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[Zone du Se 48) posa, 1958 | AMERICAN SCIENTIST AUTUMN SEPTEMBER. 1258 oxteEN CARBONDIOXIDE — NITROGEN Ê = E = , 23000 16 s2000 | : + PPS | THE BIOLOGICAL CONTROL OF ! a y | CHEMICAL FACTORS IN THE een =. ENVIRONMENT! By ALFRED C. REDFIELD E “"PHOTOSYNTHESIS ad NITROGEN FIXATION PO, NO, CO 1000 P Nas TT IS a recognized principle of ecology that the interactions of organ- isms and environment are reciprocal. The environment not only de- termines the conditions under which life exists, but the oxganisms influence the conditions prevailing in their environment. The examples on which this principle rests ave usually difficult to describe in quantita- tive terms and are frequently local in their application. In the oecan SULFATE a, NS the principal interactions between organisms and environment are . S04->S+20, chemical. Because of its unity, its fluid nature, and the intensity of the à 10000 mixing to which the water is subject their relations can be examined a N 1 Y statistically and expressed in quantitative terios, so mmemeremesmursamgemo» t TO t i The purpose of this essay is to discuss the relations between the Í statistical proportions in. which certain elements enter into the bio- "” Suas! PHOSPRORUS GARBON chemical eyele in the sea, and their relative dvailubility in the water. jogas 40000 400000 These relations suggest not only that the nibrate present in sea water . NITROGEN | COA & oil 1680000 and the oxygen of the atmosphere have beeii produced in large part by 10000 i organie activity, but also that their quantitics are determined by the Tia. 8. Tho Biochemicul Cycle. Numbers represent quentities of respective ele- requirements ot the biochemical eycle. The argument is not simple and menta present in tho atmosphere, the ocesn, and the sedimentary rocks, relative to in order that it may bo understood théimuire of the biodhemieal eyole the number of atoms of plosphorus in the ocean. : nd RA . ú and the circulation of the elements involved are reviewed. The Biochemical Cyele The production of organic matter in the sea is due to the photosyn- : thetic act roscopie floating plants, the phytoylankton, and is i limited to the surface layers where sufficient light is available. The formation of organic matter in the autotrophie zone requires all the elements in protoplasm, of which carbon, nitrogen, and phosphorus are 14 Sigma Xi National Lecture [or 1957-58. Contribution No. 976 Woods Hole Oceanographi Institution. 205 206 AMERICAN SCLENTISE of particular concern, These are drawn from the carbonate, nitrate, and phosphate ol the water. Following the death of the plants the organic matter is destroyed, cither by the metabolism of animals or the action of migroorganisms. Normaily, decomposition is completed by oxidation so that carbon, nitrogen, and phosphorus arc returned to the sea water as carbonate, nitrate, and phosphate, while requisite quantities of free oxygen ure withdrawn from the swater. E The autotrophic zone has a depth of 200 meters at most and includes less than five per cent of the volume of the ocean. Below this zone, life depends on organic matter carried down by organisms sinking from above or by the vertical migrations of animais back and forth between the depths. Although the greater part of the nutrient chemicals absorbed in the autotrophic zone complete the cycle in this layer, the portion which 'sinks as organic matter tends to deplete the surface layers of these chemicals and, with the decomposition of the organic matter in the depths, to enrich this heterotrophic zone with the products of de- composition. The existence of the vast reservoir of deep water in which organi matter may accumulate and decay out of reach of autotrophie resynthe. sis is a distinctive feature of the oceai nent which enables one toseparate, in observation and thinking, the constructive and destructive phases of the biochemical cycle. The synthesis of organic matter is a highly selective process which results in products ha composition. Differences in the compósiioa of various species -or indlividuuls exist, of Gott, bu the similarities on the whole are greater 1 iam the differences. When the population of a lar red it seems reasonable that its composition will be uniform in a statistical sense, and will be reflected in the changes in the of the water from which its materials are drawn or to which t eturned, In the decomposition of a given amount of organic matter the quantity of oxygen consumed, must be determined exactly by the quantities of carbon, nitrogen, ete., to be oxidized, and the relative changes in the quantity of oxygen, phosphate, nitrate, and carbonate in the water must depend exactly on the elementary composition of the plankton p The validity of é cept is shown by a comparison of the propor- tions in which the elements exist in the p] ion and the