124 STUDIES OF PHOTO-SYNTHESIS IN MARINE ALG^E 



that they lasted, there was no sign whatever of bacterial growth. 

 The water remained perfectly clear throughout, and there was no 

 indication of a scum on water or weed either in the jars exposed to 

 sunlight or those kept in the dark. From this, one may legitimately 

 conclude that the nitrogen-fixing bacteria, if present, had not 

 multiplied to any great extent nor sufficiently to account for the 

 results obtained. 



SUMMARY. 



1. Marine algae, like fresh- water alga?, can fix elemental nitrogen 

 from water and thus indirectly from air, in presence of sunlight, but 

 not in darkness. 



2. The store of bicarbonates of calcium and magnesium present 

 in sea water furnishes an abundant source of carbon dioxide utilis- 

 able for carbon fixation, and as fixation proceeds the sea water 

 becomes more alkaline. The limit of alkalinity is that at which all 

 bicarbonates have become converted into carbonates, and at this 

 point the potential of hydrogen-ion concentration has fallen below 

 the value P H =10- 9-1 . 



3. In the strong sunshine of spring and summer this degree of 

 alkalinity is sufficient to favour increased rapidity of cell division 

 and induce abnormal and varying forms. 



4. Marine alga3 grown in a limited volume of water and a limited 

 supply of air in sunlight and full daylight fix both carbon and 

 nitrogen rapidly into organic compounds. The amount of nitrogen 

 fixed exceeds many times the total nitrogen originally present as 

 ammonia, nitrite, or nitrate in the water. Moreover, the small 

 initial amounts of nitrogen present in these forms are not decreased. 

 It follows that the only available source is the free nitrogen of the 

 atmosphere. 



