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CHAPTER 11 A MODEL OF MARTIAN ECOLOGY* WOLF VISHNIAC, K. C. ATWOOD, R. M. BOCK, HANS GAFFRON, T. H. JUKES, A. D. MCLAREN, CARL SAGAN, and HYRON SPINRAD INTRODUCTION Although the environment on Mars differs drastically from that on Earth, the difference is not so great that the terrestrial biologist cannot envisage a group of organisms that would not only survive but flourish under Martian conditions. In attempting to describe the activities that a Martian organism must carry out in order to survive, it should be remembered that Mars, like Earth, cannot be populated by any single type of organism. Any model of a Martian ecology must describe a community of organisms, the members of which compensate for each other's activities. The sum of these activities constitutes a biological cycle of matter. Ever since the opposite net effects of photosynthesis and respiration have been known [Ingenhousz, 1779] it has been understood, at least in general terms, that a worldwide balance between these two processes must exist. The Earth is therefore a gigantic balanced vivarium in which the various populations live at steady state levels that are limited by the energy flux through the system and modified for any one organism by other members of the same food chain. "Food" is here used in the most general sense and comprises not only organic matter but also all other necessary chemical components of the environment, such * Report prepared by Professor Vishniac as chairman of a study group on this subject. 229

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230 SOME EXTRAPOLATIONS AND SPECULATIONS as oxygen, nitrogen, carbon dioxide, mineral salts, etc. On Earth the amount of food materials that is recycled maintains a biomass that approaches, within an order of magnitude, the limit imposed by the energy flux from the Sun. This relationship is exemplified by the following figures. Of the 5 X 1024 joules per year that reach the upper atmosphere from the Sun about 2 X 1024 joules reach the surface of the Earth [Rabinowitch, 1945]. Allowing for infrared radiation, which is not used in photosynthesis (50 per cent), and absorption and reflection losses (20 per cent), about 6 X 1023 joules are available for photosynthesis in forests, on prairies and arable land, and in the ocean. Photosynthesis in the lie Id is about 2 per cent efficient, so that 1.2 X 1022 joules can be converted per year. The con- version of one gram of carbon from carbon dioxide to the constituents of a living organism requires at least 5 X 104 joules (cf., Table 1). The availability of 1.2 X 1022 joules per year therefore limits the annual pro- ductivity of Earth to 2.4 X 1011 tons of carbon, of which about 2 X 1011 tons can be fixed in the ocean. The actual productivity of the oceans is estimated at 100 to 200 grams of carbon per year per square meter [Harvey, 1957; Riley}. The area of our oceans, excluding the Arctic Sea and some marginal waters is 3.5 X 1014 square meters. The annual fixation is therefore 3.5 to 7.0 X 1010 tons of carbon per year, or 18 to 35 per cent of the theoretical maximum. Considering the uncertainty of the figures, it appears that primary produc- tivity on Earth, and hence biomass, come within a factor of ten of the level at which energy flux would be directly limiting. On Mars, the biomass may be limited by the available water and a different relationship is therefore imposed on the members of the ecological community. CONDITIONS ON MARS This discussion of Martian ecology is based on the following description of the planetary environment. Mars possesses an atmosphere with a sur- face pressure between 10 and 60 mb. This atmosphere consists of about 30 to 90 per cent carbon dioxide with the remainder most likely a mixture of nitrogen and argon. The atmosphere is transparent to visible light and admits a high level of ultraviolet radiation. There is no evidence for oxygen; the spectroscopic upper limit is 50 cm-atm, or about 0.025 mb. In the following discussion two alternative assumptions are made: 1) that there is no oxygen, and 2) there is oxygen present to the detectable limit, namely a partial pressure of 0.025 mb. Surface temperatures reach 30° C but a diurnal variation of 100° must be expected in many latitudes. The atmosphere contains water vapor to the

