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The interdependence between the biosphere and the atmosphere
Respiration Physiology (1968) 5, 68-17 ; North-Hollattd Publishing Company, Amsterdam
THE INTERDEPENDENCE BETWEEN THE BIOSPHERE AND THE ATMOSPHERE
DANIEL L. GILBERT Laboratory of Biophysics National Institute of Neurological Diseases and Blindness National Institutes of Health, Bethesda, Maryland 20014, U.S.A.
Abstract. The origin of life on earth probably
occurred when there was a reducing atmosphere composed in part of hydrogen. The biosphere developed antireductant mechanisms to resist hydrogen toxicity. As hydrogen escaped from earth and the photosynthetic production of oxygen began, the atmosphere changed to an oxidizing one composed in part of oxygen. The oxygen in the atmosphere gradually increased until it reached its present value. As a consequence of the presence of oxygen, the biosphere continuously developed antioxidant mechanisms. Organisms which developed antioxidant mechanisms under relatively high oxygen concentrations and then migrated to environments of lower oxygen concentrations would possess an advantage in having very well developed antioxidant mechanisms for their low oxygen environment. The atmosphere is being changed today by the activities of man. It seems that carbon dioxide is increasing. Eventually, it is speculated that due to the continued dehydrogenation of the earth, the atmosphere will be composed of only carbon dioxide and molecular nitrogen. It is further speculated that life will not be able to survive on earth at this time.
At the present time, most of the living organisms on earth obtain their metabolic energy by consuming oxygen and carbohydrates and releasing carbon dioxide and water. The actual chemical reaction takes place within the aqueous medium of the living cell. There is a direct gaseous exchange of oxygen and carbon dioxide between terrestrial organisms and the atmosphere and an indirect exchange mediated through the hydrosphere for those organisms living in water. Thus, all organisms which utilize this respiratory process eventually depend upon the atmosphere. In addition, the biosphere, comprising all living organisms, itself causes changes in the atmosphere, both directly and indirectly. The resultant interdependence of the atmosphere and the biosphere, as they have slowly changed on earth, will be explored in this paper. Received 23 October 1967. 68
INTERDEPENDENCE BETWEEN BIOSPHERE ANDATMOSPHERE
69
The early hydrogen atmosphere It would seem most probable that the primitive atmosphere consisted of the most abundant elements in the universe (GILBERT,1964) (table 1). The predominant cosmic elements are hydrogen and the noble gas, helium. Hydrogen would then chemically combine with the other abundant elements, oxygen, carbon, and nitrogen to form respectively, water, methane, and ammonia. Hydrogen and helium, being such light elements, would continually overcome the gravitational force and escape. On large cold planets, this early atmosphere would be retained. It seems that indeed this is the present day atmosphere of Jupiter and Saturn (GILBERT, 1966b). From such an atmosphere with an appropriate energy source, it would be possible to obtain complex organic metastable compounds, the building blocks of the biosphere. It has been speculated that the origin of life occurred under such conditions (SNNEOURand OTTESEN,1966). If this is true, then perhaps someday we might be able to observe the dawn of life by traveling to Jupiter and Saturn. It has been pointed out that there is a possibility of organic compounds (LIPPINCOTTet al., 1967) and even life on Jupiter (SODEKand REDMOND,1967). The presence of hydrogen on these planets may produce a very significant greenhouse effect and have a warming influence (TRAFTON, 1967). The first organisms in such an environment would need an energy reserve, which could be in the form of carbohydrate within the organism and free atmospheric hydrogen. Then these organisms would be confronted with the problem of hydrogen toxicity; in order to overcome this, they would have had to develop defenses against hydrogen, or in other words, develop antireductant mechanisms (GILBERT, 1966b). The atmospheric hydrogen would tend to reduce the organisms to methane, ammonia, and water. Also, recent studies indicate that in the early phases of building up the earth’s biosphere, there would have been a significant concentration of hydrogen cyanide (ABELSON,1966; CALVIN, 1965; MATTHEWSand MOSER, 1967; 0~6 and KIMBALL,1961). Thus, when the biosphere originated on earth, poisons were already in the atmosphere which could destroy life itself. However, the toxicity of hydrogen cyanide is due mainly to the oxidative enzymes (HEWITTand NICHOLAS,1963), which
TABLE 1 Per cent atom abundance* Element
Cosmos
Biosphere
H He 0 C N Others
86.68 13.18 0.09 0.03 0.01 0.01
62.6 0.0 24.9 10.6 1.1 0.8
*From GILBERT (1964).
