PART I. CONDITIONS LEADING TO HIGH EFFICIENCY. -COMPOSITION OF THE CULTURE MEDIUM.-The addition to the medium of small amounts of certain substances may profoundly affect the efficiency of cells grown in the medium. Traces of organic materials (yeast extract, adenine, uric acid) sometimes increased growth but failed to improve the quantum yield. Traces of inorganic materials (particularly various heavy metals) caused increases in efficiency as well as in growth. Little attention has heretofore been paid to heavy metals in connection with the quantum yield of photosynthesis, probably because the necessary traces have usually been included in the salts and water used to prepare the medium. But failure to obtain high efficiency may be due to the use of salts and water which contain too little of the essential heavy metals. If very pure water is used, it becomes necessary to supplement the ordinary formula for the culture medium. We have found that the addition of an extra amount of ferric sulfate may greatly increase the efficiency of cells grown in a given medium. It has been possible to show that this improvement is not due to the iron itself, but to impurities contained in abundance in this particular iron salt. Indications of the identity of the impurities responsible have been obtained by direct addition of traces of eleven different elements to the culture medium.