36 THE ROLE OF ALGAE AND PLANKTON IN MEDICINE 



serve as the sole sources of nitrogen for various green algae via deamina- 

 tion or transamination. It is probable that nitrogen enters the general 

 metabolic pool most often in this manner. 



Nitrogen-fixing algae can contribute to the fertility of the soil, especi- 

 ally in tropical areas. A notable example is the utilization of blue-green 

 algae in the Usar area of northern India to decrease the excessive alkalinity 

 of the soil. 15 The algae thrive during the rainy season and decrease the 

 pH of the soil from 9.0 to as low as 7.0, and thus make possible the 

 growing of field crops during the dry season. This resembles somewhat 

 the employment elsewhere of large seaweeds as direct fertilizer. 



The cultivation of rice is also aided by microalgae. The latter flourish 

 in the wet rice paddies and, by fixing atmospheric nitrogen, enable peren- 

 nial cultivation without additional fertilizer. Nitrogen fixation here may 

 be greatly enhanced if bacteria are also present and contribute their 

 fixation activities. In addition, the algae, by producing oxygen, help to 

 aerate the rice plants. Watanabe 221 reported in 1951 that 13 species of 

 643 blue-green algae from Far East and South Sea area rice fields were 

 capable of atmospheric nitrogen fixation, chiefly in warmer climates. Most 

 of them belonged to the genera Tolypothrix and Nostoc, some to Schizo- 

 thrix, Calothrix, Anabaenopsis, and Plectonema. Aspartic and glutamic 

 acids and alanine were the systems mostly involved. A similar function 

 was reported by Douin, 222 who noted a symbiosis between the Cycadaces 

 plant and Anabaena cycadeae, which fixes nitrogen for its host. 



Much less is known about the carbohydrates of microscopic algae than 

 of the macroscopic. 40 - 41 Whereas the latter may contain large quantities 

 of carbohydrates in the cell walls (chiefly alginic complexes), the micro- 

 algae have relatively scanty structural materials, and their storage products 

 are lipid rather than starchy. Milner has isolated starch and sucrose from 

 Chlorella pyrenoidosa. Broun found 17.8-20.2 per cent (dry weight) 

 total carbohydrate in Scenedesmus obliquus. The substances included 

 mainly insoluble polysaccharide; there were also small amounts of free 

 reducing sugar, sucrose and other oligosaccharides, and water-soluble 

 polysaccharides. 



Healthy algal cells, particularly in older cultures, liberate various or- 

 ganic substances into their fluid environment. These consist primarily 

 of nitrogenous substances and pentosans. It is probable that more specific 

 knowledge of microalgal carbohydrates will emerge when higher carbo- 

 hydrate contents are achieved by varying cultural environmental condi- 

 tions. 