DANIELL. GILBERT
70
wouldn’t have developed at this early stage. Fossil studies have shown that the biosphere existed at least 3.1 x lo9 years ago (BARGHOORN and SCHOPF,1966) and the age of the earth has been estimated at 4.5 x lo9 years (ULRYCH,1967). Life has probably evolved on other planets in our universe (CAMERON,1963; MACGOWANand ORDWAY,19.66). By necessity, any biogenic storage form of energy will possess the capability to destroy life. This condition will exist in any biosphere, no matter how bizarre its development might be, even in a possible anti-matter universe (ALFVBN,1966). As the hydrogen escaped from the earth’s atmosphere, bound hydrogen in the form of water, methane, and ammonia would be released into the atmosphere as free hydrogen, thereby liberating oxygen, carbon, and nitrogen from these compounds. Oxygen, being extremely reactive, would combine with the carbon (as well as silicon, iron, and other elements) to form carbon dioxide (and other oxides) and the nitrogen would remain as molecular nitrogen. Some of the oxygen would also react with the biosphere. The existing biosphere would adjust to this action of oxygen by developing antioxidant mechanisms. However, more energy would be available to the biosphere TABLE 2 The average composition
NZ 02 Ar coa Ne He CH4
Kr NaO HZ 03
Xe
of our present day atmosphere.
Total Eg *
Mol. Wt.
Total Emoles *
3864.8 1184.1 65.5 2.41 0.0636 0.0037 0.0043 0.0146 0.0040 0.0002 0.0035 0.0018
28.0 32.0 39.9 44.0 20.2 4.0 16.0 83.8 44.0 2.0 48.0 131.3
137.9 37.0 1.64 0.0548 0.00315 0.00092 0.00027 0.00017 O.OfIOl 0.0001 0.000073 0.000014
Mole % (dry air) 78.09 20.95 0.93 0.0310 0.0018 0.00052 0.00015 0.00010 0.00005 0.00005 0.00004 O.OOOOO8
Sea Level Pressure in mm Hg 590.0 158.0 7.0 0.234 0.014 0.0039 0.0011 0.0008 0.0004 0.0004 0.0003** 0.00006
Total dry
5117.0
28.97
176.6
Hz0 Total
22.0 5139.0
18.015 28.90
1.2 177.8
*The symbol E is the abbreviation
100.0
755.0 5.0 760.0
for the prefix, Erda, and represents the multiple, 10rs
(GILBERT,
1964).
**The pressure calculated here does not correspond to the sea level pressure since the 0s concentration increases with altitude reaching a maximum at about 30 km (GILBERT, 1964). The Eg for each gas was taken from MASON (1966) except for COa and HaO, which were taken from GILBERT (1964).
I~ER~EPENDENCEBBTWEEN BIOSPDBRE AND ATMOSPHBRB
71
TABLE 3 The per cent atom abundance of our present day atmosphere. Atom
Per cent atom abundance*
Relative to neon
N 0 a Ar C Ne He Kr Xe
77.65 21.20 0.676 0.462 0.0155 0.~89 0.00026 o.OoOO49 O.OOOOO4
87000 24000 760 520 17 1 0.29 0.055 0.004
*Calculated from tabie 2.
as it would begin to oxidize metastable compounds by oxygen into water and carbon dioxide. Speculation on the initial stages of biological evolution with special reference to the energetics available to the biosphere has recently been discussed (GILBERT, 1966b). Since there is an appreciable concentration of neon in the universe, co~esponding to one-third the cosmic nitrogen atom concentration (CAMERON,1959) (see table l), it would seem most reasonable that the initial atmosphere also contained a significant amount of neon. However, our present atmosphere contains hardly any neon. The present atmospheric neon content is only 3.15 x 10e3 Eg-atoms, as can be seen in table 2, where the symbol E is the abbre~ation for the prefix, Erda, and represents the multiple lo’* (GILBERT,1964). According to DAY(1964), thereis onlyaboutO.08 cm3 of neon per ton of rock. Since there are 2.3666 x IO’ Eg of rocks in the earth’s crust, the neon content in the lithosphere (solid portion of the earth’s crust) amounts to 8.45 x 10m5 Eg-Atoms. Hence, the bulk of the neon on earth is present only in the atmosphere. In the atmosphere the amount of nitrogen is 87000 times that of neon today (table 3). In order to explain this lack of neon on earth today, it seems necessary to conclude that neon has escaped from the earth’s atmosphere. But then practically the entire primitive atmosphere must have also escaped, since the gaseous constituents (molecular hydrogen, helium, water, methane, and ammonia) which comprise the primitive atmosphere have molecular weights less than neon. However, it would be possible that the organic material and biosphere which would have been derived from this primitive atmosphere would have been retained in the hydrosphere. Even though the primitive atmosphere was lost, it left its imprint on the biosphere as evidenced from the close correlation between the abundance of the elements in the universe and biosphere as shown in table 1, with the exception of the chemically inert helium. There is a possibility that part of the heavier constituents of the primitive atmosphere would have become trapped below the earth’s crust. These trapped gases would then
72
DANIELL. GILBERT