proportions in which they vaxy in samples of water frora the open sea. “The analysis of many samples oi plankton, taken in a variety oí places with nets designed to take organisms of different size indicates that atoms of phosphorus, nitrogen, and carbon are present on the average in the ratios: 1:16:106 (2. The oxidation of this material is estimated to require 276 atoms of oxygen. In comparison, data on the phosphate, nitrate, and carbonate content of sea water collected from various con CHEMICAL FACTORS IN THE ENVIRONMENT 207. depths in the several oceans show that the available phosphorus, nitrogen, and carbon vary from sample to sample in the ateis's ratios: | 1:15:105 and that about 235 atoms of oxygen disappear for each of one phosphorus atom, ming the water to have been in euilibrium with the oxygen of the atmosphere when it was last at the face 13-41. IL is as though various quantities of material contaix phorus, nitrogen, and carbon in these ratios had decomposca water samples, The correspondence of these ratios with those obtained from the analysis of plankton, as shown in Table 1, leaves Ettie doubt that these elements vary in sea water almost entirely as the result of the synthesis or decomposition of organic matter (1. in the general stitative olved in This conclusion permits one to approach the biochemical sea in much the same way as the physiologist examines th metabolism of an individual organism. It defines the qi relations of the cycles of the separate elements which are ix the question of the relative availability in the sea of the varisus sub- stances required for the growth of organisms. In accordance with: Liebig's law of the minimum, that constituent of the sea water present ir: smallest quantity relative to the requirement for growth of orgazisms will become the limiting factor. The ratios shown in Table 1 define precisely what these relative requirements arc. When the compositiun of sea water is examined it is found that phosphorus and nitrogen aro usually available in about the proportions, 1:15, required for the formation of the plankton. Carbon, as carbonate, in contrast, is present in great excess, the phosphoras-carbon ratio being about 1:1000 whereas the required ratio is 1:105. Consequently, carbonate never bes: limitivg factor. In a similar way suliur, an important plant m: available in great quantitics relative to phosphorus and nitr: the same is true of calcium, magnesium, and potassium. TapLE 1 ATOMIC RAIOS Or ELEMENTS IN THE BiocHEMICAL Cr: de N C Annlyses of plankton 1 16 106 Changes in sen, wator 1 15 105 235 Available in sea water 1 15 1060 2300 Phosphorus and nitrogen thus appear to be the constituen:s of se: water present in limiting quantities. Tt was pointed out by Earvey in 1926 that it is a remarkable fact that in the English Channel plant growth should strip sea water of both nitrates and phosphates at about 210 AMERICAN SCIENTIST Since the distribution differs from that of the biologically inactive components of the water there must be a biochemical circulation which is different from, though dependent upon, the physical eireulation of the water noi. ITow can this difference in behavior be explained? The presumptive agency of fractionation which separates the bio- chemical from the inactive elements in sea water is the selective ab- sorption of the former in the synthesis of protoplasm near the surface, followed by the sinking of the organized matter to greater depths prior to its decomposition. In addition the differential motions of the water at different depths must be taken into account if substantial differences in concentration and their redistribution are to be explained, The forcgoing discussion is intended to bring out two points of impor- tance, namely, that the principal elementary constituents of protoplasm enter the biochemical cycle statistically in definite proportions, and that the eycle of movement of these elements between the surface layers and the deep waters runs with sufficient intensity to influence markedly their distribution on an ocean-wide scale. We may now turn to two remarkable coincidences which have led to the inquiry vlúch is our major concern. Correspondence Between Requirement and, Availability of Phosphorus, Nilrogen, and Oxygen The stoichiometrie relations summarized in Table 1 indicate that phosphorus, nitrogen, and oxygen are available in ocean water in very nearly the same proportions as those in which they enter the biochemical eycle. In discussing the remarkable coincidence in the supply and demand for nitrogen and phosphorus it has been pointed out that it might arise from: (1) a coincidence dependent on the aceidents of geochemical history; (2) adaptation on the part of the organisms; or (3) organic processes which tend in some way to control the proportions of these elements in the water 1). Ofthe first alternative not