High Temperature Systems

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Annu. Rev. Ecol. Syst. 1970.1:191-220. Downloaded from www.annualreviews.org Access provided by CONRICYT EBVC and Econ Trial on 09/24/15. For personal use only.

Department of Microbiology Indiana University Bloomington, Indiana

Temperature is one of the most important environmental factors, and be­ cause of its ease of measurement a vast body of knowledge exists concern­ ' ing its effects on living organisms. The average temperature of the earth is about 12°C ( 10 ) , and the majority of living organisms are adapted to live in a moderate range of temperatures around this mean. High temperature en­ vironments, although widely distributed across the earth, are relatively un­ common. There are a number of reasons why a study of such unusual habi­ tats is of general biological interest. It is of interest to study the systemat­ ics, distribution, and physiological adaptations of organisms which have been successful in colonizing high temperature environments in order to ex­ amine the limits to which evolution can be pushed. From an ecological point of view, high temperature environments usually have relatively simple spe­ cies composition and short food chains, which make a study of productivity, trophodynamics, population fluctuation, and species interaction more simple. From the viewpoint of applied ecology, an understanding of the biology of high temperature habitats is essential if we are to predict and control the consequences of thermal pollution by various industrial sources. It is the purpose of this essay to review recent work in these three areas and to at­ tempt to assess its significance. Although the words high temperature will often be used in this paper without qualification, the viewpoint of the observer or the group of orga­ nisms under discussion will often determine i f a given temperature is to be considered high. Thus, a temperature of 50°C would be considered critically high when referring to a multicellular animal or plant but would be consid­ ered moderate or even rather low if certain thermophilic bacteria we re 1 T h e reader is referred to a recent review by Castenholz (34) which complements the present one by providing more emphasis on the culture and physiology of the

thermophilic algae.

Supported by research grant NSF GB-781S and AEC contract COO-1804-16. For supplementary bibliographic material (444 references, S6 pages) , order NAPS Document 1164 from ASIS National Auxiliary Publicatoins Service, c/o CCM Information Corp.-NAPS, 909 Third Avenue, New York, N.Y. 10022. Remit with your order $2.00 for microfiche or $5.00 for each photocopy (10¢ for each additional page over 30). 2




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under discussion. Where necessary, I will qualify the word high by append­ ing actual numbers. Another point which should be emphasized at the outset is that one must distinguish between temperatures an organism can tolerate and those at which it can carry out its whole life processes. Thus certain invertebrates can survive exposure to temperatures approaching boiling, although they cannot grow at temperatures above 500e ( 3 1 , 56). No animal has been found which can carry out its complete life cycle at a temperature over 50°C. In terms of evolutionary success in high temperature environments only organisms able to carry out their complete life cycles are of interest. mCH TEMPERATURE ENVIRONMENTS ORIGINS OF THERMAL ENVIRONMENTS

There are four distinct causes of thermal environments: solar heating, combustion processes, radioactive decay, and geothermal activity. In man­ influenced environments, most heating is due to combustion processes, but recently heat production as a result of radioactivity has become of signifi­ cance and interest. (At one time it was thought that some natural heating might also arise from radioactivity, but that idea is apparently out of favor


Solar h eating. S olar heating can lead to soil temperatures as high as 60°C. Schramm (86) measured temperatures of this magnitude on black an­ thracite wastes in eastern Pennsylvania. Although such high temperatures occur only during the daylight hours, they are sufficient to prevent the suc­ cessful establishment of higher plants (86). Gates et al (50) measured a temperature of 600e on the surface of a desert soil at midday; in the same location the temperatures of desert leaves were in general lower. They were usually no more than 3°e above air temperature. In aquatic environments, solar heating leads to marked temperature increases only in shallow waters or in those with unusual density characteristics. In shallow marine bays, temperatures up to 400e can occur (52), although again this is a transitory effect seen only at midday. In certain aquatic environments solar heating can apparently lead to stable increases in temperature. Thus, Lake Vanda, a permanently ice-covered, density-stratified lake in Antarctica, has a bottom temperature of 25°e (57). In this case the possibility of geothermal heating has apparently been ruled out, and it is thought that the highly saline water on the bottom remains as a stratified layer which absorbs solar radiation and retains it. In a stratified pond near Eilath, Israel, temperatures as high as 500e arise as a result of solar heating ( 109). -

Combustion processes.-Combustion processes can be either biological or non biological. The study of the self-heating of hay and other organic mate-

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rials has a long history ( 55, 73, 77 ) , and it is well established that the tem­ peratures, which reach up to 70° C or higher, are the result of microbial heat evolution. It is thought that the habitat of many terrestrial thermophilic mi­ croorganisms, both fungi and bacteria, may be in locations where organic waste materials become naturally piled in windrows by wind or water and undergo self-heating ( 38 ) . Other habitats where natural self-heating exists, such as the nests of the intriguing incubator birds (Megapodiidae), are dis­ cussed by Cooney & Emerson (38 ) . Another combustion process, which is generally man-made but may also occur naturally, is the burning of coal and coal-refuse piles ( 53, 76, 89 ) . In several burning coal-refuse piles we have measured temperatures ranging from 45-1 50°C. ( One small acid stream draining such a pile had a temperature of 34-36° C even when the air tem­ peratures were less than O°C. ) Geothermal a ctivity Geothermal activity is probably responsible for the creation of the most numerous high temperature environments. The temper­ atures in active volcanoes are much too hot for living organisms ( molten lava can have a temperature of lOOO°C or over) , but hot springs and fuma­ roles associated with volcanic activity often have temperatures with a more reasonable range and are prime candidates for the development of thermo­ philic organisms. A recent survey ( 98) reveals that thermal waters are widely distributed over the face of the earth, although springs are often concentrated in restricted areas. The largest concentrations o f hot springs and fumaroles are found in Yellowstone National Park, Iceland, New Zea­ land, Japan, and the USSR. Temperatures of hot springs range from the 30s up to boiling ( 90-100°C depending on altitude). Fumaroles which consist only of steam vapor can have temperatures considerably higher than 100°C and seem to be devoid of living organisms (88; Brock, unpublished observa­ tions ) . Material on the biology of hot springs will comprise the bulk of this article. It is sometimes not realized that geothermal heating is a normal event throughout the earth's surface, although in most areas the heat production is so small that is is unrecognized. Likens & Johnson (69 ) calculated the heat production of Stewart's Dark Lake, a small meromictic lake of about 0.69 ha in northwestern Wisconsin. The bottom of this lake received essentially no heating from solar radiation, and the heat production from geothermal sources was estimated as 66 calfcm 2 year or about 7 X 106 kcalfyear for the lake as a whole. A comparison of this heat production to that of a hot spring may be of interest. For instance, a rather small Fijian hot spring produced 3 X 109 kcalfyear ( 54 ) , which is almost 3 orders of magnitude higher. Likens & Johnson (69 ) note that on a normal land surface the ,flux of solar energy is 4-5 orders of magnitude larger than the flux of geother­ mal heat, but, clearly, within the localized region of a hot spring the geo­ thermal flux is much higher. .-



