STUDIES OF PHOTO-SYNTHESIS IN MARINE ALG^E 117 



level to which this enhanced alkalinity can attain without destroying 

 the green cell which is producing it. Angelstein 1 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 alkalinity can be 

 increased until it reaches a level represented by P H =9. Moore, 

 Prideaux, and Herdman had previously fixed the limit at P H less 

 than 9-1; the last-quoted observers further point out in this 

 earlier work 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 experiments. 



If a sample of normal sea water be titrated with centinormal 

 acid in the presence of a stable indicator such as " dimethyl " 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 centi- 

 normal 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 

 determined 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 bicar- 

 bonates 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 rapid increase in hydroxyl-ion concentration and 

 fall in hydrogen-ion concentration. 



It is this accumulation of alkali which limits photo- synthesis by 

 killing the cell, for up to quite close to this " all- carbonate " point 

 the cell flourishes and synthesises rapidly, but if kept for some time 

 by too violent exposure to sunlight at this point, the algae turn 

 yellow and the growth is killed, for there is no recovery, even if 

 removed from the strong light. The degree of alkalinity reached 

 by the green cell before death is, however, remarkable. It is much 

 greater than anything which can be borne by any mammalian cell, 



1 "Cohn Beitrage," vol. x. (1911). 