auch can be said except that the probability that the ratio in the sea be what it is rather than any other is obviously small. That the coincidence applies to the oxygen as well as to the nutrient elements compounds the improbability. Tor the second alternative, it may be said that the phytoplankton do have some ability to vary their elementary composition when one element or another is deficient in the medium in which they grow. Such physiology might account for the coincidence in the nitrogen-phosphorus ratios. However, it is not evident how adaptation could determine the oxygen relation since this depends more on the quantity than the quality of the organic matter formed, and the oxygen requirement is felt only after the death of the living plant, CHEMICAL FACTOKS IN “LHE ENVIRONMENT ali For these reasons the third alternative deserves serious consideration. Mechanismas should be examined by which organic processes may have tended to control the proportions of phosphorus, nitrogen, and oxygen available for life in the sea, The Phosphorus-Nitrogen Ratio When the coincidence between the elementary composition of marine plankton and the proportions of available nitrogen and phosphorus in sea water was first noted, it was suggested that it might have been brought about by the activity of those microorganisms which form nitrogenois compounds from atmospherie nitrogen, or liberate nitrogen gas in thé course of their metabolism. The composition of such organisms must be more or less fixed in regard to their relative phosphorus and nitrogen content. When living in an environment containing a deficiency of nitrate relative to phosphate, the growth and assimilation of the nitrogen-fixing organisms might tend continually to bring the proportions of nitrogen and phosphorus nearer to that; characteristic of their own substance. Thus, in the case of Azotobacter, it has been found that; for every atom of phosphorus available in the medium about 10 atoms of nitrogen are fixed or assimilated into microbial protein gw, In an environment populated by organisms of this type, the relative proportion of phosphate and nitrate must tend to approach that characteristic of their protoplasm. Given time enough, and the absence of other disturb- ances, à relation between phosphate and nitrate such as observed in the sea may well have arisen through the action of such organisms, Nitrogen fixation is employed practically in agriculture whenever leguminous plants are used to enrich the soil. It is not unreasonable to assume that the same process has been effective on a larger scale in nature. Iutchinson has estimated that nitrogen is being fixed on the earth's surface at the annual rate of 0.0034 to 0.017 mg/em? 2). At the lesser value it would require only 40,000 years to fix the 7 X 1014 kilograms of nitrogen estimated to be available as nitrate in the ocean, Nitrogen fixation is so active that there is no difficulty in assuming that it might serve in adjusting the phosphorus nitrogen ratio in the sea. “The difficulty is rather in explaining why there is not a great excess of nitrate nitrogen in the water. The ratio of nitrogen to phosphorus in tresh waters is higher than that in ocean water, while the ratio in sedi- mentary rocks is very much lower, Consequently the ratio in the sea must tend to increase, unless some process is returning nitrogen to the atmosphere. Denitrifying bacteria might operate in this sense, in which case the phosphorus-nitrogen ratio is fixed by a complex balance. Biological mechanisms adequate to influence the phosphorus-nitrogen ratio in sea water are known. Whether they do in fact operate in a regulatory sense is a subject for future investigation. 212 AMERICAN SCIENTIST The Phosphorus-Oxygen Ratio The relation between the quantity of phosphorus present in sea water and the amount of oxygen available for the decomposition of organie matter is less obvious than the relation of phosphorus and nitrogen. The 'quantity of oxygen dissolved in sea water when it is at the sea surface appears to be fixed by equilibrium with the atmosphere, which contains about 21 per cent oxygen. Consider what would happen if a unit volume of water, containing its characteristic quantities of plant nutrients, was brought from the depths of the ocesn to the surface. There, under the influence of light, photosynthesis would convert the available nutrients to organic matter until all the phosphate is exhausted. At the same time the water is saturated with oxygen by equilibration with the atmosphere. If the unit volume of water is now returned to the depths shere the organic matter is decomposed and oxidized, will the TaBLE 2 Umuiatiox or Oxremx 1x Ocpan WATER Average S. W. North Atlantic North Pacific Phosphoras content 2.3 1.25 3.0 mg stoms/mº 8.9 3.75 9.0 Ti 6.7 7.5 saturnted S. W. º Faxcess oxygen 0.2 2.95 -1.5 Oxygen in “minimum 3.0 