have a deficiency in hydrogen. Under such conditions, the predominant gases would be water, carbon dioxide, and molecular nitrogen. These three compounds all have greater molecular weights than neon. In other words, the trapped gases would probably have the same composition as that of the atmosphere, evolving by the escape of hydrogen into space. Volcanic action releases trapped gases, which contain significant amounts of water and carbon dioxide (MASON, 1966); this degassing of the earth’s lithosphere and mantle has probably been an important source for the development of our present day atmosphere (ABELSON,1966; HOLLAND,1962). The oxygen atmosphere The next major step would be the development of the photosynthetic production of oxygen and carbohydrates by the biosphere. This would provide much more energy to the organism. It has been pointed out that oxygen is the best form of energy storage for a biosphere (GEORGE,1965; GILBERT,1964). The oxygen in the atmosphere would increase by this process (BERKNERand MARSHALL,1965; HOLLAND,1962). By necessity, the biosphere would have to produce further antioxidant mechanisms to combat oxygen toxicity (GERSCHMAN,1964; GILBERT, 1966b). Many different mechanisms for combating oxygen toxicity exist (HAUGAARD,in press). The oxygen concentration probably rose steadily due to photosynthetic activity with a resultant continuous development of antioxidant mechanisms by the biosphere. This process has evolved until the atmosphere reached its present composition as given in table 2. BERKNERand MARSHALL(1965) have emphasized the importance of the presence of our atmospheric oxygen in protecting the biosphere from ultraviolet radiation. These authors have postulated that in the past, the oxygen concentration might have fluctuated and even been greater than its present value of 158 mm Hg. However, if the partial pressure of oxygen was ever much greater than 158 mm Hg, then the biosphere should have developed antioxidant mechanisms to survive at such high values of oxygen. The evidence for such a development would be the ability of animals to withstand high pressures of oxygen; but this is generally not true (GERSCHMAN,1964). Hence, it appears that if the oxygen did fluctuate in the past, it could not have been much greater than 158 mm Hg. The dehydrogenation of the earth and the photosynthetic production of oxygen resulted in our present oxygen atmosphere which is primarily composed of nitrogen, oxygen, argon, and carbon dioxide (table 2). Argon is chemically inert, constitutes only a very small part of our atmosphere, and has probably had little influence upon the biosphere. Let us now discuss how these three other gases affect the biosphere. OXYGEN Oxygen, as has been pointed out, is toxic to all living matter. This is true in spite of the fact that some tissues and organs are exposed to naturally high oxygen pressures. Thus, the swimbladders of some fish can reach the fantastic oxygen pressure of one hundred atmospheres (GERSCHMAN,1964). Also, diving whales can develop a high
INTERDEPENDENCE BETWEEN BIOSPHERE ANDATMOSPHERE
73
pressure of oxygen in their lungs, but oxygen uptake can decrease the oxygen pressure (GILBERT, 1966a). This oxygen toxicity has undoubtedly played a major role in the development of the biosphere, in spite of the fact that the evolution of the metazoa probably could not have occurred without such an excellent energy source such as oxygen. The major effect of oxygen toxicity would be as a necessary consequence the buildup of antioxidant defenses. The development of antioxidant mechanisms would be proportional to the oxygen tension. Thus, if organisms were not exposed to oxygen, then no antioxidant defense would be developed. For example, anaerobic bacteria have not evolved with antioxidant mechanisms. If the ancestors of some organisms were exposed to a higher oxygen tension than their descendants, an interesting consequence might occur. Since the ancestor would have had to develop antioxidant mechanisms able to cope with the previous high oxygen tension, the antioxidant defense of the descendant would be better able to resist the later lower oxygen tension. The descendant would also have to be able to utilize the lower oxygen tension more efficiently than its ancestor. Perhaps such a phenomenon occurred when some organisms migrated from low altitude regions to high altitude regions. The result is that the present high altitude organism in many ways is better able to cope with its low oxygen environment than can the present low altitude organism do with its higher oxygen environment. Let us discuss the migration of part of the Camelidae family and man to high altitude regions. Part of the Camelidae family (Lama and Vicugna genera) migrated to the high altitude Andes of South America from North America (PALMER,1957). However, since both the camel and the llama possess a high blood affinity for oxygen (BARTELS et al., 1963), it appears that some mechanisms for their very successful adaptation to the low oxygen environment of the high altitude regions were present before this migration occurred. Of course, there are several other mechanisms for adaptation to high altitude (BARTELSef al., 1963; BULLARD,BROUMAND and MEYER,1966; REYNAFARJE,1966). Let us now examine the more recent migration of man to high altitude areas. Man has lived for at least 10000 years at Peruvian high altitudes (MONGEM and MONGE C, 1966). During this period, high altitude man has adapted to his low oxygen environment to a significant degree. High altitude individuals of the Andes (REYNAFARJEand VELASQUEZ, 1966) and Himalayas (MILLEDGEand LAHIRI; 1967) are better able to cope with low oxygen environments than sea level residents recently acclimatized to high altitude. The high altitude residents are also capable of performing muscular exercise most efficiently. To illustrate this point, it can be mentioned that communications could be sent by the relay runners of the high Andean Inca civilization a distance of 1250 miles in only five days. This speed required that the couriers running on the Inca roads, at altitudes ranging between 6000 and 17000 feet, had to maintain an average speed 2.5 times that of the speed accomplished by the couriers of the Roman civilization. Present day Andean Indians can run at the required rates to make such an impressive feat possible (VON HAGEN, 1957).
