THE ATMOSPHERIC GASES 



197 



gradient in carbon dioxide concentration 

 especially in thermally stratified lake waters. 

 It is difiicult to give briefly, clearly, and 

 fully an indication of the role of carbon 

 dioxide in animal ecology. It is one of the 

 major environmental factors (Chaps. 4, 11) 

 and acts as a chemical buffer to help main- 

 tain the neutrahty of aquatic habitats. In 

 addition, carbon dioxide affects varied im- 

 portant aspects of animal (and plant) 

 ecology. It acts as a retarding agent for 

 many biological reactions even if present in 

 fairly low concentrations. On the other 



other gill-breathing animals. Finally, in this 

 brief list, many aquatic animals, fishes 

 among them, react fairly definitely to an in- 

 creased amount of carbon dioxide. This may 

 be important, since regions of low oxygen 

 tension often have an increased supply of 

 carbon dioxide. 



An optimum concentration of carbon 

 dioxide often exists, below which some 

 measurable rate of activity is decreased; 

 optimum concentrations have been found 

 both for organisms exposed to the at- 

 mosphere and for those in aquatic environ- 



(16.0) 



20 



(7.1) 

 (4.2) 



(0) 

 (-5.2) 



^ 5 



? ? 



05 1.0 1.5 2.0 2.5 3.0 



CARBON DIOXIDE IN MULTIPLES OF PARTIAL 

 PRESSURE IN ATMOSPHERE 



35 



Fig. 43. Relation between carbon dioxide tension and retardation of early cleavages in frog's 

 eggs. Cleavage rate at a partial pressure of 1 atmosphere is placed at zero. (Redrawn from 

 Merwin and Allee.) 



hand, an increase in carbon dioxide up to 

 an optimum accelerates the photosynthesis 

 of green plants, hastens germination of 

 mold spores, speeds the growth rate of 

 populations of certain bacteria and some 

 protozoa, and heightens the rate of respira- 

 tion in higher invertebrates and in verte- 

 brates. Certain echinoderm plutei grow 

 larger with increased carbon dioxide ten- 

 sion, perhaps because carbonates are used 

 in the formation of their spicules (Merwin 

 and Allee, 1943). If present in some quan- 

 tity, carbon dioxide raises the threshold of 

 availability of oxygen, a process that is es- 

 pecially likely to be important for fishes and 



ments. In addition to the processes just 

 summarized, early in incubation, the hen's 

 egg develops faster at concentrations of car- 

 bon dioxide somewhat above those found 

 normally in the atmosphere. On the other 

 hand, there are equally vital processes, the 

 early cleavage of the frog's egg being an 

 example, that become steadily more rapid 

 the lower the carbon dioxide tension. The 

 latter effect is illustrated in Figure 43. It is 

 impossible with present knowledge to sug- 

 gest a general rule as to which processes 

 will be stimulated and which inhibited by 

 a slight increase above the carbon dioxide 

 tension normally encountered in nature. 



