SOIL ALGAE 231 



synthesize their protoplasm from C0 2 and from water containing inor- 

 ganic nitrogenous and mineral compounds. Some algae, however, can 

 also utilize organic materials, the extent depending on the species; some 

 may thus develop in the complete absence of light, leading a hetero- 

 trophic existence. Under those conditions, the chlorophyll may be 

 either completely lost or retained. Some species can even utilize or- 

 ganic nitrogenous compounds, and may bring about decomposition 

 of proteins (Chodat). In general, however, algae prefer nitrates as a 

 source of nitrogen; ammonium salts are less favorable. Of the nitrates, 

 Ca(N0 3 ) 2 is best, followed by KN0 3 and NaN0 3 . The secondary am- 

 monium phosphate is preferable to the other salts of ammonium, for, 

 when the ammonium is used up, the secondary phosphate will be changed 

 to the primary, which is only slightly acid, but the sulfate and chloride 

 will leave the reaction of the medium acid. The preferential utilization 

 of certain nitrogen sources may be due to a large extent to secondary 

 reactions brought about by the residual ions. As different species 

 behave differently toward the different sources of nitrogen, until further 

 work has been done with a large number of species, no general conclusions 

 can be drawn. Nitrites are not favorable, but can be utilized under 

 proper conditions of reaction and concentration (Pringsheim) . In 

 addition to water, carbon dioxide and nitrogen source, algae require 

 for nutrition K, Fe, Mg, P, S, and in most cases also Ca. These are 

 best added in the form of potassium phosphate and magnesium sulfate ; 

 a trace of iron is added in the form of chloride; and calcium, as sulfate, 

 if it is not used as nitrate. The salts are used only in very dilute solu- 

 tions 43 : Ca(N0 3 ) 2 as 0.1 per cent, MgS0 4 as 0.01 per cent and K 2 HP0 4 

 as 0.02 per cent. A faintly alkaline reaction, as given by secondary 

 phosphate and alkali bicarbonate, is best. 



A biochemical process believed to be carried out by algae, namely, 

 their ability to fix atmospheric nitrogen attracted considerable attention. 

 As most of the earlier work in this connection has been carried out with 

 impure cultures contaminated with various bacteria, results obtained 

 under these conditions were not reliable. The negative results could be 

 depended upon more than the positive results. Frank 44 suggested in 



zur Morphologie und Physiologie der Euglena gracilis Klebs. Jahrb. wiss. Bot. 

 51: 435-514. 1912; Bristol Roach, B. M. On the relation of certain soil algae to 

 some soluble carbon compounds. Ann. Bot. 40: 149-201. 1926. 



43 Richter, 1911 (p. 219). 



44 Frank, B. Ueber den experimentallen Nachweis der Assimilation freien 

 Stickstoffs durch erdbewohnende Algen. Ber. deut. Bot. Gesell. 7: 34-42. 1889. 

 Untersuchungen iiber die Ernahrung der Pfianze mit Stickstoff und iiber den 

 Kreislauf desselben in der Landwirtschaft. Landw. Jahrb. 17: 421-453. 1888. 



