[Chap. XLVI UNDER-WATER ENVIRONMENTS 603 



bacteria are largely anaerobic and spore-forming, as compared with the 

 aerobic and non-spore-forming bacteria floating in tlie water. The bot- 

 tom bacteria include the nitrifying, the nitrogen-fixing, and the humus- 

 decomposing types. Certain bacteria of the sea are of interest because 

 they secure energy by the oxidation of such substances as hydrogen 

 sulfide, sulfur, ammonia, nitrites, and methane, and use the energy in 

 the synthesis of cell substances. 



An accurate quantitative comparison between life on land and life 

 in the ocean is difiicult to make. Krogh has estimated that the entire 

 life zone on land is rarely more than 30 meters thick, from tree tops to 

 root tips inclusive, in contrast to the 4000 meters of ocean in which 

 animals live, and the 300 meters populated by green plants. For every 

 tree in a forest there may be nearly a half million animals large enough 

 to be seen by the naked eye. When the microscopic animals, bacteria, 

 and algae are added to this, the total number is prodigious. Under a 

 square meter of ocean water near the equator, down to a depth of 200 

 meters, there may be a billion or more scarcely visible or microscopic 

 plants and a million animals of various sizes, mostly microscopic. But 

 the mass of these organisms is surprisingly small, perhaps aggregating 

 only about 1/100 of a corresponding volume in a forest. Forest trees 

 stand for years, but many of the ocean plants are renewed several times 

 a year. The total biological productivity in the ocean, however, probably 

 never equals that in the forest, though locally it may be of comparable 

 magnitude. 



In sharp contrast to the land, no seed plants, no ferns, no mosses, no 

 liverworts ever grow in the open sea. The plant life characteristic of the 

 ocean is confined to algae, bacteria, and a few fungi. The species of 

 plants characteristic of the ocean rarely occur in fresh water. On the 

 land the larger plants are the chief sources of food of the animal popu- 

 lations. In the water this is not the case, for the microscopic plants are 

 the initial links in the food chains of nearly all the animals. Enormous 

 numbers of these plants grow every year, but directly thev constitute 

 only a small part of the food of the largest animals ( Fig. 273 ) . 



The food chain from green plants to the larger animals ("producer" 

 to "consumer" ) may become greatly extended because the smallest ani- 

 mals, having consumed the plankton, may become the food of crus- 

 taceans and small fishes. The latter in turn may be eaten by still larger 

 fishes ( Fig. 274 ) . It is evident that much of the energy value of the food 

 originally made in green plants is never realized by the "ultimate con- 



