316 CATALYST POISONS AND NARCOTICS CHAP. 12 



strong light) by 12%, and of IQ-^ m./l. by 72%; 10"" m./l. causes a 

 complete stoppage of oxygen production. The respiration of Chlorella 

 is stimulated by a concentration of 10~^ rn./l. H2S (oxygen consumption 

 is increased by a factor of 1.8; whereas the respiration of yeast is com- 

 pletely suppressed by the same sulfide concentration). These results 

 are closely parallel to those obtained by Warburg with hydrocyanic acid. 

 Nakamura (1938) pointed out the analogy between the effects of hydrogen 

 sulfide on photosynthesis and on catalase activity (cf. Table 11.11). It 

 is plausible that the H2S-sensitive factor in photosynthesis is the oxygen- 

 liberating enzymatic system, since plants which do not liberate oxygen 

 in light (sulfur bacteria, H2S-adapted algae) are not only uninhibited by 

 hydrogen sulfide, but even utilize this compound as an oxidation sub- 

 strate, as described in chapters 5 and 6. If this interpretation is correct, 

 experiments made with hydroxylamine should be repeated with hydrogen 

 sulfide. 



(6) Carbon Monoxide 



Padoa and Vita (1929) observed a partial or complete inhibition by 

 carbon monoxide of photosynthesis in Plantago major and in the aquatic 

 plants, Lemna minor and Elodea canadensis ; their respiration was stimu- 

 lated rather than inhibited. These authors assumed that carbon mon- 

 oxide is absorbed by chlorophyll in the same way as by hemoglobin. 

 Later (1932), they described reversible changes in the absorption spectra 

 of chlorophyll solutions in benzene, caused by saturation with carbon 

 monoxide, and considered these results as proofs of the existence of a 

 labile addition compound of chlorophyll and carbon monoxide. How- 

 ever, their photometric curves are not convincing (cf. Vol. II, Chapter 

 21); and in the opinion of Gaffron (1935), their data on the inhibition 

 of photosynthesis by carbon monoxide also are unreliable. 



Gaffron has investigated the effect of this gas on different algae (e. g., 

 Phormidium tenue), and found that carbon monoxide has no effect or 

 only a very slight effect on ordinary photosynthesis. On the other 

 hand, he found strong carbon monoxide effects — ranging from a reversible 

 stoppage of gas exchange to an irreversible injury — when this gas was 

 allowed to act on certain algae after a period of anaerobic incubation. 

 As described in chapter 6, Gaffron later (1942) found that this incubation 

 caused the transition from ordinary photosynthesis to "photoreduction" 

 involving molecular hydrogen or intercellular hydrogen donors. Gaffron 

 suggested, therefore, that carbon monoxide (in concentrations of the 

 order of 20%) is a specific inhibitor of the hydrogenase (whose activation 

 is responsible for the oxidation-reduction processes in adapted algae), 

 and does not interfere with the enzymatic system of normal photo- 

 synthesis. By inhibiting hydrogen absorption and not interfering with 

 photosynthesis, carbon monoxide prevents the adaptation and accelerates 