Man-made sources of heat.-Man-made sources of heat are indeed com­ mon in this industrial age. Space does not permit a detailed survey of such thermal habitats, although I discuss some aspects of their biology later in this article. Some habitats which have received biological attention include hot water heaters, sugar refineries, power plants and their cooling towers, paper mills, and textile mills.

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Temperature affects a wide variety of properties of water, and some of these effects are summarizled in Table 1. Certain of these temperature effects are of more practical significance than others. For instance, the marked de� crease in density of water at higher temperatures means that when hot water flows into cold, it usually floats on top and only mixes gradually. The decreased viscosity of water at high temperature means that it flows more easily than cold. ( Hot water also sounds different from cold when flowing, probably due to the viscosity effect. ) The effect of temperature on oxygen solubility may be of considerable biological significance. For instance, at 90°C oxygen will dissolve in water at less than 2% of the amount at which it dissolves at 20°C. On the other hand, silica is present in solution in hot spring waters to concentrations con­ siderably higher than in cool water ( 49 ) . At 25 ° C the pH of pure water is

TABLE 1. EHects of tempelrature on physical and chemical properties of water Property

Effect of an increase in temperature

Physical Density




Surface tension




Dielectric constant


Vapor pressure


Heat capacity




Refractive index




Chemical Ionization




Oxygen solubility


Solubility of most organic


and inorganic compounds



7.00, whereas at O°C it is 7.47, and at 40°, 6.77. However, of more interest is the effect of temperature on the pH of dilute aqueous solutions of anions and cations such as are found in nature. For instance, a solution of potas­ sium phosphate which has a pH of 6.86 at 25°C decreases to only pH 6.84 at 40°C.

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Of considerable evolutionary significance are the effects of temperature on the stability of certain biochemical and macromolecular constituents. Such key compounds as adenosine triphosphate (A TP ) and nicotinamide adenine dinucleotide (N AD) are somewhat heat labile, although the extent of such lability and its practical significance have not been investigated. Macromolecules such as RNA and DNA are heat sensitive under certain conditions ; divalent cations such as Mg++ stabilize them greatly, and it seems that at ionic concentrations found in the cell these substances may be stable even to boiling. Although molecular biologists have studied some effects of heat on macromolecules, heat has more often been looked upon as a physical­ chemical tool rather than an environmental factor. EVOLUTION AND HIGH TEMPERATURE UPPER TEMPERATURE LIMITS FOR VARIOUS TAXONOMIC GROUPS

The upper temperature limits for various taxonomic groups have been determined both by comparative observations of the biota of springs of dif­ ferent temperatures, and by the examination of the biota at different tem­ peratures along the thermal gradients created by the effluents of single springs. Although both kinds of observations have given approximately the same results, s tudies of the latter type are perhaps more meaningful since one is comparing habitats subjected to the same light conditions and chemi­ cal composition. It is of course essential to know that the spring has been flowing and available for colonization long enough for equilibrium to have been reached. At least in Yellowstone Park, the development of a hot spring community is quite rapid, probably because a large number of similar springs exist in a small area, and suitable inocula are readily available. In one location ( Mushroom Annex) where direct observation of rate of devel­ opment was possible (27), an apparently complete hot spring community was established in less than 6 months. The approximate upper temperature limits for various taxonomic groups are given in Table 2. Unfortunately, data for some groups are lacking or incomplete. Note that here we are considering not the ability of an organism to survive or endure a given high temperature, but its ability to carry out its complete life cycle at this temperature. Thus, data on the survival of higher plants or animals at temperatures above 50°C (31, 56, 67) are not relevant. It should also be obvious that measurement of the temperature of the orga­ nism, rather than its environment, is essential. The body temperatures of the



TABLE 2. Approximate U1)per temperature limits for various taxonomic groups'

Annu. Rev. Ecol. Syst. 1970.1:191-220. Downloaded from www.annualreviews.org Access provided by CONRICYT EBVC and Econ Trial on 09/24/15. For personal use only.