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Model of Martian Ecology 231 n 3 1 1 5 W M U o 0 O O o c X X X X X g U i s_ s 8. o A § U .8 B c 0 X >l 1 cu i_ U i Required PC Cofactors eu Q approximati 0.5 ATP a. | g 0 •/., X A q o ri O U. U 0> P>J !• .ii g '3 ea e. «- 8 u 0 e "e o o | V 8 r— i l*s 2 09 •-*- U -C u g o o\ 3 0. version of unit weight of iving organism is based < lated as follows: Calvin.Benson Pathway Survey of known pathw material per mole ATP [Bauchop and Elsden, 1 i •f. m .-' o O g 3 cr a ? { 6 w 2 "13 feb 1 * IS a. o c 3 c o « 1 I H 1* "o U to is | .w a H 00 T3 .2 4> fcj en M c s CX U a U 3 2 •9 CJ "o O 0 o O S : in Biosynthe arbohydrate C en imum requir 01 a t Jj f ea | •3 '3 a ea a I I O 1 p1 « E Ja U o cd H

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232 SOME EXTRAPOLATIONS AND SPECULATIONS extent of 2 X 10-3 gm cm-2. Hoarfrost may form during the Martian night, and water is frozen out at the Martian poles. The waxing and waning of the polar caps with the seasons implies an atmospheric water transport. The light areas of Mars are thought to be covered with limonite of an aver- age particle diameter of 100/« or less and the dark areas may contain related material [Dollfus, 1957]. To form a stable ecological system a community of living organisms must not only survive in this environment, but the activities of the organisms must tend to maintain this environment. The following outline of possible biological activities on Mars is an extrapolation based on the principles of terrestrial biochemistry. PRIMARY PRODUCTIVITY The external source to support a community of organisms is the radiant energy of the Sun. Two types of mechanisms may exist to convert the radiant energy into chemical energy. There may be a non-biological se- quence of reactions, similar to those that occur in experiments that produce organic matter from primitive atmospheres (Miller [1957]; and cf., Chapter 2), or there may be a biological energy conversion, namely photosynthesis. The incidence of ultraviolet radiation may bring about a reaction between carbon dioxide, water, and other atmospheric constituents to form carbohy- drates or related compounds. The laboratory simulation of this process has now been performed by Young, Ponnamperuma and McCaw [1965] and both five- and six-carbon sugars have been produced, among other com- pounds. The rates of synthesis, while low, are increased in the presence of limonite. These organic compounds may accumulate on or precipitate to the surface and serve as the primary source of organic material on which Martian organisms could grow. The alternative would be the existence of photosynthetic organisms that assimilate carbon dioxide in the light. Since oxygen is either absent from the Martian atmosphere, or at best present in extremely low concentration, the Martian photosynthesis would resemble that of terrestrial bacteria, rather than plants. Green plants and algae on Earth use water as the ulti- mate electron donor in photosynthesis with the consequent liberation of oxygen—which, indeed, is thought to be the major source of oxygen in the terrestrial atmosphere. Photosynthetic bacteria use compounds other than water as their electron donors, such compounds including hydrogen, a variety of reduced sulfur compounds, and other substances. In the absence of direct information, it is nevertheless possible to advance arguments for or against the production of organic compounds by non-

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A Model o] Martian Ecology 233 biological photochemistry, based on a consideration of the selective advan- tage that such a process may or may not have for the remainder of the Martian population. Should organic matter be produced by non-biological photochemistry at a non-limiting rate, so that there is always an excess of organic matter available to heterotrophic organisms, then there might be no selective pressure favoring the evolution of photosynthetic organisms. On Earth, accumulations of non-living organic matter are consumed by organisms which develop as rapidly as organic matter is formed, but on Mars life is limited by water and therefore organic matter might conceivably accumulate in excess of the biomass. However, water is also one of the raw materials of non-biological photochemical synthesis and the rate of formation of organic matter by this mechanism must therefore be restricted, much as life itself might be restricted. Another disadvantage of non- biological photochemical synthesis might be that only a fraction of the organic matter so produced might be useful to Martian organisms. In other words, the efficiency of energy conversion in the food chain would be low. Under these circumstances, selective advantages might favor the for- mation of photosynthetic organisms that would be the primary producers of organic matter and the converters of radiant energy into chemical energy. However, the activity of such photosynthetic organisms may be supple- mented by the occurrence of "extraorganismal photosynthesis," so that the primary food supply might be in part organic matter synthesized by photo- synthetic organisms and in part "manna from heaven." RESPIRATION On Earth, photosynthetic processes are counterbalanced by respiratory activities in which oxygen is the terminal electron acceptor. Whether even a small part of the respiration of Martian organisms is linked to oxygen will depend on whether the Martian atmosphere contains any oxygen at all. Present observations place an upper limit of 0.025 mb on the partial pres- sure of oxygen in the Martian atmosphere. Although on Earth the partial pressure of oxygen is nearly four orders of magnitude greater, even such a small amount of oxygen as might exist on Mars could be of biological significance, as the following calculation will show. At 0.025 mb the con- centration of oxygen is 2.5 X 10'5/22.4 = 1.13 X 10"* moles per liter of oxygen in the atmosphere. The solubility of oxygen in water at NTP is 4.89 X 10"2 ml per ml. The concentration of oxygen in water at 0° C would therefore be 1.13 X 10'6 X 4.89 X 10"2 = 5.46 X 10"8 moles per liter. This concentration is marginal for terminal oxidative processes of some terrestrial organisms. Thus a micrococcus species has been re-

