Life at High Temperatures Live at a Slightly Higher Temperature

55°C to the boiling point. As the temperature increases, whole taxonomic groups of organisms are eliminated from the habitat (Table 1) (1, 3). No multicellular animals live at temperatures greater than 50°C, although a few protozoa can Life at High Temperatures live at a slightly higher temperature. Multicellular plants also show upper Thomas D. Brock temperature limits around 50°C. Thus, above 50°C only microorganisms are found. Eukaryotic microorganisms are much more restricted in their distribu- Organisms capable of living at high level. Stetter et al. (3) described a bacte- tion than prokaryotic microorganisms; temperatures, called thermophiles, have rial genus, Pyrodictium, capable of the upper temperature limit for eukary- long fascinated biologists and earth sci- growth up to 1 10°C; isolates were ob- otes seems to be about 60°C (8). Thus, entists. Natural high-temperature envi- tained from a submarine thermal area off above 60°C only prokaryotic organisms ronments are widely distributed on the the coast of Italy. However, discovery of are found. It seems likely that structural earth, being found in association with the deep-sea hydrothermal systems ex- characteristics of eukaryotes, perhaps in volcanic activity. Living bacteria are tended the temperature range available nuclear membrane systems, are incom- patible with thermostability. Not all kinds of prokaryotes are able Summary. Water environments with temperatures up to and above boiling are to grow at temperatures greater than commonly found in association with geothermal activity. At temperatures above 600C, above 60°C. Only a few genera are repre- only bacteria are found. Bacteria with temperature optima over the range 650 to 1 050C sented, and even among these genera we have been obtained in pure culture and are the object of many research projects. The find that only certain species are capable upper temperature limit for life in liquid water has not yet been defined, but is likely to of high-temperature growth. Further- be somewhere between 1100 and 200°C, since amino acids and nucleotides are more, certain kinds of prokaryotes seem destroyed at temperatures over 2000C. Because bacteria capable of growth at high incapable of very high temperature temperatures are found in many phylogenetic groups, it is likely that the ability to grow growth. For instance, the photosynthetic at high temperature had a polyphyletic origin. The macromolecules of these orga- prokaryotes show a well-defined upper on November 14, 2012 nisms are inherently more stable to heat than those of conventional organisms, but temperature limit of 700 to 73°C (9). In only small changes in sequence can lead to increases in thermostability. Because of many parts of the world, photosynthetic their unique properties, thermophilic organisms and their enzymes have many prokaryotes are not found even at tem- potential biotechnological uses, and extensive research on industrial applications is peratures as high as 70°C (10, 11). under way. Although photosynthetic organisms do not live at temperatures greater than 73°C, autotrophic organisms do, but found in most boiling-water environ- for biological colonization from 1 10°C to these are autotrophs capable of using ments, where they often reproduce ex- above 350°C (4). The latter temperature inorganic energy sources such as sulfide, www.sciencemag.org tremely well (1). A question of consider- is clearly too hot for life, as peptide and elemental sulfur, and ferrous iron. In able interest concerns the upper tem- phosphodiester bonds (5) and amino ac- addition to these chemolithotrophs, het- perature limit for life in liquid water ids (6) are destroyed even at 250°C (7). erotrophic bacteria grow rapidly in water environments. Until a few years ago, the Thus the upper temperature limit for life at or near the boiling point. The diversity liquid water environments with the high- is somewhere between 1100 and 250°C. of bacteria living in boiling water is sur- est known naturally occurring tempera- Since undersea hydrothermal vents exist prisingly high: sulfur bacteria, hydrogen- tures were close to sea level, and hence at water levels from near sea level to the oxidizing bacteria, elemental sulfur-re- Downloaded from had boiling points of around 100°C. Over deep ocean, it should be possible to find spiring bacteria, obligate anaerobic het- the past decade, however, remarkable high-temperature vents emitting sterile erotrophs, and aerobic heterotrophs. habitats with temperatures as high as boiling water with thermal gradients in Some of these bacteria are capable of 350°C have been discovered at the bot- the cooling outflow of such vents where switching from an aerobic to an anaero- tom of the oceans, opening up the possi- organisms