Group Eucaryotic organisms Animals (including protozoa) Vascular plants Mosses Fungi Algae

Te mperature

",,50°C ""4S-S0°C ",50°C 55-60°C 55-60°C

Procaryotic organisms Blue-green algae Bacteria •

Data from Brock (13) and from unpublished observations

by Brock.

small aquatic organisms found in hot springs are the same as the water in which they are immersed. In terrestrial high temperature habitats such as deserts, the physiological and behavioral activities of the organism may per­ mit it to achieve a temperature lower than its surroundings. The limits given are not for all members of a group but only for certain members. For instance, there are many species of bacteria which are very heat sensitive, even though we know of species of bacteria with optimum temperatures from 25° C to about 90°C. The most interesting aspect of the data of Table 2 i s that the structurally complex organisms are eliminated before the struc­ turally simple ones. I h ave discussed elsewhere (13, 17) the evolutionary and molecular significance of this observation. When should a temperature be considered extreme? As I have pointed out ( 1 7 ) , an environmental extreme must be defined taxonomically by ex­ amining the whole assemblage of species, microbial and multicellular, living

under various conditions. When this is done for temperature, it is observed that species diversity is lower at high temperatures than at low, and at the very highest temperatures only single species may be found. Thus, when the species diversity is low, the temperature is defined as extreme. In this way, extreme temperatures can be defined not only for living organisms as a whole, but for different taxonomic groups such as vertebrates, invertebrates, angiosperms, and blue-green algae. The species found at the highest temper­ atures are not necessarily struggling to survive, but may actually be opti­ mally adapted to their environments ( 14 ) . The relationship between species diversity and temperature for a single taxonomic group can best be illustrated by data of Brues ( 29 ) on water bee­ tles ( Table 3). A similar relationship shifted about 20 degrees higher can be derived from Copeland's data (39) ( see also 66) on blue-green algae, al­ though the taxonomy of this group is such (16, 102) that the significance o f this curve i s not as great as the one for beetles. One can draw the conclusion,



TABLE 3. Number of species of water beetles collected in hot springs at diHerent temperatures'

Annu. Rev. Ecol. Syst. 1970.1:191-220. Downloaded from www.annualreviews.org Access provided by CONRICYT EBVC and Econ Trial on 09/24/15. For personal use only.


30°C 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 •

Data of

Number of species 60 58 55 52 47 46 45 35 33 33 22 15 10 6 4 2

Brues (29).

however, that a temperature which would be considered extreme for beetles, and at which only a few of the species of beetles are present, can be consid­ ered nonextreme for blue-green algae. Is THERE AN UPPER TEMPERATURE FOR LIFE?

This is a question of considerable fundamental importance which was first posed by Cohn ( 36 ) and amplified by Hoppe-Seyler ( 58) . I have exam­ ined the literature and discussions of this question ( 13 ) and showed that the upper temperature for life has not yet been defined. Bacteria have been re­ ported living at the boiling point in many alkaline hot springs, and recently Batt and I ( 11 ) have measured in situ growth rates in typical springs at temperatures of 90°C or over and have found surprisingly rapid growth rates, with doubling times of 2-7 hours. In unpublished studies in Boulder Spring ( 90-92°C ) at Yellowstone we have shown by isotope methods ( B rock, Bott & Brock, unpublished ) that the temperature optimum of the organisms found there is 88-90°C. HIGH TEMPERATURE, PH, AND SALINITY

Although virtually every boiling hot spring of neutral or alkaline pH has been found to have extensive bacterial growth, in acidic springs this is not always the case. As Gary Darland's recent work in my laboratory has shown, there is a definite effect of pH on the upper temperature at which

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bacteria are found. At pH values below 4 there is a progressive lowering of the upper temperature limit from 90°C, and at pH 2 the upper limit is about 70°e. By following the growth rate in the thermal gradient of a single acidic spring at pH 2 ( Roaring Mountain), Darland showed that the growth rate of bacteria living at temperatures below 70°C is relatively rapid, and thus demonstrated that acidity itself is not inhibitory to bacterial growth. The upper temperature for algal growth is also lower in acidic waters. Although algae grow at temperatures of up to 73-75°C in neutral or alka­ line waters, in acidic waters the upper temperature for algal growth is about 55°C ( Doeme! & Brock, submitted). These observations suggest that the combined effects of low pH and high temperature are more than evolution­ ary processes can overcome. A knowledge of the relationship between temperature and salinity would also be of interest but no systematic studies have been made. Unfortunately, most hot springs are of rather similar salinities ( around 1000-2000 ppm total dissolved solids), and hypersaline springs are often not very hot ( 75 ). There is one sea water hot spring in Iceland ( 7), near Cape Reykjanes. When I visited this spr i ng in 1965, it had an excellent thermal gradient from over 75°C to less than 40°C, but I could find no evidence of algal growth. Although a conductivity measurement on water from this spring

showed that it did indeed have a salinity similar to sea water, the pH was 5.3, much lower than that of sea water. This suggested that the sea water had undergone alteration underground. Many waters from deep wells in geothermal regions are hypersaline ( 101), but biological studies of free flow­ ing wells have not been done. Studies of the biological significance of other chemical factors of thermal waters, such as radioactivity, ,fluoride, boron, silica, heavy metals, H 2 S and CH4, have not been done but would be of great interest. GENETIC ANn PHYSIOLOGICAL ADAPTATION TO THE THERMAL ENVIRONMENT

Studies using radioactive isotope techniques have shown that both the blue-green algae and the bacteria found at different temperatures along a thermal gradient created by the effluents of hot springs are optimally adapted to the temperatures at which they are found ( 14, 24). Whether such adaptation is genetic or physiological can be determined only by cul­ tural studies. Castenholz and his students ( 34, 80) have done the only sig­ nificant work on this problem with hot spring organisms. In the case of Sy­ nechococcus, the blue-gn:en alga which is the most common photosynthetic organism in many western North American springs at temperatures of 5075° C, the data show clearly that temperature optimum is genetically deter­ mined. Peary & Castenholz ( 80) found at least four distinct temperature strains with optima at about 45, 50, 55, and 65 ° C. Interestingly, the tempera­ ture strains show inherent differences in growth rates even at their optima. The high temperature strain grows most slowly ( about 2 doublings per

