THE CYCLES 379 



the consumption of larger animals. The decomposing and transforming 

 organisms in the marine habitat are, as on the land, micro-organisms. 



Considered as a whole, the biosphere is nourished essentially by COg, 

 nitrogen, water, sulphates, nitrates and phosphates. The greater part of 

 the energy entering it is of solar origin. 



One important aspect of the metabolism of the biosphere is the topo- 

 graphy of its distribution. On land, the products of the activity of degrad- 

 ing organisms may be retained in the soil, although there is always a loss 

 which may be considerable in acid soils or where there is excessive rain. 

 In the sea, dead bodies end by sinking into the depths and they may be 

 displaced by the currents so that the products of their transformation may 

 be situated far away from the place where the macromolecules were 

 synthesized. In the depths of the ocean phosphates and nitrates may be 

 deposited far away from the places where photosynthesis is occurring 

 but they may be brought to the surface again by vertical currents. Although 

 during the year the displacement downwards and the displacement upwards 

 may approximately cancel each other out, mineral salts are being con- 

 tinuously poured into the sea and a considerable amount is accumulated 

 on the sea bottom and is inexorably removed from the terrestrial habitat. 



On the other hand, the return to the inorganic form of what has been 

 assimilated by the biosphere may be blocked by hold-ups in the mineral- 

 ization process. The humus in the soil is one of these hold-ups and it is 

 beneficial in its influence on the physical properties of the soil. But peat, 

 lignite and coal show how there has been a long hold-up in the carbon cycle 

 in certain areas. Chalk, coral, etc., are further examples of stagnation in the 

 same cycle. Another example is the accumulation of guano in certain islands 

 in Peru. No doubt the nitrogen of the atm^osphere is an inexhaustible 

 source, but the COg of the air is not. Even in the equilibrium state of 

 the biosphere at the present time, the low level of COg in the atmosphere 

 under bright conditions limits the intensity of photosynthesis which would 

 be much greater if more COg was available. 



The low concentration of COg in the atmosphere is the result of an 

 equilibrium between producers and consumers. If this small amount was 

 no longer present, life would stop. 



As for phosphorus, it can only be Hberated slowly from rocks and marine 

 deposits. The most rapidly available source is provided by the decomposi- 

 tion of organisms. This is the vulnerable point in the metabolism of the 

 biosphere. The increasing removal of phosphorus from the land to 

 the sea is a menace to all terrestrial life, and it may make the colonization 

 of the continents a mere episode between two eras of marine existence, 

 unless the return of phosphorus to the land is increased by man, perhaps 

 by the realization of his old dream of utilizing the vast resources of the 

 oceans. 



