WATER 



177 



enzyme action, and extracellular enzymes 

 and other catalysts have an importance in 

 ecology the extent of which has not yet 

 been measured. 



Ordinary chemical reactions may follow 

 surface adsorption. 



Adsorption on the external body surface 

 is especially important in the life of bac- 

 teria, protozoans, and of small metazoans. 

 It may also produce significant eflFects in 

 gilled animals generally, especially in those 

 such as the lamellibranch mollusks that 

 have much surface in relation to bulk. 



Our discussion of adsorption is inade- 

 quate and must remain so, pending basic 

 advances in surface chemistry and the ap- 

 pearance of monographic studies of adsorp- 

 tion as a physiological and finally as an 

 ecological process. 



The bacteria of the sea and, to a certain 

 extent, those of fresh water are adsorbed 

 on, or attached to surfaces, usually to the 



surfaces furnished by larger planktonic 

 forms. Aquatic bacteria are relatively rare 

 as free-floating organisms; they may be 

 classed as only pseudoplanktonic. Even the 

 cleanest surface of almost any kind pro- 

 motes the growth of bacteria in dilute solu- 

 tions, especially in dilute solutions of some 

 10 mg. per liter or less. The surfaces act 

 (a) by adsorbing organic nutrients, espe- 

 cially colloids or poorly dissolved solutes, 

 and (b) by retarding the diffusion of 

 exoenzymes and of nutrients that must be 

 hydrolyzed extracellularly before ingestion 

 by the bacteria. The surface thus acts as a 

 concentrator of nutrients from the extremely 

 dilute solutions found in most waters. 

 Particles larger than the bacteria themselves 

 are most effective concentrators; in fact, 

 particles smaller than the bacteria, adsorbed 

 on the bacteria, may retard or accelerate 

 the work of the latter organisms (ZoBell, 

 1943). 



12. WATER 



The hydrological cycle consists of the varied 

 events happening to a particle of water 

 from the time it is a bit of vapor in the 

 atmosphere until it is again evaporated. 

 Water enters the atmosphere from the sur- 

 face of bodies of water and from the soil, 

 from plants in transpiration or in drying, 

 and, in smaller amounts, from animals. It 

 is disseminated through the atmosphere by 

 winds, precipitated as liquid or frozen rain, 

 or as snow. Once precipitated, there may 

 be immediate evaporation, or this may be 

 delayed until after a run-off in streams or 

 a slow creep-off as seepage or in glaciers. 

 The water may be stored in soil, swamps, 

 lakes, or oceans before re-evaporation takes 

 it back to the beginning of the cycle. Plants 

 and animals take water out of the inorganic 

 hydrological cycle and make a subsidiary 

 organic one from which the water returns 

 sooner or later by diffusion, excretion, or as 

 a result of organic decomposition. The main 

 features of the cycle are suggested diagram- 

 matically in Figure 39. The hydrological 

 cycle is the central concept of the sub- 

 science of hydrology. 



The oceans are the great reservoirs of 

 water. They occupy 70.8 per cent of the 

 510.1 X 10° square kilometers of the earth's 



surface and have an average depth of 3795 

 meters. Their total volume is about 1370 

 X 10" cubic kilometers. The amount of 

 water frozen into the ice of glaciers and 

 ice sheets equals some 9.3 per cent (9.3 X 

 10"') of this amount. That found in the air 

 as vapor is only about 9 X 10° of that in 

 the sea. In more direct terms, if all moisture 

 in the air were precipitated and collected in 

 the ocean, the sea level would be raised 

 only 3.5 cm. The best available estimate 

 suggests that the amount of fresh water is 

 about thirty-three times that in the atmos- 

 phere (Wiidt, 1942). 



Estimates vary widely as to the total 

 amount of free water in the earth's crust. 

 They range from less than 1 per cent to a 

 quarter or even a half of the total amount 

 in the oceans. Meinzer and Wenzel, com- 

 menting on these estimates (1942), regard 

 both extremes as erroneous. They indicate 

 that "the quantity of water in the rocks is 

 much less than the quantity in the ocean 

 but many times as great as the quantitv in 

 lakes, streams and the atmosphere." About 

 half of this underground water is held in 

 molecular attraction; the remainder h 

 free to flow out into springs and wells. 

 There is no available estimate of the quan- 