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day). This suggests that the organism has had to discard its ability to grow rapidly in order to be able to grow at all at a higher temperature. Of course, in nature the organism can maintain itself at high temperatures, even if it grows slowly, as long as faster-growing organisms are not available to re­ place it. A series of growth rate measurements on the same organism have also been made in the field by means of a newly devised technique (23). Doubling times of about 1 2 hours ( 2 doublings per day) were measured at temperatures of 70-72°C; such rates arc similar to those of the laboratory cultures. A number of attempts, with somewhat controversial results, have been made to acclimatize organisms to temperatures other than their optima. The early literature in this field was reviewed by Davenport & Castle (41 ) and some of the later literature by Farrell & Rose (46). Davenport & Castle (41) accept as valid the 1 887 report by Dallinger that he was able to induce a flagellated protozoan, which normally would not grow above 23°C, to grow at 70°C by gradually increasing the temperature during culture over a 2-year period. I personally would like to see this experiment repeated in a number of different laboratories before accepting it as valid. As Farrell & Rose ( 46) note, attempts to adapt most bacteria to higher temperatures have failed, but amongst bacteria of the genus Bacillus, adaptation to higher temperature has been possible (2). Recent studies with genetically marked strains (42) have shown that this adaptation is not genetic but physiologi­ cal : The ability of the bacterium for continued growth at high temperature is quickly lost upon transfer to the lower temperature. Bacillus subtilis, the organism used in the studies of Dowben & Weidenmiiller ( 42), could grow without adaptation at 53-55°C but could be induced to grow at temperatures as high as 72°C by slow adaptation. Care was taken to ensure that the orga­ nism growing at these higher temperatures was indeed derived from the parent and was not a contaminant. ( Thermophilic Bacillus strains are com­ mon in normal soil and can probably be isolated from the air, thus it was important to rule out contamination.) The molecular mechanism of this physiological adaptation should be of considerable interest. Selection of organisms with temperature optima lower than the parent has also been attempted. It is well established that mere growth at low tem­ perature does not select for organisms with low temperature optima ( 46). The difference in effects of low and high temperature on selection is under­ standable when it is realized that low temperature is not a lethal factor to an organism in the same way high temperature is and hence will not immediately eliminate the parent organism. Temperature-sensitive mutants unable to grow at the high temperature at which the parent can grow are easily isolated and have been the subject of widespread genetic and bio­ chemical analysis. In many of these mutants only a single enzyme has been altered to a form which is heat labile, and since the enzyme is inactive at the high temperature, the organism cannot grow. Such mutants may behave normally at the temperature optimum for the parent, but usually they grow


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more slowly, probably because the enzyme is slowly inactivated even at the optimum temperature. The effect of temperature on the growth rates, physiology, and biochem­ istry of organisms has received wide study both in microorganisms (46, 47, 6 1 ) and in higher organisms (67, 83, 94 ) . Although the concepts derived from this work are relevant to an understanding of high temperature sys­ tems, space does not permit even a cursory treatment of this vast body of knowledge. Although organisms from moderate to high temperature environments usually have temperature optima similar to their environmental tempera­ tures ( 3, 1 4 ) , this is not necessarily true for organisms from low tempera­ ture habitats. Many organisms isolated from perpetually cold environments have temperature optima of 15-25°C (46, 63, 64), suggesting that evolution of organisms optimally adapted to low temperatures has not generally been possible. It might be suggested that metabolism is limited at low temperature by the physical process of diffusion, which even the most perfectly adapted organism cannot overcome. However, there are a few organisms which have their optima at temperatures less than lOoC ( 30, 46) . The fact that organisms living in hot spring gradients have temperature optima close to their environmental temperatures may reflect the general stability of hot spring temperatures. It is dangerous for an organism in an unstable thermal environment to have its temperature optimum exactly at its environmental temperature since the lethal temperature is always only a few degrees higher than its optimum and a random excursion by the habitat to temperatures above the optimum would lead to death. Thus in unstable habi­ tats organisms would be advised to establish themselves at temperatures considerably below their optima, and of course at such suboptimum tempera­ tures growth rate and productivity will be less than optimum. In thermal springs, where seasonal and diurnal fluctuations in temperature are minimal, organisms can become established at their optimum temperatures, as our di­ rect field measurements ( 14 ) have shown to be the case. TEMPERATURE ADAPTATION OF COMMUNITIES

It seems likely that the adaptation of communities to altered temperature regimes occurs primarily by changes in the species composition rather than through physiOlogical or genetic changes in the species already present. Thus because of the slow rate of either genetic or physiological adaptation ( see above ) , it seems more likely that when temperature is altered, existing organisms will be killed or cease to grow and will be replaced by colonizing organisms with optima matching the new thermal regime. The rate of com­ munity adaptation will then probably depend on the distance the source of inoculum is from the habitat, but measurements of rate of colonization have not been done. Such measurements arc especially important if we are to pre­ dict the consequences of thermal pollution. One experimental study of the adaptation of heterotrophic microecosys,

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tems to different temperatures has been reported. S. D. Allen in my labora­ tory ( 3 ) set up heterotrophic microecosystems at temperatures ranging from 2 to 75°C. He used material derived from soil, water, sewage, and hot springs as inocula. Despite extensive cross mixing of material from systems developed at various temperatures, each system developed its own character­ istic microflora. Since conditions other than temperature were the same in all systems, this experiment showed that temperature alone is responsible for selecting the kinds of organisms which develop and that mixed popula­ tions which are optimally adapted to a wide range of temperatures can be developed. ECOLOGY OF HOT SPRINGS THE HOT SPRING ECOSYSTEM