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234 SOME EXTRAPOLATIONS AND SPECULATIONS ported to respire at 1 ° C under 0.1 mb oxygen as rapidly as under 200 mb oxygen [Warburg and Kubowitz, 1929], and light emission by luminescent bacteria can be observed at concentrations considerably below 0.1 mb [Hastings, 1952]. A variety of intestinal parasites is reported to respire at extremely low partial pressures of oxygen [Bueding, 1963] and germina- tion of certain plant seeds has been observed by Siegel et al. [1963] at about 1 mb oxygen. Thus if the Martian atmosphere should contain 0.025 mb oxygen, Martian microorganisms may carry out respiratory activities comparable in their net balance to those of terrestrial organisms. In the absence of any direct evidence for oxygen, we must proceed on the assumption that Mars is anaerobic, but it is worthwhile to consider, in passing, the biological significance of aerobic respiration. In the oxidation of organic matter with oxygen as a terminal electron acceptor, a greater change in free energy takes place than in the use of any other commonly available electron acceptor. Although such thermodynamic considerations bear no necessary relationship to the biological efficiency with which such energy is utilized, it is clear that more energy is available in aerobic respi- ration and that some organisms, at least, will take advantage of this pos- sibility. In a competition for organic substrates, aerobic respiration provides an organism with a selective advantage since it gains the same amount of energy at the expense of less organic matter. Experiments on the growth of microorganisms [Bauchop and Elsden, 1960] show that growth is di- rectly related to the biologically significant energy that is made available in substrate dissimilation, provided that the raw materials for cell synthesis are present. There is then first of all the advantage in numbers or mass bestowed on those organisms that utilize oxygen. Secondly, the availability of more energy and the use of an electron acceptor that diffuses readily through the living tissue makes possible the evolution of larger multicellular structures than is possible in fermentative organisms. It can be stated as a generalization that all multicellular organisms that we know are aerobic. It is the use of oxygen that enables an organism to devote part of its energy to the maintenance of a constant temperature, that is, to the maintenance of a constant activity, largely independent of temperature fluctuations in the environment. This means that an organism so endowed can be active in an environment in which some of its competitors are dormant. Undoubt- edly, all these faculties are the prerequisite for the development of intelligent life, so that the prerequisites for intelligence were developed when photo- synthetic organisms first learned to utilize water as electron donor and thereby discharge oxygen into the atmosphere. Nevertheless, it is prema- ture to exclude the existence of anaerobic metazoa on Mars. In the absence of oxygen, Martian respiratory organisms must use other electron acceptors. Terrestrial examples of anaerobic respirations include

