378 UNITY AND DIVERSITY IN BIOCHEMISTRY 



From the biochemical point of view the organisms which make up an 

 ecosystem are of four types : producers, consumers, those which bring 

 about decomposition, and those which accompHsh transformations of 

 material. In the terrestrial region, for example in soil, the producers are 

 mainly green plants although autotrophic bacteria also play a part. All the 

 consumers are heterotrophes, i.e. herbivores consuming green plants 

 (insects, rodents, ruminants, etc.), primary carnivores who consume the 

 herbivores, and secondary carnivores who feed on primary carnivores, etc. 

 Parasites are also consumers. The decomposing organisms are those that 

 attack the dead bodies of producers and consumers and turn their sub- 

 stance into the inorganic state of COg, ammonia, HgS, etc. This is accom- 

 plished by bacteria and fungi. 



The transforming organisms alter the inorganic compounds resulting 

 from the activities of the above and convert them to compounds which 

 can be used by the producers ; nitrates and sulphates for example. 



It appears therefore that an ecosystem can exist very well if it is made 

 up of producers and decomposing and transforming organisms. The 

 consumers, including man, are only an unessential intermediate in the 

 metabolism of the ecosystem. 



The components of marine ecosystems differ from those of terrestrial 

 ecosystems. In the open sea for example, the principal producers are the 

 plankton, especially diatoms, which form what has been called "the ocean 

 pasture". The amount of growth depends on seasonal conditions and 

 photosynthesis only occurs where light is able to penetrate. Also, in 

 addition to COg, nitrates, sulphates and phosphates are required. In 

 winter when the ocean waters become agitated and are mixed, these salts 

 are brought to the surface and rapid growth can begin as soon as there is a 

 sufficient number of hours of sunlight. But gradually the inorganic salts 

 are used up and the herbivorous consumers, especially the copepods, 

 graze the ocean pasture and devour it. During the summer, owing to 

 thermal stratification of the water and because the dead organisms sink into 

 the depths, the situation remains stationary. When autumn comes, the 

 surface temperature falls and agitation of the waters increases ; the upper 

 layers are resupplied with inorganic nutrients and a new growth of phyto- 

 plankton begins which is soon arrested by being consumed and by the fall 

 in temperature and the amount of light. Winter brings about the great 

 mixing of waters which, in the sea, corresponds to the tilling of the soil. 



We have already listed the copepods among the marine herbivorous 

 consumers. They are not the only examples, many molluscs and even 

 certain fish belong to this group. But herbivorous consumers are less 

 common than on land and they are almost always small in size (copepods, 

 etc.). These small herbivores are eaten by numerous marine animals and 

 notably by the whale. However most marine carnivores are dependent on 