It seems to me that hot springs make admirable model ecosystems for the investigation of certain fundamental problems of ecology. Just as a simple organism such as Escherichia coli was used as a model for working out fun­ damental aspects of molecular biology, so might a simple ecosystem make possible the study of fundamental and generalizable aspects of ecology. In­ deed, there is a precedent in using springs as models in the pioneering work of Odum ( 78) on the trophodynamics of Silver Springs, Florida. [There are those who prefer to model complex ecosystems with computers. This is un­ doubtedly a valuable approach, but it becomes unsatisfying when one wants to check the computer model against reality and is then faced with really studying the complex system one has been avoiding. The hot spring ecosys­ tem can also be modeled with a computer, but in this case the results can be quickly tested in the field ( R. Wiegert, personal communication) .J Hot springs have a number of attributes which make them of value. Constancy of temperature is one of the most obvious. Water temperature does not alter with insolation so that effects of the two factors tempera­ ture and light can be studied separately. Flow rate and chemical characteris­ tics also remain relatively constant, in contrast to the situation in most aquatic habitats. ( However, as not all hot springs exhibit constancy, it is important to verify this in preliminary studies before initiating detailed investigations. ) Almost as important as constancy is the fact that hot springs are very easy to sample. When one considers the great expense involved in sampling marine habitats, or even most fresh water habitats, the ease and cheapness of quantitatively sampling the biota of a hot spring or its effluent can be appre­ ciated. Especially the ability to repetitively sample the same location is of considerable value. Sampling equipment consists of no more than cork bor­ ers, forceps, and glass vials or bottles. For adult hot spring insects, a simple vacuum system permits sampling, and counting of animals on the algal mats can be done photographically. As the water cools in the effluent channel draining a flowing hot spring, a thermal gradient is set up. It permits a study of the biota which become

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established at different temperatures under conditions where other environ­ mental characteristics remain constant. This is of considerable importance for studies on evolutionary success, species diversity, and trophodynamics as a function of a single environmental factor. It is of considerable value that at higher temperatures species diversity is low, and at the greatest extremes often only a single species may be pres­ ent. Thus,species identifieation, one of the greatest problems of the ecolo­ gist, is simplified. This is ,especially fortunate in hot spring studies since the taxonomy of the microbial groups which are dominant in hot spring habitats is rather poorly understood. Another facet of species limitation by high tem­ perature is that whole taxonomic groups ( e.g. animals) or whole levels in food chains ( e.g. herbivores ) may be completely absent, making it possible to analyze some steps without interference from others. When the taxo­ nomic complexity of even simple cold water springs is considered ( 78,92 ) , the advantage of hot springs can b e greatly appreciated. The constancy and antiquity of many springs is noteworthy. Many Euro­ pean springs existing today were known to the Romans and Greeks. We can identify from the descriptions some of the Yellowstone springs studied by Peale in 1878 ( 79) and find that the temperatures are the same today. How­ ever, although constancy is the rule, change also occurs as a result of

earthquakes, volcanic eruptions, or unknown causes. For instance, Mush­ room Spring, which we studied extensively for 4 years and which had ap­ parently not changed significantly for at least 90 years, suddenly quit flow­ ing around January 1969, as a result of the development in its vicinity of a new geyser ( Brock & Doemel, unpublished data ) . However, in terms of the life spans of the organisms present, even 90 years can be considered long lived. As habitats for colonization, hot springs are analogous to islands: They are usually surrounded by vast areas in which the hot spring organisms can­ not develop. MANIPULATION OF THE HABITAT

One of the great virtues of the spring habitat is the ease with which it can be manipulated. In many cases the hot spring effluent is confined to a well-defined channel, but if a more reproducible habitat is desired, artificial channels can be constructed. We have used wooden channels about 6-8 feet long for a number of years in our Yellowstone work. Steady state condi­ tions are reached in a few months. Studies can be made of the successional aspects of the approach to steady state ( 1 9 ) , or the population in the steady state can be manipulated.'iVe have used opaque shades (23) and neutral den­ sity filters ( 25 ) to evaluate various aspects of the response of the organisms to light. Flow rate can be modified by construction of dams or weirs. R. G. Wiegert (personal communication ) has constructed large channels 4 feet wide by 8 feet long for use in his studies on the animal communities of hot springs. To control flow rate he has conducted the water into a plastic pipe

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to which a valve is attached. In one study, water from a single spring was split at the source and allowed to run down two sets of channels, one set receiving water at 30 liters/min and the other at 60 liters/min. Wiegert also devised a simple device for cooling water without exposing it to the air, so that temperature could be reduced without concomitant loss of CO 2 or in­ crease in O2, For studies on gas exchange, a split channel, one half of which is maintained free of organisms, can be used to measure gas diffusion rates. These engineering jobs, simple and fairly inexpensive to complete in hot springs, would be complicated and expensive to undertake in most other aquatic habitats. They have the additional virtue of making possible experi­ mentation from which may be drawn firmer conclusions than from mere ob­ servational study. CHEMISTRY OF HOT SPRING WATERS

Chemically, hot springs fall into several quite distinct groups. The wa­ ters of many hot springs of neutral or alkaline pH consist essentially of di­ lute sodium chloride-bicarbonate solutions with small amounts of trace ele­ ments and biologically important anions. The pH of these springs is con­ trolled by a bicarbonate buffer system and varies with the partial pressure of CO2 gas. Often such springs may be weakly acidic at the source (pH 66.5 ) , and the pH will rise quickly to values greater than 8 in the effluent channel as CO2 diffuses into the air. Another major group of hot springs consists of dilute solutions of sulfuric acid with pH values in the range o f 2-4. These springs are often high i n metal ions, ammonium ion, and phos­ phate. A few hot springs are mixed chloride-sulfate waters, usually with pH between 4 and 6, and represent a transition between the sulfuric acid and the chloride-bicarbonate systems. One rather interesting group of springs, called travertine springs, are rich in CO 2 and supersaturated with CaCOa and produce extensive deposits of travertine as CO2 diffuses out of the sys­ tem. The presence of minor chemical constituents such as H2 S, CH4, F, B, As, and Hg may be of considerable significance either in promoting or in­ hibiting biological development. There is marked variation in the presence and concentration of such constituents from spring to spring, but the biolog­ ical significance of such variation has not been investigated. Of the key algal nutrients, phosphate is often unusually high in hot spring waters ( 1-3 ppm is not uncommon ) whereas nitrate and ammonium may be high, low, or virtually absent. Although the salinity (or total ion concentration) of most hot spring waters is not especially high ( 1 ppt ) , some springs have consid­ erably higher salinity. These include a sea water ( 35 ppt ) hot spring in Ice­ land (7) , Warm Mineral Springs ( formerly Warm Salt Springs ) , Florida, at about 17 ppt ( 48 ) , and one Utah warm spring at 50 ppt ( 75 ) . The cold springs of Salsomaggiore, Italy, have total dissolved solids of 180 ppt ( 93 ) . To date, most work has been done comparing the biota along the thermal gradient of single spring systems, but wide-scale comparisons of springs of different chemical properties should be of considerable interest. The pH



