204 SULPHUR BACTERIA 



showed that the relative efficacy of daylight, red and blue 

 light was about as 6 : 3 : i respectively. 



The Function of Light : Photosynthesis. 



It is universally agreed that light is essential to the sulphur 

 bacteria, but there is considerable diversity of opinion as to 

 its precise function. It has been maintained by Engelmann 

 and others that light is a source of energy for an anabolic 

 process comparable to the photosynthesis of green plants. It 

 will be recollected that the chlorophyllous plant tissues utilize 

 solar energy for the synthesis of carbohydrates from carbon 

 dioxide and water. During the process oxygen is liberated 

 as a waste product. 



Now Engelmann has demonstrated that the purple bacteria 

 liberate oxygen in the light, and it is from this that he deduces 

 that a similar photosynthetic process takes place in them. 

 Both Engelmann and Molisch have made the liberation of 

 oxygen the test of photosynthesis, whereas the essential proof 

 is the demonstration of the synthesis of more complex from less 

 complex compounds. Engelmann proclaimed the occurrence 

 of photosynthesis chiefly because of the liberation of oxygen, 

 whilst Molisch denied its occurrence because he considered 

 oxygen was not liberated. It is possible to imagine a photo- 

 synthetic process in which oxygen is not liberated.* Hence 

 as neither advanced proof of synthesis the question must 

 still be regarded as an open one. Subsequent opinions on 

 the occurrence of photosynthesis have mainly centred on the 

 question whether oxygen is liberated, and are therefore not 

 relevant. 



Whilst definite proof of photosynthesis is not available the 

 following facts form strong indirect evidence of its occurrence 

 in the purple bacteria. 



* As an analogous case may be mentioned the fact that respiration 

 is in essentials the katabolism of complex compounds, and not only may the 

 process be imagined without the intake of oxygen, but there are actually 

 organisms (the anaerobic bacteria) in the respiration of which oxygen 

 is not taken in. 



