10 Brown and Davidson 1977 Competition ants rodents

Once in the pool the two equivalents combine in the

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Unformatted text preview: . Once in the pool, the two equivalents combine in the dark to produce hydrogen through hydrogenase. An alternate, although less probable, mechanism is one in which the first reducing equivalent is supplied by a dark reaction and the second by a light reaction (or vice versa). The sensitivity of the detection apparatus is sufficient to deter880 mine that less than 10 percent of the reaction centers are evolving hydrogen between flashes in a dark reaction. The size of the photosynthetic unit for hydrogen evolution in Chlorella is chlorophyll: H2 ; 1400: 1. The photosynthetic unit size for oxygen evolution (the Emerson and Arnold unit) for Chlorella coupled to benzoquinone is chlorophyll : 02 z 1700 : 1. These data allow a determination of the ratio of hydrogen to oxygen, both normalized to the chlorophyll content of the algae. The ratio of H2 to 02 is 1.2. Were the movement of electrons by chlorophyll as efficient for hydrogen as for oxygen, the stoichiometric ratio would be 2. These data indicate that with regard to the photophysical apparatus of photosynthesis, the ability to utilize absorbed visible quanta for the light-driven reaction is at least 60 percent as efficient for photosynthetic hydrogen evolution as for photosynthetic oxygen evolution. One can think of several reasons why the ratio of H2 to 02 is only 1.2 rather than 2. One possibility is that not all the reducing equivalents are captured by the hydrogenase. Measurements on Scenedesmus D3 give a unit size of chlorophyll : H2, 800 : 1. However, attempts to measure the oxygen unit size have resulted in a very low yield of O2, most likely caused by losses of intermediates. The concept of photosynthetic unit stems from the pioneering work of Emerson and Arnold (1). The experiments described in this report are, to my knowledge, the first determination of the photosynthetic unit size based on hydrogen evolution. This unit size is com- parable to the size based on oxygen evolution. Note added in proof: Since the initial submission of this report flash experiments identical to those performed on Chlorella as described above have been performed on Chlamydomonas reinhardtii. For Chlamydomonas the 02 oscillations are essentially the same as for Chlorella. However, the H2 yield is larger, such that the ratio of H2 to 02 is 1.9. This is close to the theoretical maximum. ELIAS GREENBAUM Rockefeller University, New York 10021 References and Notes 1. R. Emerson and W. Arnold,J. Gen. Physiol. 15, 391 (1932); ibid. 16, 191 (1932). 2. H. Gaffron and K. Wohl, Naturwissenschaften 24, 81 (1936); ibid., p. 103. 3. P. Joliot, G. Barbieri, R. Chabaud, Photochem. Photobiol. 10, 309 (1969); P. Joliot and B. Kok, in Bioenergetics of Photosynthesis, Govindjee, Ed. (Academic Press, New York, 1975), pp. 387-412. 4. B. Kok, B. Forbush, M. P. McGloin, Photochem. Photobiol. 11, 457 (1970); B. Forbush, B. Kok, M. P. McGloin, ibid. 14, 307 (1971). 5. E. Greenbaum and D. C. Mauzerall, ibid. 23, 369 (1976). 6. E. Greenbaum, ibid. 2...
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