74
DANIELL. GILBERT
NITROGEN What is the effect of nitrogen on life? In a non-nitrogen atmosphere it has been shown that there is a greater chance of producing pulmonary atelectasis (DuBors et al., 1966; MACHATTIEand RAHN, 1960). However, at high pressures it has a narcotic effect (FENN, 1967). Although this relatively inert gas is more abundant than oxygen in the atmosphere, its action at normal atmospheric pressure (table 2) has only recently become a practical problem. When the astronauts of the U.S.A.were exposed to nitrogen free atmospheres, it was found that nitrogen does play a major role in reducing the fire hazard of an oxygen environment (DENISON,ERNSTINGand CRESSWELL,1967; FENN, 1967; ROTH, 1966a,b). The tremendous fire hazard that would be presented by the absence of some inert gas in the atmosphere could in itself prevent the development of life on earth. In this respect alone, nitrogen plays an important role in the survival of the biosphere. CARBONDIOXIDE Since the biosphere constitutes such an infinitesimal part of the earth’s crust, it is really astounding that man has actually been changing the composition of the earth’s crust within the past hundred years. There is evidence to show that the carbon dioxide concentration in the atmosphere has increased about 7 % since 1900 due to the burning of fossil fuels and the removal of forests (GILBERT, 1964). The carbon dioxide in the atmosphere absorbs the long wave heat waves and thus prevents heat waves from leaving the earth resulting in a greenhouse effect. Hence, increasing the carbon dioxide increases the earth’s temperature, which has been observed during the first half of this century. However, the liberated carbon dioxide is stabilized to a large degree by the ocean (GILBERT, 1964). To complicate matters, man has also been polluting the air which blocks the radiation from the sun and, in the past ten years, there appears to be a reversal in the temperature rise due to this factor (MCCORMICKand LUDWIG,1967). Other factors which can decrease the earth’s temperature are an increase in the atmosphere of water (MUELLER, 1963) and volcanic debris (PUTNAM,1964). Temperature changes on even a small scale can have a very significant effect on man. The future carbon dioxide atmosphere Eventually the dehydrogenation of the earth will probably result in the absence of all water. The atmosphere will then be composed of only nitrogen and carbon dioxide, and life will no longer be possible. The evolution of planetary atmospheres and its relationship to possible life stages, is illustrated in fig. 1 (GILBERT, 1964). It seems that Mars contains a sizable amount of carbon dioxide (BARKER,1967; SAGAN, 1966), an inferred amount of nitrogen and a barely detectable amount of water (SAGAN,1966). It might be that when Mars is examined more closely, evidence of a previous biosphere will be found. A hot planet, such as Venus (CHASE,KAPLAN and NEUGEBAUER, 1963), would have a tendency to lose its atmosphere; it could be
I~~EPB~BNCE
75
BETWEEN BIOSPHEREAND AT~OSP~
EVOLUTION
OF ATMOSPHERES
i”“‘“““““““‘““““““““““‘~~~~~~~~~~~_~-~~~~~~______
* i f H2
y _-_..__
i
outerspce
:
He
! i
2 H2
______ _I ___________-__“““..
I----
2 H2
! ______________-_
3H2 : cm-J
xv j2 ~_!____--_ x02
I f
2
t post-life sta* I ____-_..* ____-_--~YJ
N2
f
i
X represents other substances oxidized by oxygen
Fig. 1. Evolution of atmospheres. Taken from GILBERT(1964). Acknowledgment
is given to the
American Physiological Society for permission to use this figure.
in a possible post-life stage. A signifhzant concentration of carbon dioxide does exist there (DAYIIOFF et aE., 1967) which would support this hypothesis. However, it has been suggested that perhaps there has been a migration of life to the clouds where the temperature is not so high (MOROWITZ and SAGAN, 1967). The detection of any possible life on these planets is difficult to determine at the present time. It should be noted that our earth would not, with our present tools of observation, yield signs of life to an observer from outer space (KILSTON, DRUMMO~ and SAGAN, 1966; SHKLOVSKII and SAGAN, 1966).
Eventually first the nitrogen and then the heavier carbon dioxide will disappear and the earth will be devoid of an atmosphere like Mercury and our moon. It has been proposed that even the moon has undergone some biogenic activity ~GILVARR~, 1966). References ABELSON,P. H. (1966). Chemical events on the primitive earth. Proe. Nut. Acad. Sci. 55: 1365-1372. A&N, H. (1966). Worlds-~tiworlds. ~ti~tter in Cosmology. San Francisco, W. H. Freemen and Co. BAROHOORN, E. S. and J. W. SCHOPF(1966). Microorganisms three billion yeers old from the Precambrian of South Africa. Science 152: 758-763. BARKER,E. S. (1967). A determination of the Martian COs abundance. Astrophys. J. 147: 379-381. BAR~?LS,H., P. H~LPERT,K. BARBEY,K. BETKE,K. RIEQEL, E. M. LANG and J. METCALFE(1963). Respiratory functions of blood of the yak, llama, camel, Dybowski deer, and African elephant. Am. J. Physiol. 205: 331-336. BERKNER,L. V. and L. C. MARSHALL (1965). History of major atmospheric components. Proc. Nat. Acad. Sci. (U.S.) 53: 1215-1226.
