THE CYCLES 377 



IV. THE CIRCULATION OF PHOSPHORUS 



The circulation of this element so necessary to life takes place exclusively 

 by way of compounds in which the phosphorus is in its highest state of 

 oxidation and hydration. Orthophosphoric acid, originating in the in- 

 organic world, during its passage through the living world undergoes no 

 chemical change apart from the formation of salts and esters (calcium 

 phosphate in bones, phosphatides, nucleoproteins, sugar phosphates, etc.). 



Before entering the living world, the phosphoric acid in inorganic com- 

 pounds must first be mobilized. This is brought about by the liberation 

 of various strong acids into the soil by numerous aerobic and anaerobic 

 micro-organisms. The COg liberated by the roots of plants and by 

 bacteria also plays a part. Once rendered soluble the phosphates may be 

 reprecipitated or assimilated by plants and then passed to animals in 

 organic or inorganic combination. The excreta of animals and the dead 

 bodies of plants and animals return to the soil the phosphates which have 

 been removed. In this way a local accumulation of phosphate may be 

 obtained and used as fertilizer. 



One remarkable aspect of phosphorus metabolism in the biosphere is 

 the increase in phosphate concentration with depth in the ocean. It 

 appears that the phosphorus gradually passes into the sediment at the 

 bottom and it can be calculated that several times the amount present in 

 the ocean has been removed in this way and been replaced by phosphorus 

 from the lithosphere brought down by the rivers. Compared to the removal 

 of this element the return of phosphorus to the continents is ver}^ much in 

 arrears since it depends on the uptake of food from the sea by animals, 

 especially man and birds. The fishing industries of the world take around 

 60,000 tons of phosphorus (25 to 30 million tons of fish). Sea birds bring 

 back to the land around ten times this figure. 



Thus there is a considerable deficit between the 137 x 10^ tons of 

 phosphorus added to the ocean each year from the continents and the 

 amount returned to the land by the feeding and excretion of marine 

 animals. As a result there is a constant accumulation of phosphorus in the 

 depths of the oceans and it can no longer be used to support terrestrial life. 



V. THE METABOLISM OF THE BIOSPHERE 



The cycles we have just described show, for the principal elements con- 

 cerned, the entry of these elements into the biosphere from the inorganic 

 world and their return to the inorganic world. Taken together these cycles 

 describe the metabolism of the biosphere and they regulate its mass and 

 distribution. Each region of the biosphere, made up of a community of 

 organisms and their surroundings, is a functional unit and is termed an 

 ecosystem. 