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values which we found in an extensive but by no means exhaustive study of Yellowst one hot springs ranged from 1.3 to 9.5. Castenholz ( 34 ) has recently reviewed the chemistry of hot springs as related to effects on thermophilic algae. In recent years, a number of studies concerning primary productivity, nutrient cycling, trophodynamics, succession, and related questions have been undertaken in hot spring ecosystems, and the results so far available suggest that these systems have great promise for future work. PRIMARY PRO:OUCTIVITY OF HOT SPRING ECOSYSTEMS

In principle, any method used for the study of primary productivity of ordinary aquatic environments could be used with hot springs. In practice, certain methods have proved difficult to apply due to certain peculiarities of the hot spring habitat. Oxygen change by chemical methods has been mea­ sured (66) and oxygen electrodes have been used in warm springs (43, 90), but oxygen electrodes may not function properly at really high tempera­ tures. Since hot spring waters are virtually devoid of oxygen as they issue from the ground, large corrections need to be made for diffusion of oxygen into the water from the air. Stockner ( 90 ) , using an algal-free channel to correct for diffusion, attempted to overcome this problem by studying algal productivity on an artificial channel. The in situ pH method for study of primary productivity (8) can appar­ ently not be used in most hot springs ( P. Fraleigh & R. G. Wiegert, per­ sonal communication ) . Hot spring waters in the neutral to alkaline range are oversaturated with CO2 and lose this rapidly as they flow from the source. This results in a rise in pH which generally masks pH changes due to photosynthesis. The 14C method, on the other hand, can be used (22,68), and we have used it ext-75°C), standing crop is low; it rises to a peak in the temperature range of 55-60° C and then falls sharply, and at temperatures below 40° C the algal mats are very reduced in size ( 15, 20) . (The temperatures stated apply to the Yellowstone hot springs ; in Iceland the values are shifted to a lower temperature. ) One does not even need quantitative measurements to observe this phenomenon since the relationship is so dramatic that it can be seen. Indeed, it is this marked algal development in the 55-60° C range which makes the Yellowstone algal mats so dramatic and attractive that even the early explorers noticed them ( reviewed by Brock 18 ) . Although standing crop is highest around 55-60°C,primary productivity, measured either by the uC method ( 1 5 ) or the washout method (Brock & Brock 23, and unpublished), is higher at temperatures below and above this value. One possible explanation for this discrepancy is that when the mats are developed to their fullest extent, they are so thick and compact that ex­ change of nutrients and gases in and out of the mat may be inhibited. At higher temperatures, where the mats are thin, nutrient and gas exchange should be good. At temperatures between 40 and 50° C, although the mats are fairly thick, they are kept open and porous by the action of animal gra­ zers ( see below ) .

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Why is there a maximum standing crop at 55-60°C? We really need to turn thi s question around and ask why standing crop is less at higher and lower temperatures. At the highest temperatures it seems possible that in­ herent thermal limitations on photosynthetic life enter in, although our growth rate data from the washout method have shown that growth rate is faster at temperatures near the upper limit ( 70°C) than at lower tempera­ tures. ( Although growth rate is faster, productivity is lower because the standing crop is lower. ) Much more work needs to be done on algal growth at these high temperatures. We have suggested elsewhere ( 12, 1 5 ) that, at temperatures below the 5560° C optimum, standing crop drops because the lower temperature permits the extensive development of animal grazers. In Yellowstone these grazers are larvae and adult ephydrid flies ( 12 ) , whereas in eastern Oregon hot springs they are ostracods ( 33 ) . Another explanation of decreased standing crop at low temperature, suggested by the data of Armitage ( 6 ) and Wright & Mills ( 106) and currently being tested by R. G. Wiegert, is that at the lower temperatures growth is limited by CO2 deficiency. Many of the spring waters are high in CO2 as they issue from ground but lose this fairly rapidly due to diffusion into the air, so the pH of spring water can rise from 6.5 to 8.5. Another possibility, suggested but not tested by Stockner ( 90 ) , i s that algal mats produce antibiotic materials which affect the growth of the algae downstream. This hypothesis could be tested by completely removing the algal mat upstream from a low temperature site and observing the behavior of the low temperature mat. ANIMAL GRAZERS IN HOT SPRINGS

The upper temperature for animal life is about 50° C. In certain hot springs, ostracods are found in large numbers at temperatures around 47°C ( 33 ) , and the ephydrid flies at Yellowstone are extremely common at tem­ peratures of 30-40°C. Above 50°C only algae and bacteria are present, and we thus have a truncated food pyramid ( 1 5 ) . The absence of grazers at the higher temperatures may have important consequences in terms of both en­ ergy :flow and nutrient cycling ( 12) . Two general types of effects of grazers can be envisaged: 1. biological and biochemical effects of the animals on the mats by way of feeding and excretion processes, 2. mechanical effects of the animals on the mats. Mechanical effects are readily observed in the hot springs. At temperatures above 50°C, where animals are absent, the algal mats are even, compact, and clearly stratified into layers; gas and nutrient exchange is probably low. At temperatures around 40°C, where animals are plentiful, the mats are rough and corrugated, and much less compact ; gases and nutrients probably exchange well. The hot spring animals may also affect the species distribution and growth form of hot spring algae. Castenholz ( 33) has some evidence that the lowest temperature at which Oscillatoria terebriformis is found is con­ trolled by the grazing activities of an ostracod ; when the ostracod is abs€nt,

