THE ATMOSPHERIC GASES 



195 



for respiration, and also act as collectors of 

 dissolved oxygen from the suirounding 

 water and, in so doing, illustrate several 

 physical principles. The oxygen-collecting 

 mechanism works as follows: When first 

 captured, the bubble holds 21 per cent oxy- 

 gen, the partial pressure of the atmosphere. 

 As the insect consumes oxygen, the partial 

 pressure of this gas falls, and the partial 

 pressure of nitrogen is relatively increased. 

 Under many conditions this eventually re- 

 sults in a diffusion of oxygen from the water 

 into the bubble and a diffusion of nitrogen 

 out into the surrounding water. The carbon 

 dioxide given off into the bubble diffuses 

 out into water so rapidly that we need take 

 no serious account of it. The bubble con- 

 tinues to act as an oxygen collector until its 

 gases are dissolved, and loses its functional 

 significance as an oxygen-collecting device 

 only after the use of oxygen by the insect 

 sufficiently exceeds the rate of its diffusion 

 from water. Considering solubiHties and 

 partial pressures (p. 191), oxygen will dif- 

 fuse into the bubble about three times as 

 fast as the nitrogen diffuses out, and the 

 underwater bubble may finally yield some 

 thirteen times the amount of oxygen it orig- 

 inally contained (Ege, 1918). 



Some mammals trapped below ice make 

 use of a variant of this device. They exhale 

 air just below the surface of the ice to form 

 a flat bubble with a large air-water surface, 

 and after a short time they inhale. As with 

 the insects, such a bubble collects oxygen, 

 disposes quickly of carbon dioxide, and so 

 enables the trapped animals to swim under 

 ice for relatively long distances. 



OXIDATION-REDUCTION POTENTIALS 



Oxidizing and reducing substances exist 

 in the environment and in the organism, of- 

 ten in close proximity to each other. Oxida- 

 tion in chemistry means not only reactions 

 in which free oxygen actually is used up, or 

 even the transfer of combined oxygen from 

 one substance to another, but may mean the 

 introduction or increase of one or more 

 electronegative elements or, conversely, the 

 reduction or removal of one or more electro- 

 positive elements. In simplest temis, oxida- 

 tion is the process of removing electrons. 

 Reduction is the opposite of oxidation, and 

 hence it consists primarily in the addition 

 of electrons (Kendall, 1923). One and the 

 same substance can give up electrons (oxi- 

 dation) or accept them (reduction), de- 



pending on its relative position on the 

 oxidation-reduction scale in comparison 

 with other available reacting systems. Posi- 

 tion on this scale indicates the oxidation- 

 reduction potential (redox potential) of the 

 given material. The position is expressed as 

 an electric potential in terms of Ei, recorded 

 in volts. A system with a high potential can 

 oxidize one that stands lower in the scale 

 and itself undergoes reduction in the proc- 

 ess. 



The oxidation-reduction potential of the 

 physical environment offers a promising 

 field of study that has received relatively 

 little attention. The redox-potential of 

 sea water is correlated with the amount 

 of dissolved oxygen and with the pH of the 

 water. When there is Uttle dissolved oxygen, 

 or in the presence of hydrogen sulfide, 

 dissolved organic substances apparently 

 need to be considered also. 



Oxidation-reduction relations have two 

 aspects: intensity, as measured directly by 

 the potential, and the buffering of the sys- 

 tem. Buffering in this sense refers to the 

 abihty to carry on a given amount of oxida- 

 tion (or reduction) without a significant 

 change of potential; it is sometimes called 

 the capacity of the oxidation-reduction sys- 

 tem. 



The importance of the oxidation-reduc- 

 tion potential of the environment is most 

 obvious for microorganisms. Aerobic bac- 

 teria require a high potential and cannot 

 live long in the low one supphed by stag- 

 nant waters or muds where oxygen is ab- 

 sent. They tolerate an Eh of from -f 0.4 to 

 — 0.2 volts. Anaerobic bacteria are limited 

 to oxygen-free waters and bottom sedi- 

 ments. They five in microhabitats where the 

 Eh may go below —0.42 with a pH of 7.0. 

 As a general rule, the Eh of the bottom sedi- 

 ments decreases with depth, with a zone of 

 rapid change in a few upper centimeters. 

 Active aerobic bacteria are consequently 

 hmited to or near the upper surface of such 

 sediments; below come the facultative and 

 obligate anaerobes (Sverdrup, Johnson, and 

 Fleming, 1942). 



The presence of electrical potentials has 

 been demonstrated in fresh water and soil 

 surveys as well as in sea water and bottom 

 mud (Burrows and Cordon, 1936; Allgeier, 

 Hafford, and Juday, 1941), and 't is be- 

 lieved that such potentials are oxidation- 

 reduction (redox) potentials. If this is true, 

 such readings are destined to play an in- 