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A Model of Martian Ecology 235 the reduction of sulfate to sulfide, in which the sulfur atom accepts electrons in terminal respiration, the reduction of nitrate to nitrogen, and the reduction of carbon dioxide to methane. Thus a number of common pseudomonads and enteric bacteria will grow at the expense of the reaction: 5 CH3COOH + NO3 -» 10 CO2 + 4 N2 + 6 H2O + 8 OH~. Some of the acetic acid is assimilated to make bacterial matter, but the reaction above summarizes the energy metabolism. Sulfate reduction, which is largely carried out by Desulfovibrio, proceeds according to the reaction: 2 CH3CHOHCOOH + SO4= -» 2 CH3COOH + S= + 2 CO2 + 2 H2O. As a final example a reaction in which methane bacteria produce methane is: 2 CH8CH,,OH + CO2 -> 2 CH3COOH + CH4. All three processes are thermodynamically spontaneous reactions and the change in free energy supports the growth of the microorganisms, although the change in energy is not so great as it would have been if oxygen had been the electron acceptor. Should reactions of this type occur on Mars, that is, should organic matter produced in photosynthesis be oxidized with electron transfer to nitrogen, sulfur, or carbon, there must be balancing reactions that reoxidize the nitrogen, sulfide, and methane. This recycling of the reduced electron acceptors may be carried out by photosynthetic organisms. Terrestrial photosynthetic microorganisms include the purple sulfur bacteria that reduce carbon dioxide in the light and derive the requisite electrons from sulfide or thiosulfate. Such a metabolism can be ecologically linked to that of the sulfate-reducing bacteria as is shown in Figure 1. On Earth, such oxidations can also take place at the expense of oxygen. Thus there are microorganisms that will oxidize sulfide or methane and ammonia to sulfate, carbon dioxide and nitrate. Light has the same significance for photosynthetic organisms that oxygen has for respiratory organisms; in photosynthesis light is used to create an electron donor and an electron acceptor, and thus an electron flow is initiated. The electron acceptor is capable of oxidizing compounds, the oxidation of which was at one time thought to proceed only with oxygen. It was found by Scher [1964] that non-sulfur purple bacteria could photosynthesize with aromatic compounds such as benzoic acid as an electron donor. The oxida- tion of methane has also been observed [Vishniac, 1963]; it leads to a photosynthesis that probably proceeds as follows: CH4 + CO2 -» 2 (CH2O).

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236 SOME EXTRAPOLATIONS AND SPECULATIONS KM S COz CH4 F«20j FiO /(CH.O) V Pool of Orgomc \Mott«r I ^ 1' PhoIotynttivtif Figure 1. Hypothetical cycles of matter on Mars. ECOLOGICAL ROLE OF LIMONITE The presence of limonite on the Martian surface is potentially of great ecological significance. Iron in its oxidized or reduced form may serve as an electron acceptor in respiration, or an electron donor in photosynthesis, while limonite, because of its water content, may serve also as a water reservoir on Mars. Limonite is a non-crystalline iron ore, containing chiefly goethite or lepidocrocite (Fe2O3 • H2O) and additional adsorbed water [Deer et a/, 1963]. Its average composition is (Fe2O3)2 • 3H2O, but only about % of the water is water of crystallization. The ferric iron of limonite may be thought to serve as a respiratory substrate for Martian organisms, a respiration in which iron serves as the terminal respiratory electron acceptor. This respiration can be described by 2n Fe2O3 + (CH2O)n -* 4n FeO + n CO2 + n H2O In the reduction of limonite, that is ferric oxide, to ferrous oxide the bound water would be set free, in addition to whatever water is formed in the oxidation of organic substrates. Terrestrial organisms are known, at least in crude culture, that can live by the oxidation of organic substrates with ferric hydroxide as the terminal electron acceptor [Vishniac, 1965]. The reoxidation of the ferrous oxide to the ferric form can reasonably be expected to support a photosynthesis which would take the following form

