Studies of Photo- synthesis in Marine Algce. 



59 



It is to be regretted that we had not the results above quoted before us 

 when starting our series of experiments, and that in consequence no special 

 precautions were taken to exclude nitrogen-fixing bacteria from our jars. Yet 

 in our opinion it may be reasonably assumed that their presence did not 

 seriously affect the results obtained, for the following reasons : — 



.(1) What we consider to be definitely proven as a result of these and 

 former experiments above referred to is the utilisation of solar energy for 

 the purpose of nitrogen fixation. Column 5 in the Table shows a marked 

 difference in the amount of nitrogen, fixed according as the sea-weeds wer^e 

 exposed to sunlight, or kept in weak light, or in the dark. It appears to'lis 

 to be a point of secondary importance whether that energy is so ut5i§^f^ Ubra^I? 

 the alga itself or by an associated bacterium. If the effects observed are^o^ 

 be ascribed to the latter, it would be a novel experience to find a chlorophyll- 

 less organism so functioning in relationship to hght, and the proof that "it 

 could do so would be of unsurpassed interest in relation to the problem 

 of the origin of life in a chlorophyll-less world. 



(2) It is to be noted that, in order to get nitrogen fixed in appreciable 

 quantities, previous observers have found it necessary to grow their bacteria 

 in appropriate culture media containing small quantities of glucose, mannitol, 

 or other organic compounds, as well as the inorganic phosphates, etc. These 

 media in due course became turbid, and bacterial scums were formed. Our 

 experiments were carried out in pure sea water only, and, during the eight 

 days 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 

 suflficiently to account for the results obtained. 



Summary. 



1. Marine algee, like fresh-water algse, 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 utilisable 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 Ph = 10~^'\ 



3. In the strong sunshine of Spring and Summer this degree of alkalinity 

 VOL. xcii. — B. w 