0.01 oxygen layer” * Calculated allowing 276 atoms of oxygen to be used in oxidizing a quantity of or- ganie matter containing ono atom of phosphorus in «shich esse 1 mg atom P/mS is equivalent to 3 milliiters Os/liter. dissolved oxygen present be sufficient for the purpose, will there be a large excess of oxygen, or will the oxygen be deficient? The answer can be given only within approximate limits, for two reasons. Tirst, the quantities of nutrients are different in the deep waters of the several occans, as we have seen. We can consider the extreme cases of the North Atlantic containing phosphorus in the concentration of 1,25 mg atoms per m?, the North Pacific conta ng 3.0 mg atoms per mê, and an “average” sea water containing, say, 2.8 mg atoms per mº. Second, the quantity of oxygen dissolved in the water when it is at the surface will vary depending upon the temperature, and to a lesser degree upon the salinity. The lowest oxygen concentrations are not found at the bottom of the ocenns, but at some intermediate depth, where also maximum con- i i | | Ê CHEMICAL PACTORS IN 'THE ENVIRONMENT 218 ; centrations of phosphate and nitrate occur. The temperature aí. this depth in the North Atlantic is about 8ºC. and sehen at the suvface at this temperature the water would have taken up 6.7 milliliters vf Aygen per liter. The corresponding temperature of the minimum açygen layer in the North Pacific is about 3ºC. and the oxygen soluisfriy 7.5 milliliters per liter. Trom these numbers Table 2 has been prepared to show thz: guess oxygen which might be expected to rémain in the minimum. oxygen layer if all the phosphorus present has been derived from the gxicttion of organic matter. In the case of the “average” sea water it appszs that the quantity of organic matter which can be formed from the nwizafis in a unit of volume of “average” sca water is almost exactly thoé ubich can be completely oxidized by the oxygen dissolved at the suifs, In the North Atlantic, where the phosphorus content is reduced, osty Zbcut one half of the dissolved oxygen would be consumed and tiz Açõess oxygen corresponds well with the quantities observed to remain in the zone of minimum oxygen content. In the North Pacific, the pAvsphate content of the deep water is so great that the dissolved oxygsk would be more than exhausted if the process went to completion &$ zostu- lated. Actually the Pacific Ocean does not appear to be anaerobis st any depth. However, large volumes of water at intermediate depth: contain aly small traces of oxygen 41 In high latitudes where the degp srater of the oceans originates the photosynthetic processes do not cos Vert all of the available nutrients into organic matter before the water sinks. Consequently, the deep water contains phosphates which have p4t been liberated by oxidation during the preceding turn of the cycle. Presumably this effect accounts for traces of oxygen which remain in the minimum oxygen layer of the North Pacific. Although the oxygenation of the oceans appears to be adeguate to oxidize the products of the biochemical cycle at present the margin is not large. Actually anaerobic conditions exist in a number cf Isolated areas, such as the Black Sea and the Sea of Azoy, Kaoe Bay ix iÃe East Indies, the Cariaco Trench, and numerous fjords in Norway ed clse- where (4). À decrease in the oxygen content of the atmosphere, aá Increase in surface ocean temperatures or reduetion in the vertical ci on of the water might lead to the extension of anaerobic conditions over auch wider areas, Tt is widely held among geochemists that the primitive gm Osphere was devoid of oxygen, or at least contained very much less oxy9 en than at present. During the course of geological history atmospheric oxygen is thought to have been produced by the photochemical dissecation of water in the upper atmosphere and by the photosynthetic reduction of carbon dioxide, previously present in much greater quantifios pal. 216 AMIBRIC: N SCIENTIST oxygenated. cas of marsh and estuary present anaerobie conditions, as do quite generally marine muds. At present the production of oxygen through sulfate reduction in such situations may not be more than sufficient to balance the losses due to the oxidation of eroding terrestrial surfaces. IH, however, in the past the oxygen of the atmosphere were lower than at present, anserobic conditions may have been much more prevalent. The reduction of sulfates may then have served to bring the oxygen content of the atmosphere and sea into correspondence with the requirements set by the quantities oí phosphorus available. ln considering the influence of the biochemical cycle on the environ- inent it should be remembered that the cycle can, and apparently for the most part does, run its course without adding more to the environ- ment at one phase than it withdraws at another. Photosynthesis cannot increase