82 PHOTOSYNTHESIS 



after the alga has been in anaerobic conditions for two or 

 more hours (i.e. after 'adaptation'). In the dark the alga then 

 absorbs hydrogen from an atmosphere of high hydrogen 

 content or evolves hydrogen if the external concentration 

 of hydrogen is small; in the light these reactions are 

 accelerated but if carbon dioxide is present it is reduced. 

 In the dark in the presence of oxygen and hydrogen both 

 gases may be consumed and the energy of the oxy-hydrogen 

 reaction coupled to the reduction of carbon dioxide. This 

 represents a chemosynthetic reaction such as is found in 

 Bacillus py en oticus. At high light intensities and with carbon 

 dioxide available the cells deadapt, i.e. lose their capacity for 

 photoreduction and revert to normal photosynthesis. De- 

 adaptation is prevented in the presence of hydroxylamine 

 or o-phenanthroline, both of which have been presumed to 

 inhibit the enzyme concerned in oxygen liberation in photo- 

 synthesis, and thus it seems probable that deadaptation 

 occurs simply as a result of the production of oxygen or 

 excess of the precursor of oxygen. The ability to adapt to 

 photoreduction has now been shown to be present in a wide 

 range of algae representing many different families. In con- 

 trast with Athiorhodaceae, hydrogen production in Scene- 

 desmus is not affected by the presence of nitrogen. 



Growth of Scenedesmiis cannot be maintained by photo- 

 reduction or the oxy-hydrogen reaction; their importance 

 lies in emphasizing the close relationship between the photo- 

 synthetic system in these plants and that in the bacteria. 



THE GENERALIZED NATURE OF CARBON DIOXIDE REDUCTION 



Some of the Athiorhodaceae, as mentioned earlier, are 

 able to grow in the dark heterotrophically provided oxygen 

 is present, oxidation supplying the necessary energy for 

 growth. In the dark under anaerobic conditions Rhodo- 

 spirillum rubrum shows the normal propionic acid fermenta- 

 tion of glucose; under aerobic conditions in the dark or an- 

 aerobic conditions in light a compound such as acetate is in- 

 corporated into the cells. Experiments with labelled acetate 

 show that the products in light and darkness are essentially 

 the same although the rate of formation is slower in the dark. 