might first appear. This type bic metabolism. Finally, some are not bility of organisms living at even higher of habitat would then provide a suitable only extremely thermophilic but also aci- temperatures than previously imagined. location for determining the upper tem- dophilic. In 1967 I noted, "Bacteria are able to perature limit for life. In recent years a number of new and grow ... at any temperature at which interesting thermophilic bacteria have there is liquid water, even in pools which been isolated and characterized (Table are above the boiling point" (1, p. 1014). Diversity of Thermophilic Organisms 2). Many of these bacteria are now avail- In the years since, this statement has able from the German Collection of been amply confirmed, and many arti- Three broad classes of organisms have Microorganisms. The organisms can be cles on organisms living at or near the been recognized on the basis of tempera- cultured readily, even in large-scale fer- sea-level boiling 'point of water have ture optima for growth: psychrophiles, menters, using well-defined culture me- been published (2). capable of growing at low temperatures; dia. To extend such observations to higher mesophiles, growing in the temperature it was Thomas D. Brock is E. B. Fred professor of temperatures, necessary to find range 25 to about 45°C; and thermo- natural sciences, Department of Bacteriology, Uni- boiling springs at locations below sea philes, growing at temperatures from versity of Wisconsin, Madison 53706. 132 SCIENCE, VOL. 230 Effect of Temperature on phile, Thermoplasma acidophilum, does stable (13). Supramolecular structures Species Diversity not have a metabolism linked to sulfur. such as ribosomes and membranes are This heterotroph is found in self-heating also thermostable in thermophiles, al- The relation between species diversity coal refuse piles. It can live at moderate- though inactivated by heat in meso- and temperature for several different sin- ly high temperatures (550 to 60°C) and philes. Further, the catalytic activity of gle taxonomic groups is illustrated in low pH even though it lacks a cell wall. thermophilic enzymes is low or absent at Fig. 1. As shown, the population struc- moderate temperatures at which conven- ture becomes progressively simpler with tional enzymes of similar function are increasing temperature. Prokaryotes are Molecular Basis of Thermophily optimally active. The temperature opti- currently divided into two major king- ma of thermophilic enzymes are fre- doms, the Eubacteria and the Archae- In conventional organisms macromol- quently at or above the optima for the bacteria (12). One interesting fact, possi- ecules such as proteins and nucleic acids growth of the organisms. bly of evolutionary significance, is that are inactivated irreversibly by heat, but The most direct approach to determin- at temperatures greater than 90°C, all in thermophiles these components are ing the molecular basis of thermostabili- species capable of reproducing are mem- bers of the Archaebacteria. However, not all Archaebacteria are thermophiles, Table 1. Upper temperature limits for growth (2, 3). and most Archaebacteria live at meso- Approximate upper philic temperatures. Among one Archae- Group temperature (°C) bacteria group, the methanogens, species are known that live at low mesophil- Animals moderate thermophilic, and ther- Fish and other aquatic vertebrates 380 ic, high Insects 450 to 50" mophilic temperatures. Another fact Ostracods (crustaceans) 49" to 50" illustrated in Fig. 1 is that the spore- Plants forming bacteria that are thermophilic Vascular plants 450 have generally lower temperature maxi- Mosses 50" Eukaryotic microorganisms ma than the species that do not form Protozoa 56" spores. The Eubacteria living at the high- Algae 55" to 60" est temperatures (up to 90°C in some Fungi 60" to 62" cases) almost without exception do not Prokaryotic microorganisms on November 14, 2012 form are Blue-green algae (Cyanobacteria) 70" to 73" spores; they Gram-negative and Photosynthetic bacteria 70" to 73" of uncertain taxonomic affiliation. Chemolithotrophic bacteria > 100" The Archaebacteria were character- Heterotrophic bacteria > 100° ized initially from the nucleotide base sequence in the 16S RNA of the ribo- some, but have subsequently been shown to differ from other bacteria in a Table 2. Species and genera of thermophilic bacteria discovered in recent years (25). variety of properties (12). On the basis of Archaebacteria 16S RNA nucleotide sequences a phylo- Aerobic acidophiles, autotrophs www.sciencemag.org genetic scheme for all living organisms Sulfolobus acidocaldarius type species has been constructed The rela- Sulfolobus brierleyi (Fig. 2). Sulfolobus solfataricus tion of Archaebacteria to eukaryotes

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