the oxygen content of the environment unless the products of photosynthesis are in some way withdrawn from reoxidation. The reduction of sulíates will not make more oxygen available in the sea if the sulfides produced arc reoxidized in the sea water. Fractionating mechanisms are requiz h separate the products of the cycle, so that they cannot re-enter it. Such mechanisms exist at the sea bottom, where materials may be buried in the sediments, and at the surface where volatiles may pass into the aimosphere. Considor the fate of the sulfide and oxygen which are produced in anaerobic basins by the activity of sulfate-reducing organisms. When the water containing these products is brought to the surface layers of the sea by mixing processes the sulfide will be reoxidized to sulfate, consuming a quantity of free oxygen equivalent to that produced by the original reduction. No change in the total oxygen in the water column would result if this were the complete picture, However, if a portion of the sulfide formas insoluble compounds, such as iron sulfide, which settle into the bottom sediments, or if a portion of the oxygen liberated near the sea surface passes into the atmosphere, these portions cannot re-enter the cyele immediately and a net increase in the oxygen of the environment will be produced. Conditions on the land, in contrast with the sea, appear to be much less favorable for producing changes in the free oxygen of the atmosphere, either by photosynthesis or in response to anacrobic conditions through sulfate reduction. The predominance of erosion does not permit the permanent entrapment of large quantities of organic matter on land. Soils are usually well aerated so that organic carbon does not remain long out of circulation. Anaerobic conditions do exist in swamps, and many coal deposits must represent reduced carbon formed in such places which have contributed oxygen to the atmosph but such withdrawals must be small compared to the quantities of organic matter accumulated in marine sediments. ed w PACTORS IN THE ENVIRONMENT 217 CHEMICAL The exchange of oxygen across the sea surface must be imy this connection. The solubility of oxygen in water is such thas under equilibrium conditions more than twenty times as much oxyger occurs in a unit volume of air than in a comparable volume of watez at the sen surface, Of any increase in the quantity of oxygen in the water, whether produced by the reduction of sulfates or the photosynthetic reduction of carbonates, only 4 small portion would remain in the water since a larger fraction would pass into the atmosphere.* In tais way the biochemical eycle of the sea can continue to add to the oxygen content of the environment until a new equilibrium is established between the atmosphere, the sea, and the available nutrient materials. It appears, then, that the biochemical cycle in the sea may have produced the major amount of oxygen im the atmosphere snd that conditions in the sea have adjusted its level to that which occurs at present. We think of the atmosphere as determining the oxygen content, of the sea. “This is because the atmosphere is the great reservoir of oxygen on the earth's suríace and be c the motion of the air gives it a conveniently constant composition. Perhaps it is more correct, how- ever, to think oi the sea, and particularly of its nutrient content, as determining the composition of the atmosphere. paus Geochemical Considerations As a final check on these speculations, we can look at th availability of the principal materials of the biochemical eycle on the f they conform to the postulates, In Table 3 earth's surface, to see estimates are given of the total quantities of Lhe pertinent elements in the atmosphere, the occan, and the sedimentary rocks, The content of the atmosphere is quite accurately known; that of the ocesn can be" approximated since the greater part of its water is of relatively uniform composition. The values for the sedimentary rocks are quite uncertain, partly because of the variability in their composition and the incomplete- ness in sampling, and partly because the volumes in question are arbi- trarily chosen. However, we are concerned with orders of magnitude so the numbers are useful. The first column of numbers in Table 3 ave the estimated weights of cach element on the earth's surface. Divided by the phosphorus content of the ocean and adjusted for atomic weights, the second column gives the numbers of atoms relative to the atoms of phosphorus in the ocean, * The intensity of the exchange of oxygon across the sea surface is indicated by the observations that in the Gulf of Maine 4 quantity of oxygen sufficient to form a | 0.9 meters in depth enters the utmosphere cach spring as the result of photosynthetie activity by phytoplankton. This is about 15 per cent. of the oxygen locally present to a depth of 200 m. This quantity