THE INTERDEPENDENCE BETWEEN THE BIOSPHERE AND THE ATMOSPHERE
DANIEL L. GILBERT Laboratory of Biophysics National Institute of Neurological Diseases and Blindness National Institutes of Health, Bethesda, Maryland 20014, U.S.A.
Abstract. The origin of life on earth probably
occurred when there was a reducing atmosphere composed in part of hydrogen. The biosphere developed antireductant mechanisms to resist hydrogen toxicity. As hydrogen escaped from earth and the photosynthetic production of oxygen began, the atmosphere changed to an oxidizing one composed in part of oxygen. The oxygen in the atmosphere gradually increased until it reached its present value. As a consequence of the presence of oxygen, the biosphere continuously developed antioxidant mechanisms. Organisms which developed antioxidant mechanisms under relatively high oxygen concentrations and then migrated to environments of lower oxygen concentrations would possess an advantage in having very well developed antioxidant mechanisms for their low oxygen environment. The atmosphere is being changed today by the activities of man. It seems that carbon dioxide is increasing. Eventually, it is speculated that due to the continued dehydrogenation of the earth, the atmosphere will be composed of only carbon dioxide and molecular nitrogen. It is further speculated that life will not be able to survive on earth at this time.
At the present time, most of the living organisms on earth obtain their metabolic energy by consuming oxygen and carbohydrates and releasing carbon dioxide and water. The actual chemical reaction takes place within the aqueous medium of the living cell. There is a direct gaseous exchange of oxygen and carbon dioxide between terrestrial organisms and the atmosphere and an indirect exchange mediated through the hydrosphere for those organisms living in water. Thus, all organisms which utilize this respiratory process eventually depend upon the atmosphere. In addition, the biosphere, comprising all living organisms, itself causes changes in the atmosphere, both directly and indirectly. The resultant interdependence of the atmosphere and the biosphere, as they have slowly changed on earth, will be explored in this paper. Received 23 October 1967. 68
INTERDEPENDENCE BETWEEN BIOSPHERE ANDATMOSPHERE
69
The early hydrogen atmosphere It would seem most probable that the primitive atmosphere consisted of the most abundant elements in the universe (GILBERT,1964) (table 1). The predominant cosmic elements are hydrogen and the noble gas, helium. Hydrogen would then chemically combine with the other abundant elements, oxygen, carbon, and nitrogen to form respectively, water, methane, and ammonia. Hydrogen and helium, being such light elements, would continually overcome the gravitational force and escape. On large cold planets, this early atmosphere would be retained. It seems that indeed this is the present day atmosphere of Jupiter and Saturn (GILBERT, 1966b). From such an atmosphere with an appropriate energy source, it would be possible to obtain complex organic metastable compounds, the building blocks of the biosphere. It has been speculated that the origin of life occurred under such conditions (SNNEOURand OTTESEN,1966). If this is true, then perhaps someday we might be able to observe the dawn of life by traveling to Jupiter and Saturn. It has been pointed out that there is a possibility of organic compounds (LIPPINCOTTet al., 1967) and even life on Jupiter (SODEKand REDMOND,1967). The presence of hydrogen on these planets may produce a very significant greenhouse effect and have a warming influence (TRAFTON, 1967). The first organisms in such an environment would need an energy reserve, which could be in the form of carbohydrate within the organism and free atmospheric hydrogen. Then these organisms would be confronted with the problem of hydrogen toxicity; in order to overcome this, they would have had to develop defenses against hydrogen, or in other words, develop antireductant mechanisms (GILBERT, 1966b). The atmospheric hydrogen would tend to reduce the organisms to methane, ammonia, and water. Also, recent studies indicate that in the early phases of building up the earth’s biosphere, there would have been a significant concentration of hydrogen cyanide (ABELSON,1966; CALVIN, 1965; MATTHEWSand MOSER, 1967; 0~6 and KIMBALL,1961). Thus, when the biosphere originated on earth, poisons were already in the atmosphere which could destroy life itself. However, the toxicity of hydrogen cyanide is due mainly to the oxidative enzymes (HEWITTand NICHOLAS,1963), which
TABLE 1 Per cent atom abundance* Element
Cosmos
Biosphere
H He 0 C N Others
86.68 13.18 0.09 0.03 0.01 0.01
62.6 0.0 24.9 10.6 1.1 0.8
*From GILBERT (1964).