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the alga can grow in nature at lower temperatures. He has also shown ( per­ sonal communication ) that the development of nodules consisting of the algae Pleurocapsa and Calothrix is determined by the feeding activities of the ostracod. These two algae cannot compete normally with Oscillatoria terebriformis but can do so when the latter is eliminated by ostracod feed­ ing. The nodular growth form of Pleurocapsa and Calothri:r makes them re­ sistant to ostracod feeding'. The hot spring mats in the animal-free zone develop thick, clearly strati­ fied layers. The surface component consists of algae embedded in a matrix of filamentous bacteria, and only filamentous bacteria, which produce exten­ sive, brilliantly orange-colored mats (16, 19), are present beneath this layer. Bacterial layers of 2-4 cm are common. These mats take months or years to form (27) . We have shown ( Brock 19, and unpublished ) that the nutrients for the bacteria are derived from the algae. It appears as if some of the energy and nutrients produced by the algae are assimilated by bacteria and are only slowly released. When adult or larval ephydrid flies are added, the mats are rapidly minerali:�ed ( 12) , which suggests that animals promote the mineralization process. The hot spring mats can hence be viewed as inter­ esting systems for studying the role of animals in nutrient cycling. SUCCESSION

Succession can be studied admirably in hot springs. The rapidity with which colonization and establishment of new mats occms (27, 99) means that results are obtained quickly, and the use of artificial channels makes it possible to time events. In one study of succession in a Synechococcus mat at 58°C ( 19 ) , the unicellular alga developed only after a layer of orange, filamentous bacteria was first laid down. I have suggested that the bacteria formed a matrix within which the algal cells could become trapped, thus making it possible for them to maintain themselves in the rapidly flowing water. DISPERSAL

The island nature of hot springs makes them excellent habitats for studying microbial dispersal, but they have not as yet been studied experi­ mentally in this way. W'e do know that within a given thermal area ( e.g. Yellowstone ) dispersal from one spring to another is rapid, since new springs are rapidly colonized (27 ) . Dispersal may be by air or via ani­ mals. Ephydrid flies excrete large numbers of viable microbial cells and must be important dispersal agents. Buffalo and elk frequently walk across quite hot water and can transport microorganisms on their feet. Although air dispersal may be common, no data are available. The hot spring algae are not especially resistant to drying ( Castenholz, personal communication ) . I was unable to culture algae from dried algal mats taken from the channel of the now defunct Mushroom Spring about 6 months after it stopped flow­ ing, although mats in the same channel which had remained moist from seepage yielded viable cultures.



The colonization of isolated hot springs may be slow. The thermal flora of the hot springs of the Azores is quite impoverished (21 ) , and Castenholz (35 ) has suggested that the thermophilic alga Synechococcus may be absent from Iceland because there was insufficient time for colonization after the glaciers retreated from the thermal regions.

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In contrast to those of other environments, the temperatures of springs do not vary throughout the year with insolation. Thus, seasonal changes of the biota of springs may be directly attributed to effects of light intensity and duration, and such effects of light can be more easily studied in springs than in habitats where temperature also varies. In thermal springs an addi­ tional factor may be operative : high respiratory rate at high temperatures may mean that the photosynthetic compensation point is not exceeded at all during the short days of winter. This is apparently the case in Iceland, where Tuxen (95) has calculated that because of the high latitude ( 64°S'N at Reykjavik) an alga receives 1 /5000 the light energy on the shortest day as on the longest. Schwabe ( S7 ) has stated that in Iceland : "In den licht­ armsten Monaten ( November-Januar) kommt das Leben in Thermalbioto­ pen nach meinen Beobachtungen . . . zum Stillstand." According to Schwabe, algal mats virtually disappear in winter. We found that an Icelan­ dic hot spring which had an extensive algal mat in August (20 ) had a much reduced standing crop early the following May, when day length was already quite long. This suggested that the community had not yet had time to recover from the previous winter. At lower latitudes hot spring algae can maintain themselves throughout the winter. At about 45° N latitude available light is reduced only about lO-fold [from 500 to 50 caljcm2 min (90 ) ], in contrast to the 5000-fold re­ duction at Iceland. Although Stockner reported a considerably reduced algal growth rate in the winter at Ohanapecosh Hot Springs ( 46° 44'N ) our winter observations at Yellowstone (44° 30' in the main geyser basins) showed no great difference in standing crop between summer and winter (25 ) . At least one reason why the Yellowstone populations do not change markedly during reduced light in the winter is that they increase signifi­ cantly their pigment content, as shown in Table 4. In summer green pigment concentrations are low, and the mats appear yellow or orange. In winter, or in summer when covered with neutral density glass ( 25 ) , the green pigment concentrations go up and the mats appear dark green. An increase in green pigment in summer can be observed visually within hours or days after neu­ tral density filters are installed ( 25 ) . In addition to changes in light intensity, an additional winter change at Yellowstone, which affects the ephydrid flies but not the microorganisms, is the long period of cold weather. The ephydrid fly populations, especially of the species Ephydra bruesii, are actually much larger in winter than in sum­ mer. There are apparently several reasons for this. We have shown ( B rock & Brock, unpublished ) that the relative humidity just above the algal mat



TABLE 4. Quantitative chlorophyll assays at selected stations of

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Mushroom, Spring, Yellowstone National Park"

Stationb (in order of decreasing temperature)

Summer average

Winter average


26 1 87 687 315 503

417 507 1153 583 710


Values expressed as J'g chlorophyll/ms. location of stations.