of oxygen returns to the water during the following fall and winter to replace that used in the oxidation of the summer crop of organic matter [15]. relative dire 2is AMERICAN SCIENTIST So reduced, the relative quantities are indicated in Figure 3 which presents in diagranunatic form the biochemical eyele as described TABLE 3 Quantr AND PRropORTIONS OP EL que Eanrr's Sunrace* Ocean r 1x 10H 1 Nas NO; Tx10% 15 as N, 2.3x1018 510 c ax 0 1,000 s 1xI0% 10,000 (o) Ldxige 270 Atmosphere Nas Ny 3.8x105 62,000 (o) 64x 104 16 o 1.2x10% 23,000 Esrth's crust tary rocke P' 40,000 A 10,000 8 10,000 E 460,000 Cost and ei ex 108 160,000 petroleura * Kstimates aro based on following sources: Atmosphere [19]; Ocean, volume X 10º Km? [20], P 71 X 1078 gr/Ke(originat), N as NO: 15 XP, N as No 17.3 X 10-" gr/Kg nesuming eaturation at 20, and 34,83 % 8 [21], O 85 Os, including cor sumption in biochemical cycle, 10.7 X 10-3 g:/Kg tnration 34,5 % 8 [22], O us LICO; 0.028 gr/Kg [20] 8 as SO, 0.884 gr/Kg [20]; Earthvs eru: area 6.1 X 10'/em?, Sedimentory rocks 170 Kg/emº, P 0.46 per cent (arbitraxy wei ing of analyses by Stokes and Steiger) N 0,051 per cent, S = 0,12 per cent (arbitrary weighting of analyses by Stokes), O 3 Kg/em?, and coal and petroleum 1.28 Kg/em? after Kalle [19]. The diagram shows phosphate, nitrate, and carbonate entering the organic phase of the biochemical eyele near the sea surface, throngh the process of photosynthesis. Phosphorus, nitrogen, and carbon are selected by the synthetic process in the proportions of 1:15:105. step which coordinates the eycles of the several clemen! a unique way and gives meaning to thc comparisons, The elements are carried in these proportions to the point ol decomposition where they are oxidized to their original state as phosphaté, nitrate, and carbonate. “Phe oxygen required is just that set free by photosynihesis. Such a eycle could run indefinitely im am otherwise closed system so long as light is supplied. To account for the correspondence in the ratios of phosphorus and nitrogen in the organic phase of the cycle and in the inor ment, bacterial processes of nitrogen fixation indicated at the upper right and, similarly, the s is shown at the lower left. This latter is a his is the nie environ- and denitrification are ulfate reduction process sumed to operate cffectivel 2MICAL PACLORS IN THE ENVIRONMENT 219 only when the environment becomes anacrobie. Vinally, the exchanges with the atmosphere and the sediments of the sea botiom are shown. If these processes are operative it is necessary tha supplies are adequate and that their products exist in suitable quantities. Considering first nitrogen, there exist in sea water for each atom of phosphorus 15 atoms of nitrogen available as NO; and a reserve of 510 ivalent to 86,000 atoms of phosphorus, which is available he sea were it to he drawn on, The nitrogen about one-sixth Lat in the atmosphere and filths of this is fossil nitrogen srhich ma med to be derived from organic matter 2). large quantitics of nitrogen have passed through the age from the atmospheric reserves to be at the sea bottom. The quantity withdrawn in n comparison to the reserve in Lhe atmosphere. rogen supply is adequate. one of the most abundant ions jo sen water. In this form » equivalent to 10,000 atoms of phosphorus. It would pplying oxygen equivalent to 40,000 atoms of phos- i Clearly, the sullate reduction, mechanism could or a long time. If il has operated as postulated in have been removed from Lhe sea. Sedimentary imated é io contain sulfur equivalent to 10,000 atoms of ie phosphorus. Tf this were all the product of sulfnte reduction, vould have produced oxyreu equivalent to 40,000 atom of oceanie pios vice that present in the atmosphere. Tt is not clear how niuch o! the 8 limentary rocks is present as lides, but mueh of it is. Clesrly, much oxygen cam have been produced pasé by sulfate reduction ar ibly this process has contributed n important degree in producing the oxygen of the atmospher Carbon is present in the sca, chiely us carbonate ions, in about ten times the quantity required for the biochemical cycle, Mueh of the large deposits cf carbon in the sedimentary rocks is present carbonates avd cannot have contributed to the production of free oxygen. The estimated carbon present as cos] and petroleum, equivalent to 160,000 atoms of oceanic phosphorus, is suflicient to yield oxygen on reduction equivalent to 320,000 atoms of oceunie phosphorus, which is more than ten times the present content of the atmosphere. “The Inown facts of geochemistry do not appear to contradiet the sup- position nted on the mechanism which may have controlled the relativo availability of phosphate, nitrate, and oxygen in the sea. Sources of nitrogen and sulfate are available in great excess and the by- of do ser Iwenty time: thst in the ocean. More than four y he ass EO 
