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.
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,
6. E. Greenbaum, ibid. 2...
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