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A Model of Martian Ecology 237 light 4n FeO + n CO2 + n H2O—> 2n Fe2O3 + (CH2O)» There would therefore exist an ecological coupling between a respiration transferring electrons to ferric iron and a photosynthesis deriving electrons from ferrous iron. The limonite would at the same time serve as a water buffer, in the sense that its reduction to ferrous iron would give up water while the newly formed ferric oxide would gradually take up water to reform limonite. Martian organisms might therefore be thought of as swimming in an ocean of limonite. It is worth noting in this connection that the partial pressure of water in the Martian atmosphere corresponds approximately to that observed above limonite at typical Martian tempera- tures [Adamcik, 1963]. There is an additional consequence of such an iron cycle. Wherever living organisms are active on Mars there would be present simultaneously both oxidized and reduced iron compounds. In the presence of chelating compounds such a mixture is likely to produce intensely colored com- plexes of the type of which Prussian blue is an example. Should the dark areas on Mars be the result of biological activity their color (if real) may well find its explanation in the formation of such complexes. ORGANIC MATTER ON MARS We are at present in no position to estimate the Martian biomass, if there is any. Nevertheless it is instructive to carry out some speculative calculations that may lead to an estimate of the magnitude of the Martian biomass and a realization of the amounts that must be dealt with in any "life detection" experiment. As has been mentioned before, life on Earth is probably primarily limited by carbon dioxide and on Mars by water. A direct comparison between the terrestrial and a presumed Martian biomass is therefore not possible, unless we make certain assumptions regarding the water balance. On Earth the biomass is approximately 10 g per cm2 or about ten times the amount of atmospheric water. On Mars a similar relation would give for 2 X 10"8 g water per cm2 a biomass of 2 X 10~2 g living matter per cm2. Whether this figure is regarded as high or low depends on what assumptions one wishes to make concerning under- ground water resources. The amount of organic matter turned over by such a population can be calculated in terrestrial terms. For example, a cell of Escherichia coli weighs 10"12 g wet or 10"13 g dry. In order to synthesize that amount of material the cell requires 10"14 moles of ATP (based on Bauchop and Elsden [1960]), which are made in a glycolytic path from 5 X 10"15

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238 SOME EXTRAPOLATIONS AND SPECULATIONS moles or 9 X 10~13 g of glucose. Adding to this amount the weight of material needed to synthesize a new cell mean that E. coli turns over 10-J2 g of matter for every new cell. The total turnover of matter is therefore equal to the biomass per generation time. The generation time may be as short as twenty minutes, while the surface area of an Escherichia coli is about 10~7 cm2. Thus a terrestrial turnover rate for a bacterium can be as great as 10~8 g cm-2 sec-' under favorable laboratory conditions. However, for soil microorganisms, with starvation the rule and abundance the exception, average turnover rates are likely to be lower by about three orders of magnitude. For unit weight of microorganisms, these turnover rates are 0.8 X 10~3 g sec-1 in the laboratory or 0.8 X 10~6 g sec-1 in soil. Assuming the Martian biomass to be 2 X 10~2 g cm-2, the rate of turnover of organic matter on Mars becomes 1.6 X 10'8 g cirr2 sec \ Attempts to measure the activity of soil samples on Mars must therefore be prepared to measure rates of this order of magnitude. An argument can be made for comparable rates of metabolism in Martian and terrestrial organisms if we assume a biological basis for the seasonal darkening of certain areas on Mars. The wave of darkening on Mars travels at 35 km per day, while spring in the U.S. Middle West advances 40 km per day. STRUCTURE, MORPHOLOGY, AND GENERAL PHYSIOLOGY OF MARTIAN ORGANISMS The structure of Martian organisms will be dictated by the need to withstand extremes in temperature and the need to exploit those conditions under which the fluid of the internal environment is liquid. Organisms must also compete for water and specialize in its conservation. In addition the high ultraviolet flux may require the evolution of shielding or other protective devices. Organisms may maintain the liquid state of their body fluids by several techniques. Pigmented organisms absorb radiation and raise their tem- perature. The freezing point of their body fluids may also be depressed by the incorporation of solutes. Terrestrial halophilic microorganisms possess internal salt concentrations that approximate those of the external medium [Larsen, 1963]. Depending on the identity of the salts, the freezing point of an aqueous medium may be depressed to —50° C. Alternatively, organic solutes such as glycerol may not only depress the freezing point but also prevent the formation of crystalline ice that would otherwise disrupt the structures of the organism. Such a mechanism is