DANIELL. GILBERT
70
wouldn’t have developed at this early stage. Fossil studies have shown that the biosphere existed at least 3.1 x lo9 years ago (BARGHOORN and SCHOPF,1966) and the age of the earth has been estimated at 4.5 x lo9 years (ULRYCH,1967). Life has probably evolved on other planets in our universe (CAMERON,1963; MACGOWANand ORDWAY,19.66). By necessity, any biogenic storage form of energy will possess the capability to destroy life. This condition will exist in any biosphere, no matter how bizarre its development might be, even in a possible anti-matter universe (ALFVBN,1966). As the hydrogen escaped from the earth’s atmosphere, bound hydrogen in the form of water, methane, and ammonia would be released into the atmosphere as free hydrogen, thereby liberating oxygen, carbon, and nitrogen from these compounds. Oxygen, being extremely reactive, would combine with the carbon (as well as silicon, iron, and other elements) to form carbon dioxide (and other oxides) and the nitrogen would remain as molecular nitrogen. Some of the oxygen would also react with the biosphere. The existing biosphere would adjust to this action of oxygen by developing antioxidant mechanisms. However, more energy would be available to the biosphere TABLE 2 The average composition
NZ 02 Ar coa Ne He CH4
Kr NaO HZ 03
Xe
of our present day atmosphere.
Total Eg *
Mol. Wt.
Total Emoles *
3864.8 1184.1 65.5 2.41 0.0636 0.0037 0.0043 0.0146 0.0040 0.0002 0.0035 0.0018
28.0 32.0 39.9 44.0 20.2 4.0 16.0 83.8 44.0 2.0 48.0 131.3
137.9 37.0 1.64 0.0548 0.00315 0.00092 0.00027 0.00017 O.OfIOl 0.0001 0.000073 0.000014
Mole % (dry air) 78.09 20.95 0.93 0.0310 0.0018 0.00052 0.00015 0.00010 0.00005 0.00005 0.00004 O.OOOOO8
Sea Level Pressure in mm Hg 590.0 158.0 7.0 0.234 0.014 0.0039 0.0011 0.0008 0.0004 0.0004 0.0003** 0.00006
Total dry
5117.0
28.97
176.6
Hz0 Total
22.0 5139.0
18.015 28.90
1.2 177.8
*The symbol E is the abbreviation
100.0
755.0 5.0 760.0
for the prefix, Erda, and represents the multiple, 10rs
(GILBERT,
1964).
**The pressure calculated here does not correspond to the sea level pressure since the 0s concentration increases with altitude reaching a maximum at about 30 km (GILBERT, 1964). The Eg for each gas was taken from MASON (1966) except for COa and HaO, which were taken from GILBERT (1964).
I~ER~EPENDENCEBBTWEEN BIOSPDBRE AND ATMOSPHBRB
71
TABLE 3 The per cent atom abundance of our present day atmosphere. Atom
Per cent atom abundance*
Relative to neon
N 0 a Ar C Ne He Kr Xe
77.65 21.20 0.676 0.462 0.0155 0.~89 0.00026 o.OoOO49 O.OOOOO4
87000 24000 760 520 17 1 0.29 0.055 0.004
*Calculated from tabie 2.
as it would begin to oxidize metastable compounds by oxygen into water and carbon dioxide. Speculation on the initial stages of biological evolution with special reference to the energetics available to the biosphere has recently been discussed (GILBERT, 1966b). Since there is an appreciable concentration of neon in the universe, co~esponding to one-third the cosmic nitrogen atom concentration (CAMERON,1959) (see table l), it would seem most reasonable that the initial atmosphere also contained a significant amount of neon. However, our present atmosphere contains hardly any neon. The present atmospheric neon content is only 3.15 x 10e3 Eg-atoms, as can be seen in table 2, where the symbol E is the abbre~ation for the prefix, Erda, and represents the multiple lo’* (GILBERT,1964). According to DAY(1964), thereis onlyaboutO.08 cm3 of neon per ton of rock. Since there are 2.3666 x IO’ Eg of rocks in the earth’s crust, the neon content in the lithosphere (solid portion of the earth’s crust) amounts to 8.45 x 10m5 Eg-Atoms. Hence, the bulk of the neon on earth is present only in the atmosphere. In the atmosphere the amount of nitrogen is 87000 times that of neon today (table 3). In order to explain this lack of neon on earth today, it seems necessary to conclude that neon has escaped from the earth’s atmosphere. But then practically the entire primitive atmosphere must have also escaped, since the gaseous constituents (molecular hydrogen, helium, water, methane, and ammonia) which comprise the primitive atmosphere have molecular weights less than neon. However, it would be possible that the organic material and biosphere which would have been derived from this primitive atmosphere would have been retained in the hydrosphere. Even though the primitive atmosphere was lost, it left its imprint on the biosphere as evidenced from the close correlation between the abundance of the elements in the universe and biosphere as shown in table 1, with the exception of the chemically inert helium. There is a possibility that part of the heavier constituents of the primitive atmosphere would have become trapped below the earth’s crust. These trapped gases would then
72
DANIELL. GILBERT
