Studies of Photo- synthesis in Marine Algce. 



53 



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.* The increased alkalinity can be used as a measure of 

 the photo-synthetic activity, and was so employed by Moore, Prideaux, and 

 Herdmanf in determinations made at Port Erin during the years 1912-1915, 

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

 level to which this enhanced alkalinity can attain without destroying the 

 green cell which is producing it. Angelstein§ found that, in solutions 

 containing one part of bicarbonate to two of carbonate, the plants continued 

 to give off oxygen. Osterhout and Haas determined by the titration method 

 that the alkahnity can be increased until it reaches a level represented by 

 Ph = 9, Moore, Prideaux, and Herdman having previously fixed the limit at 

 Ph— less than 91, the last-quoted observers further pointing out that this 

 corresponds to the point at which all the bicarbonates have been converted 

 into carbonates, and this has again been confirmed in the present experi-^ 

 ments. 



If a sample of normal sea water be titrated with centinormal acid in the 

 presence of a stable indicator such as " di-methyl " or " methyl-orange," a 

 figure defining the entire content of alkali present as bicarbonate is obtained. 

 This figure scarcely ever varies with season or otherwise, and amounts to 

 about 24 c.c. of centinormal alkali per 100 c.c. of sea water. If, now, some 

 sea water be taken, and a green algal growth be exposed in it to bright 

 daylight or sunshine, so as to produce as intense a degree of photo-synthesis 

 as possible, and then the amounts of normal carbonates formed from 

 bicarbonates, as carbon is synthesised into organic compounds, be deter- 

 mined by titrating the alkalinity to an indicator such as phenolphthalein, it 

 will be found that the limit is just about half the preceding value, namely, 

 12 c.c. of centinormal alkali per 100 c.c. of sea water. This limit marks the 

 point at which all the bicarbonates of magnesium and calcium present in the 

 sea water have become converted into normal carbonates. If photo-synthesis 

 passed this point, free hydrates of magnesium and calcium would commence 

 to be present in the sea water, and there would be a correspondingly 



* See Czapek, ' Bio-chemie der Pflanzen,' 2nd edition, vol. 1, pp. 518, 519, where 

 numerous references may be found. 



t "Report on the Lancashii'e Sea-Fisheries Scientific Investigations for 1914," 

 Herdman, 'Trans. Biological Society of Liverpool,' vol. 29, p. 171 (1915). 



l "On the Dynamics of Photo-synthesis," ' Jnl. of General Physiology,' vol. 1, p. 1 

 (1918). 



§ ' Cohn Beitrage,' vol. 10 (1911). 



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