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A Model of Martian Ecology 239 known to contribute to the protection of insects that survive winters with extreme temperatures [Salt, 1959; 1961]. Because of the lower temperatures, Martian life may have evolved in a manner that makes far more diversified use of light reactions than do living things on Earth. Any chemical enzymatic reaction which because of low temperature would take a long time to proceed, could be speeded up when coupled to a light absorbing pigment fit to deliver the activation energies regardless of temperature. In other words, the Q10 of 2 that is typical for metabolic reactions on Earth may be lowered considerably by the right combination of light-dependent and light-independent reactions. While little is known of their action, catalysts that are active at low temperatures are known to exist. For example, isolated chloroplasts continue to de- teriorate at —20° C and the process responsible for this deterioration does not stop until the temperature has been lowered to —40° C. Metabolism on Mars may therefore be more dependent on light than on temperature. The necessity to conserve water is compatible with the maintenance of a temperature above that of the environment. Thus transpiration in photo- synthetic organisms would be kept to a minimum. In the absence of tran- spiration photosynthetic organisms would not be cooled, but their tem- perature would be raised by the absorbed radiation. Without transpiration pull, water could rise only by osmotic and capillary mechanisms, but it is not clear to what extent this restriction limits the height of a plant. Transpiration may be minimized by the absence of structures analogous to stomata and by the presence of a lipid cuticle. Similarly to known hydrophobic membranes, cuticle of this sort could allow the passage of carbon dioxide but be largely impermeable to water. In addition the cuticle might serve as a shield against ultraviolet radiation. Generally waxy or fatty structures would be more important on Mars than carbohydrate structures, if one can judge by terrestrial experience, in which carbohydrate storage is more frequently an aerobic process, while fats are stored in the absence of oxygen. Hence cellulose and lignin may be absent and plants either will be without rigid structural materials or will elaborate a silica skeleton. Fats are also a more efficient store of hydrogen than are carbo- hydrates. For protection against ultraviolet radiation, Martian organisms may develop three kinds of defenses. Highly absorbent organic material may be incorporated in the cell wall or in a waxy cuticle, such as mentioned above. The radiation so absorbed would also serve to raise the tempera- ture of the organism. An absorbent inorganic material such as the limonite of Martian soil, could be combined with the silica shell to make a rigid iron glass with strong ultraviolet absorbing properties. Finally, an organism might be shielded by fluorescent material and the emitted light could sup- port photosynthesis.

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240 SOME EXTRAPOLATIONS AND SPECULATIONS The ecological niches that Martian organisms might occupy are sum- marized in Figure 1. The occurrence and significance of many of the individual reactions have been outlined above; only the nitrogen cycle requires additional discussion. The immediate source of biologically sig- nificant nitrogen is ammonia. This ammonia can be provided either as soluble ammonium compounds or it may arise by reduction of other nitro- gen compounds. Many microorganisms are capable of reducing nitrate and nitrite to ammonia, and others are able to fix atmospheric nitrogen, which is reduced to ammonia. In the respiratory utilization of nitrate the end product is gaseous nitrogen, which then enters the nitrogen cycle by nitrogen fixation. The recycling of ammonia to nitrate is, in our experi- ence, a strictly aerobic reaction, carried out by the autotrophic nitrifying bacteria that derive their energy by the oxidation of ammonia to oxides of nitrogen. Should the Martian atmosphere be entirely devoid of oxygen, the nitrogen metabolism of the Martian organisms may follow either of the two following patterns: 1) There may be no nitrogen cycle in the redox sense, but all biologically significant nitrogen shuttles back and forth be- tween free ammonia or ammonium compounds and organic amines. 2) There may be an anaerobic oxidation of ammonia to nitrate, for which one likely candidate would be the utilization of ammonia as an electron donor in bacterial photosynthesis. An alternative would be an autotrophic organism using ammonia as the electron donor and ferric oxide as electron acceptor. No organisms of this type are as yet known on Earth. CONCLUSIONS Mars may be populated by a community of microorganisms and plants that utilize sunlight as the primary energy source and catalyze a cycle of matter on the surface of the planet. Microorganisms may vary from forms that live a few millimeters below the surface in a microclimate affording some protection from ultraviolet radiation and favoring retention of water and organic matter, to shielded organisms that expose themselves on the very surface of the soil. One attractive model for such shielded organisms are the Testacidae, the armoured sarcodina, such as Arcella or Difflugia. The shells of such organisms on Mars may be largely opaque to ultraviolet radiation and their amoeboid character could cause attachment to the substrate or to each other. Such attachment is suggested by the observation that Martian storms whirl up material from the bright areas and occa- sionally deposit it as a visible bright spot on a dark area, while aeolian transport of dark material has not been observed. If the seasonal darkening of certain Martian regions is indicative of