have a deficiency in hydrogen. Under such conditions, the predominant gases would be water, carbon dioxide, and molecular nitrogen. These three compounds all have greater molecular weights than neon. In other words, the trapped gases would probably have the same composition as that of the atmosphere, evolving by the escape of hydrogen into space. Volcanic action releases trapped gases, which contain significant amounts of water and carbon dioxide (MASON, 1966); this degassing of the earth’s lithosphere and mantle has probably been an important source for the development of our present day atmosphere (ABELSON,1966; HOLLAND,1962). The oxygen atmosphere The next major step would be the development of the photosynthetic production of oxygen and carbohydrates by the biosphere. This would provide much more energy to the organism. It has been pointed out that oxygen is the best form of energy storage for a biosphere (GEORGE,1965; GILBERT,1964). The oxygen in the atmosphere would increase by this process (BERKNERand MARSHALL,1965; HOLLAND,1962). By necessity, the biosphere would have to produce further antioxidant mechanisms to combat oxygen toxicity (GERSCHMAN,1964; GILBERT, 1966b). Many different mechanisms for combating oxygen toxicity exist (HAUGAARD,in press). The oxygen concentration probably rose steadily due to photosynthetic activity with a resultant continuous development of antioxidant mechanisms by the biosphere. This process has evolved until the atmosphere reached its present composition as given in table 2. BERKNERand MARSHALL(1965) have emphasized the importance of the presence of our atmospheric oxygen in protecting the biosphere from ultraviolet radiation. These authors have postulated that in the past, the oxygen concentration might have fluctuated and even been greater than its present value of 158 mm Hg. However, if the partial pressure of oxygen was ever much greater than 158 mm Hg, then the biosphere should have developed antioxidant mechanisms to survive at such high values of oxygen. The evidence for such a development would be the ability of animals to withstand high pressures of oxygen; but this is generally not true (GERSCHMAN,1964). Hence, it appears that if the oxygen did fluctuate in the past, it could not have been much greater than 158 mm Hg. The dehydrogenation of the earth and the photosynthetic production of oxygen resulted in our present oxygen atmosphere which is primarily composed of nitrogen, oxygen, argon, and carbon dioxide (table 2). Argon is chemically inert, constitutes only a very small part of our atmosphere, and has probably had little influence upon the biosphere. Let us now discuss how these three other gases affect the biosphere. OXYGEN Oxygen, as has been pointed out, is toxic to all living matter. This is true in spite of the fact that some tissues and organs are exposed to naturally high oxygen pressures. Thus, the swimbladders of some fish can reach the fantastic oxygen pressure of one hundred atmospheres (GERSCHMAN,1964). Also, diving whales can develop a high
INTERDEPENDENCE BETWEEN BIOSPHERE ANDATMOSPHERE
73
pressure of oxygen in their lungs, but oxygen uptake can decrease the oxygen pressure (GILBERT, 1966a). This oxygen toxicity has undoubtedly played a major role in the development of the biosphere, in spite of the fact that the evolution of the metazoa probably could not have occurred without such an excellent energy source such as oxygen. The major effect of oxygen toxicity would be as a necessary consequence the buildup of antioxidant defenses. The development of antioxidant mechanisms would be proportional to the oxygen tension. Thus, if organisms were not exposed to oxygen, then no antioxidant defense would be developed. For example, anaerobic bacteria have not evolved with antioxidant mechanisms. If the ancestors of some organisms were exposed to a higher oxygen tension than their descendants, an interesting consequence might occur. Since the ancestor would have had to develop antioxidant mechanisms able to cope with the previous high oxygen tension, the antioxidant defense of the descendant would be better able to resist the later lower oxygen tension. The descendant would also have to be able to utilize the lower oxygen tension more efficiently than its ancestor. Perhaps such a phenomenon occurred when some organisms migrated from low altitude regions to high altitude regions. The result is that the present high altitude organism in many ways is better able to cope with its low oxygen environment than can the present low altitude organism do with its higher oxygen environment. Let us discuss the migration of part of the Camelidae family and man to high altitude regions. Part of the Camelidae family (Lama and Vicugna genera) migrated to the high altitude Andes of South America from North America (PALMER,1957). However, since both the camel and the llama possess a high blood affinity for oxygen (BARTELS et al., 1963), it appears that some mechanisms for their very successful adaptation to the low oxygen environment of the high altitude regions were present before this migration occurred. Of course, there are several other mechanisms for adaptation to high altitude (BARTELSef al., 1963; BULLARD,BROUMAND and MEYER,1966; REYNAFARJE,1966). Let us now examine the more recent migration of man to high altitude areas. Man has lived for at least 10000 years at Peruvian high altitudes (MONGEM and MONGE C, 1966). During this period, high altitude man has adapted to his low oxygen environment to a significant degree. High altitude individuals of the Andes (REYNAFARJEand VELASQUEZ, 1966) and Himalayas (MILLEDGEand LAHIRI; 1967) are better able to cope with low oxygen environments than sea level residents recently acclimatized to high altitude. The high altitude residents are also capable of performing muscular exercise most efficiently. To illustrate this point, it can be mentioned that communications could be sent by the relay runners of the high Andean Inca civilization a distance of 1250 miles in only five days. This speed required that the couriers running on the Inca roads, at altitudes ranging between 6000 and 17000 feet, had to maintain an average speed 2.5 times that of the speed accomplished by the couriers of the Roman civilization. Present day Andean Indians can run at the required rates to make such an impressive feat possible (VON HAGEN, 1957).
