ALGAE — MEIER 411 



thus providing oxygen for the respiration of animals which in turn 

 throw off carbon dioxide for the plants. Algae also use nitrogen in 

 the form of nitrates, nitrites, or ammonium compounds. A small 

 quantity of iron is essential to their growth. Under certain condi- 

 tions 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 peoples 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 respect to 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 absorbs 

 energy in the infra-red and red region to a much greater extent than 

 in the blue. As a consequence plants in clear water receive a rela- 

 tively large percentage of light within the region 4400 to 5800 angs- 

 troms. Most plants cannot live indefinitely in light intensities too 

 low to permit sufficiently rapid photosynthesis to balance the carbo- 

 hydrates used up in respiration. The depth at which the compensation 

 point occurs depends on the species as well as on the quantity of light 

 available. The point at which photosynthesis just balanced respira- 

 tion for certain algae was found to occur from 7 to 20 meters in 

 turbid water and at 30 meters in clearer water. The optimum location 

 for photosynthesis in the lakes of northern Wisconsin was found to 

 be at the surface on cloudy days and at a depth of about 5 meters on 

 fair, bright days. The brown and green algae require higher light 

 intensities for a photosynthetic balance than the red algae. The 

 ability of the red algae to live at greater depths than the green or 

 brown algae may be due to the fact that the red algae absorb a 

 greater percentage of blue light. 



Temperature has an important effect on the acceleration or retarda- 

 tion of growth and reproduction, and under exceptional conditions 

 the temperature of the habitat restricts the algal population to certain 

 species. 



The quantity of water or moisture necessary for algal growth varies, 

 as may be seen, from the large amount required by the plants that 

 live submerged in the ocean to the infinitesimal quantity at the 

 disposal of the aerial algae. 