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A Model of Martian Ecology 241 biological activity, one would ascribe a rate of metabolism to Martian organisms that is comparable to that of terrestrial organisms. This is judged from the darkening of the nuclei, which takes place in a matter of days, and from the rate of advance of the wave of darkening. These rates are compatible with a generation time measured in hours rather than in days or weeks. The largest organisms on Mars may be plants that do not transpire, and that lack rigid support unless they elaborate a silicious skeleton. It has been suggested that the disappearance of light areas that form in dark regions by the settling of dust after a "sandstorm," may be due to the soil particles sliding off the leaves or limbs of such plants. An alternative is that the soil microorganisms multiply and overgrow such soil deposits. Soil from light colored areas, carried by the wind into dark regions, may be a fertilizer and stimulate growth of organisms by supplying fresh limonite with additional water or trace nutrients. For the Martian organisms, spring begins when, the rise in average tem- perature makes water available for photosynthesis. If we assume that photosynthetic organisms are pigmented, while the heterotrophic organisms by and large are not, then the occurrence of photosynthesis and the ab- sorption of light by multiplying photosynthetic organisms contributes to a further temperature rise in the microclimate. The heterotrophic part of the population follows suit and respires accumulated organic matter with the reduction of iron compounds. The formation of ferrous iron in the presence of ferric compounds and organic compounds, among which there may be chelating agents, will result in the formation of complexes that are strongly light-absorbing, and so the soil will darken. In the autumn, the falling temperature will inactivate heterotrophic organisms first, while the photosynthetic organisms, owing to their ability to warm themselves with absorbed light, remain active longer. Consequently iron compounds con- tinue to be oxidized (see page 236), ferrous compounds disappear, and the ground turns lighter. As winter descends, the ferric oxide slowly reacts with water and returns to limonite, thus storing water for the following spring. CONSEQUENCES This speculative outline of Martian ecology suggests those possibilities for which "life detection" experiments should be prepared. Instrument landings should take place in a dark area and sampling should take place during and after the wave of darkening has passed. If more than one land- ing is feasible then light and dark areas should be compared. Attempts to cultivate microorganisms should make use of sampling devices that are

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242 SOME EXTRAPOLATIONS AND SPECULATIONS capable of gathering particles as large as 100/t in diameter and culture media should accommodate the major ecological niches, with allowance for halophilic organisms. REFERENCES Adamcik, J. (1963), Planetary Space Sci., 11, 355. Bauchop, T., and Elsden, S. R. (1960), J. Gen. Microbiol, 23, 457. Bueding, E. (1963). In: B. Wright, ed., Control Mechanisms in Respiration and Fermentation, Ronald, New York, pp. 167-177. Deer, W. A., Howie, R. A., and Zussman, J. (1962), Rock-Forming Minerals, 5, Longmans, London, p. 124. Dollfus, A. (1957), Ann. Astrophys., Suppl. 4. Harvey, H. W. (1957), Chemistry and Fertility of Sea Water, Cambridge. Hastings, J. W. (1952), J. Cell. Comp. Physiol., 39, 1. Ingenhousz, J. (1779), Experiments Upon Vegetables, London. Larsen, H. (1963), Halophilism. In: I. C. Gunsalus and R. Y. Stanier, eds. The Bacteria, 4, Academic Press, New York, p. 297. Miller; S. L. (1957), Biochim. Biophys. Acta, 23, 480. Rabinowitch, E. (1945), (Recalculated from) Photosynthesis and Related Processes, 1, Interscience, New York. Riley, G., personal communication. Salt, R. W. (1959), Can. J. Zool., 37, 59. Salt, R. W. (1961), Ann. Rev. Entomol., 6, 55. Scher, S. (1964), In: M. Florkin and A. Dollfus, eds., Life Sciences and Space Research II, North-Holland Publishing Co., Amsterdam. Siegel, S. M., Rosen, L. A., and Giuarro, C. (1963), Nature, 198, 1288, and unpublished report: Qtly. Report No. 1, Sept. 30, 1963, submitted to NASA, Contract No. NASW-767. Vishniac, W. (1963), Presented at Fourth International Space Science Sym- posium ( COSPAR), Warsaw. Vishniac, W. (1965), In: M. Florkin, ed., Life Sciences and Space Research III, North-Holland Publishing Co., Amsterdam. Warburg, O., and Kubowitz, F. (1929), Biochem. Z., 214, 5. Young, R. S., Ponnamperuma, C., and McCaw, B. K. (1965), to be published.