74
DANIELL. GILBERT
NITROGEN What is the effect of nitrogen on life? In a non-nitrogen atmosphere it has been shown that there is a greater chance of producing pulmonary atelectasis (DuBors et al., 1966; MACHATTIEand RAHN, 1960). However, at high pressures it has a narcotic effect (FENN, 1967). Although this relatively inert gas is more abundant than oxygen in the atmosphere, its action at normal atmospheric pressure (table 2) has only recently become a practical problem. When the astronauts of the U.S.A.were exposed to nitrogen free atmospheres, it was found that nitrogen does play a major role in reducing the fire hazard of an oxygen environment (DENISON,ERNSTINGand CRESSWELL,1967; FENN, 1967; ROTH, 1966a,b). The tremendous fire hazard that would be presented by the absence of some inert gas in the atmosphere could in itself prevent the development of life on earth. In this respect alone, nitrogen plays an important role in the survival of the biosphere. CARBONDIOXIDE Since the biosphere constitutes such an infinitesimal part of the earth’s crust, it is really astounding that man has actually been changing the composition of the earth’s crust within the past hundred years. There is evidence to show that the carbon dioxide concentration in the atmosphere has increased about 7 % since 1900 due to the burning of fossil fuels and the removal of forests (GILBERT, 1964). The carbon dioxide in the atmosphere absorbs the long wave heat waves and thus prevents heat waves from leaving the earth resulting in a greenhouse effect. Hence, increasing the carbon dioxide increases the earth’s temperature, which has been observed during the first half of this century. However, the liberated carbon dioxide is stabilized to a large degree by the ocean (GILBERT, 1964). To complicate matters, man has also been polluting the air which blocks the radiation from the sun and, in the past ten years, there appears to be a reversal in the temperature rise due to this factor (MCCORMICKand LUDWIG,1967). Other factors which can decrease the earth’s temperature are an increase in the atmosphere of water (MUELLER, 1963) and volcanic debris (PUTNAM,1964). Temperature changes on even a small scale can have a very significant effect on man. The future carbon dioxide atmosphere Eventually the dehydrogenation of the earth will probably result in the absence of all water. The atmosphere will then be composed of only nitrogen and carbon dioxide, and life will no longer be possible. The evolution of planetary atmospheres and its relationship to possible life stages, is illustrated in fig. 1 (GILBERT, 1964). It seems that Mars contains a sizable amount of carbon dioxide (BARKER,1967; SAGAN, 1966), an inferred amount of nitrogen and a barely detectable amount of water (SAGAN,1966). It might be that when Mars is examined more closely, evidence of a previous biosphere will be found. A hot planet, such as Venus (CHASE,KAPLAN and NEUGEBAUER, 1963), would have a tendency to lose its atmosphere; it could be
I~~EPB~BNCE
75
BETWEEN BIOSPHEREAND AT~OSP~
EVOLUTION
OF ATMOSPHERES
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y _-_..__
i
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:
He
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Fig. 1. Evolution of atmospheres. Taken from GILBERT(1964). Acknowledgment
is given to the
American Physiological Society for permission to use this figure.
in a possible post-life stage. A signifhzant concentration of carbon dioxide does exist there (DAYIIOFF et aE., 1967) which would support this hypothesis. However, it has been suggested that perhaps there has been a migration of life to the clouds where the temperature is not so high (MOROWITZ and SAGAN, 1967). The detection of any possible life on these planets is difficult to determine at the present time. It should be noted that our earth would not, with our present tools of observation, yield signs of life to an observer from outer space (KILSTON, DRUMMO~ and SAGAN, 1966; SHKLOVSKII and SAGAN, 1966).
Eventually first the nitrogen and then the heavier carbon dioxide will disappear and the earth will be devoid of an atmosphere like Mercury and our moon. It has been proposed that even the moon has undergone some biogenic activity ~GILVARR~, 1966). References ABELSON,P. H. (1966). Chemical events on the primitive earth. Proe. Nut. Acad. Sci. 55: 1365-1372. A&N, H. (1966). Worlds-~tiworlds. ~ti~tter in Cosmology. San Francisco, W. H. Freemen and Co. BAROHOORN, E. S. and J. W. SCHOPF(1966). Microorganisms three billion yeers old from the Precambrian of South Africa. Science 152: 758-763. BARKER,E. S. (1967). A determination of the Martian COs abundance. Astrophys. J. 147: 379-381. BAR~?LS,H., P. H~LPERT,K. BARBEY,K. BETKE,K. RIEQEL, E. M. LANG and J. METCALFE(1963). Respiratory functions of blood of the yak, llama, camel, Dybowski deer, and African elephant. Am. J. Physiol. 205: 331-336. BERKNER,L. V. and L. C. MARSHALL (1965). History of major atmospheric components. Proc. Nat. Acad. Sci. (U.S.) 53: 1215-1226.