410 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



It should also be emphasized that the presence of a plentiful supply 

 of plant life (algae) in waters where there is an abundance of fish 

 is also of importance in regulating the balance of carbon dioxide and 

 oxygen. Since the algae use in the production of their food a large 

 amount of carbon dioxide which is given off by the fish, and the 

 fish need a large amount of oxygen which is given off by the algae, 

 the two forms of life derive mutual benefit from their association. 



Of great importance in nature is the effect of algal growth on 

 air and water. But before developing this subject further, it seems 

 necessary to describe briefly the food substances and conditions 

 necessary for the growth of algae. We already know that algae, like 

 other plants, need certain essential elements for their growth. Cal- 

 cium is not essential for many algae, but certain of them are unable 

 to develop without it. Calcium, potassium, and magnesium are im- 

 portant because their bicarbonates furnish a supplemental supply of 

 carbon dioxide for photosynthesis, which is the production of sugar 

 from water and carbon dioxide taking place by the action of 

 chlorophyll in light. During this process, a part of the oxygen is 

 set free, thus providing oxygen for the respiration of animals which 

 in turn throw off carbon dioxide for the plants. It should also be 

 mentioned here that algae also use nitrogen in the form of nitrates, 

 nitrites, or ammonium compounds. A small quantity of iron is also 

 essential to their growth. Under certain conditions, the nature and 

 quantity of the available calcium, magnesium, potassium, nitrogen, 

 and iron compounds have a direct influence upon the existing type 

 of algal flora just as the varying diets of the different races of people 

 affect their characteristic appearance and habits. 



Light, owing to the fact that it is essential for photosynthesis, 

 would seem to be an important factor in the environment of algae. 

 But algae differ markedly in their tolerance of light intensity, as has 

 been shown by our experiments here at the Smithsonian Institution. 

 Provided their food is prepared and in available form, some algae 

 exist in a green condition in the depths of the earth and the ocean 

 with a very small amount of light. The intensity of light that is 

 available for plants growing under water below a depth of 1 meter 

 decreases more or less uniformly with the depth. The turbidity of 

 the water also has an effect on the quality of the light. Water ab- 

 sorbs energy in the infrared and red regions of the spectrum to a 

 much greater extent than in the blue region. As a consequence plants 

 in clear water receive a relatively large percentage of light within 

 the region 4,400 to 5,800 Angstroms. Most plants cannot survive 

 indefinitely in light intensities too low to permit sufficiently rapid 

 photosynthesis to balance the carbohydrates used up in respiration. 



