116 STUDIES OF PHOTO-SYNTHESIS IN MARINE 



necessary to feed the algse with nitrogen, were this the source, would 

 be immense. Thus, Gebbing 1 found the amount of nitrogen as 

 ammonia present in sea water to be only 0-05 mgrm. per litre, and the 

 amount as nitrite plus nitrate as 47 mgrm . per litre. 2 These results 

 are confirmed by some to be given later in this chapter. 



This paucity of nitrogenous compounds in sea water, while it 

 indicates that the nitrogen of the plant tissues is probably derived 

 from the dissolved elemental nitrogen of the atmosphere, is not, 

 however, clear proof, for the sea water, for example, contains but a 

 trace of that silica from which the skeletons of the diatoms are 

 derived. It might be argued that in the restless movement of the 

 sea the volume of water which daily laved the plants was ample to 

 compensate for the small amount of dissolved nitrogenous com- 

 pounds in the water. To settle this query it is obviously necessary 

 to grow marine algae in a limited volume of sea water and then to 

 determine the amount of nitrogen fixed. If this latter many times 

 exceeds the amounts of nitrogen present as ammonia, nitrites, and 

 nitrates in the sea water used, then clearly for here there is no soil 

 to obscure the issue this fixed nitrogen must come from the dis- 

 solved nitrogen in the sea water, which in turn came from the 

 nitrogen in the air. 



This has been done in the experiments recorded below, with the 

 clearest proof that marine algae do so fix their nitrogen. 



As photo- synthesis proceeds, and the supply of carbon is drawn 

 from the dissolved bicarbonates, the reaction of the nutrient 

 medium becomes all the time more alkaline. 3 The increased 

 alkalinity can be used as a measure of the photo-synthetic activity, 

 and was so employed by Moore, Prideaux, and Herdman 4 in deter- 

 minations made at Port Erin during the years 1912-1915, and 

 subsequently by Osterhout and Haas. 5 It is interesting to note the 



1 J. Gebbing, Centralb.f. Bacterial., 2 Abth., vol. xxxi. 



2 Cf. also Moore. Edie, Whitley, and Dakin, " The Nutrition and Metabo- 

 lism of Marine Animals," Biochem. Journ.,vol. vi., p. 255 (1912); Raben, 

 Wissensch. Meeresuntersuch.. Kiel, vol. viii. (1905). 



3 See Czapek, " Bio-chemie der Pflanzen," 2nd edition, vol. i., pp. 518, 519, 

 where numerous references may be found. 



* " Report on the Lancashire Sea-Fisheries Scientific Investigations for 

 1914," Herdman, Trans. Biological Society of Liverpool, vol. xxix., p. 171 

 (1915). 



5 " On the Dynamics of Photo -Synthesis," Journ. of General Physiology, 

 vol. i. ; p. 1 (1918). 