b See Brock (15) for

surface is much higher in cold than in warm weather, and the flies congre­ gate and lay their eggs in such humid environments. Additionally, in sum­ mer the terrestrial habitats surrounding the springs are warm, and the flies disperse from the mats onto the soil, although they return to the mats at night ( N. Collins, personal communication ) . In winter the flies are re­ stricted to the mats, since if they leave the mats, they freeze. At the edges of the hot spring channels, where snow does not accumu­ late, the vascular plant Mimulus guttatus ( Scrophulariaceae ) grows through­ out the winter as a low rosette form, spreading by runners. When longer and warmer days return in the spring, this plant converts to an upright form and it flowers. The onset of flowering occurs sometime in April and extends through the summer. In habitats away from the thermal basins, both in Yellowstone and in other areas of the West, the same plant flowers in July and August. ECOLOGY OF ACID HOT SPRINGS

Most of the previous discussion has centered around the ecology of neu­ tral and alkaline springs, where most investigation has been done. In very acid hot springs, the biota is quite different, as are some of the ecological r elationships. As noted earlier, the upper temperature for algal growth in acid springs is about 55°C, which is a lower temperature than that found in other springs. The alga living at this temperature, Cyanidium caldarium, is the sole photosynthetic component until the temperature drops below 40° C, at which temperature the algae ChJamydomonas sp. and Euglena mutabilis ap­ pear. Euglena mutabilis i s also characteristic o f nonthermaI acidic waters, such as acid mine drainages. The virtual absence of Cyanidium at temper­ atures below 35 °C is p robably due to its inability to compete with these other algae, since it will grow, :although slowly, at temperatures as low as 20° C in unialgal culture. W. Doemel in my laboratory has shown clearly that the tem­ perature optimum of this organism, both in nature and in laboratory culture, is 45°C and that its maximum growth temperature under both conditions i s

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about 55°C. One of the interesting findings i s that strains of C. caldarium optimally adapted to temperatures from 35°-55 °C do not exist, although, as noted earlier, temperature strains have been found for SynechococcusJ the dominant alga of alkaline springs. The pH range in which C. caldarium lives is from 0-5, although it grows only slowly at pH 5. The pH optimum is fairly broad, from 2-4. In nature this organism lives not only in acid springs but in warm acid soils. The latter are quite common in thermal basins, and the alga is usually found just below the surface, where moisture is appar­ ently available and where sufficient light is still obtained. Although Cyanidium is sometimes found in association with bacteria and fungi, in many habitats it is found as a virtually pure culture. One cannot really speak of a food chain or an ecosystem in such situations. Because the cells are relatively dense and the water flow rates are not especially rapid, the organism builds up mats with quite high population densities. Cells in the lower portions of these mats become brown and are apparently dead. Some exporting of C. caldarium cells does occur continually, as visual ob­ servations have shown, but it probably increases greatly when the flow rate of the effluent suddenly rises as a result of heavy rains. At temperatures below 40°C, chironomid larvae are frequently seen in the Cyanidium deposits, but no work has been done on their effect on the alga. Cyanidium caldarium seems not to thrive best in full sunlight ; it has de­ veloped better at Nymph Creek, which is heavily shaded by surrounding trees, than it has at the other Yellowstone sp rings. Doemel has shown that if the light available to populations living in full sunlight is reduced by use o f neutral density filters, the population density increases. I n its habitat in soil, it probably receives light of considerably reduced intensity. Another alga common in acidic habitats is a species of Zygogonium ( 71 ) which forms extensive dark purple mats on moist acid soils. However, Zygogonium is not found at temperatures much above 30° C and hence could hardly be called a thermal alga. As we have shown (71 ) , the extensive mats of this alga provide the base for the development of an ecosystem which includes other algae (Euglena mutabilis and Chlamydomonas sp. ) , ephydrid flies (Ephydra thermophila, not E. bruesi) and killdeer (Charadrius voci­ ferus). Work on the feeding relationships of the insects of these Zyg.ogonium mats is now under way by Nicholas Collins in R . G. Wiegert's laboratory. APPLIED ECOLOGY OF mGH TEMPERATURE ECOSYSTEMS

The principles developed in work on hot springs may have wide applica­ tion in applied work. To date, the study of practical high temperature sys­ tems has been mostly empirical, but I hope that theoretical input will soon be available. I would like to consider here only two areas, sewage treatment and thermal pollution. ORGANIC WASTE TREATMENT

Although the effect of temperature on the decomposition rate of organic matter in sewage treatment processes has been frequently studied, these

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studies have often been made without reference to whether the organisms were optimally adapted to the temperatures employed. As we mentioned ear­ lier, at low temperatures organisms are often not optimally adapted, and no matter what temperature they come from, they show optima around 2530°C. It is thus reasonable that in the temperature range from 0-35 ° C the decomposition rate of organic matter follow the Arrhenius relationship (81, 1 07, 108 ) . If temperatures higher than 30°C are used, decomposition is not necessarily faster unless the population is adapted to the temperatures. In some studies on the thermophilic treatment of organic wastes (51, 70) no attempt has been made to adapt the populations first although in two studies, one anaerobic and the other aerobic, the populations were adapted. The anaerobic study is perhaps the more interesting because the effect of temperature on the solubility of oxygen is not a factor. Fair & Moore (44, 45 ) showed that the rate of sludge digestion increased as temperatures rose to 35-40°C ; after a plateau the rate increased again as temperatures rose to 5 5-60°C, after which it dropped sharply. Unfortunately, as no inocula from high temperature environments such as hot springs were used, the reduction in rate at temperatures above 60° C may be due to the lack of a suitable pop­ ulation. In Allen's studies ( 3 ) no microecosystems could be developed at temperatures above 55°C until inocula from hot springs were used ; then mi­ croecosystems adapted to 60-75°C were obtained. Thus, the suggestion of Fair & Moore ( 45 ) that there m