GENERAL I ARTICLE Life at High Temperatures Ramesh Maheshwari Amazing microorganisms thrive at high temperatures in­ compatible with the familiar forms of life. A few species Ramesh Maheshwari is at from hot springs or from the vicinity of hydrothermal vents the Department of Biochemistry, lISe, at the floor of the oceans are the basis of a multimillion Bangalore. His research dollar industry. An entirely new form ofanimal life has been interests have included discovered around the volcanic eruptions from the sea thermophilic organisms. floor. This animal life thrives in the permanently dark environment only because it harbors symbiotic bacteria that synthesize food molecules from inorganic chemicals in the emissions. Thermophilic microorganisms are involved in composting and humification in terrestrial habitats. Introduction For most terrestrial organisms, the optimum temperature of growth is around 2S-30°C. Among the few exceptions in the higher forms of life is the Pompeii worm which survives at SO°C or more in the vicinity of geothermal vents in the deep sea and the plant Tidestromia oblongifolia (Amaranthaceae) found in Death Valley in California, where the hottest temperature on earth ever recorded during 43 consecutive days in 1917 was >48 °C (Guinness Book of W orId Records, 1999). Temperature stress is accompanied by water stress and since all organisms need liquid water, life at high temperatures is found only in the terrestrial hot springs or at the bottom of ocean in the vicinity of volcanic eruptions. This life is comprised of unicellular bacteria or organ­ isms resembling bacteria. Since 1960s, new discoveries keep pushing the upper tempera­ ture limit of life higher and higher (Figure 1). Because of the lability of crucial biomolecules, for example adenosine triphos­ phate, ATP - an energy-rich molecule which is the principal Keywords donor of energy in biological systems, and of nicotinamide Temperature, life, thermo­ adenine dinucleotide phosphate, NAD(P)H - a molecule which philes, extremophiles. -RE-S-O-N-A-N-C-E--I-~--Pt-em-~--r--20-0-5----------~-----------------------------~- GENERAL I ARTICLE -150- +--Predicted upper limit for life -140- -130- -120- ...-Upper temp limit for life, 2000 -110- +--upper limit for life, 1990 -100- . +--Upper limit for life, 1970 90- +-- Upper limit for life, 1960 +--Highest temp. Death Valley, California,JuIy 10, 1913;; ~AI Azizia, Libya, September 13,1922 ........... Protoplasmic streaming ceases after 5 min heating in most e Highest temp., Bangalore, 22 May 1931--+ 0- ~ Upper limit for aquatic vertebrates Annual average, Dalol, Denakil Depression, Ethiopia {30-} ~ - +-Topt for most animals, plants and microorganisms Average annual temp. in tropics -20 Average temp. of earth--+ 10-+--T min for E. coli and most plants Figure 1. Thermometer of functions as a coenzyme in several biochemical reactions in the life. The upper temperature cell, the upper temperature limit of life is predicted to be around limit of life has been raised 150°C, prompting researchers to search for the champion higher and higher. Modified from R Maheshwari, 2005, organism. Fungi: Experimental Meth­ Organisms that have an optimum temperature (T )of 45-50 °C ods in Biology. CRC Press, opt Boca Raton, USA. are called thermophiles. The T opt is the temperature at which growth rate is fastest. The so-called 'hyperthermophiles' have a T opt of 90°C or even higher. Thermophiles and hyperthermo­ philes have not only an elevated T max' hut also an elevated minimum temperature of growth (Tmin)' However, the tempera­ ture range in which these organisms grow is similar to that of the mesophiles which grow at 'normal' temperatures. This means that the overall temperature range at which any organism can grow is rather narrow, approximately 30°C. The T opt is always closer to T max than it is to T min' The most heat-enduring are the single-celled organisms which superficially resemble bacteria, 2-4---------------------------------~-----------R-ES-O-N--A-N-C-E--I-s-e-Pt-e-m-b-e-r--2-0-0-S GENERAL I ARTICLE but are now classified in the domain of life called Archaea (previously Archaebacteria). The differences between Archaea and Bacteria are in the chemistry of their membrane lipids, structure of cell walls, structure of ribosomes and growth sensi­ tivity to antibiotics. Lethal Effect of Heat Why is prolonged exposure to temperatures >45 °C generally lethal? The integrity of the cell and of the cellular compartments known as organelles depends on the structure of membranes which is a lipid-based sheet. Macromolecular structures depend on the three-dimensional structure of the molecules of which they are composed of. For example, a protein in the cell which performs the task of catalysis of metabolic reactions or the transport of molecules has folded polypeptide chains called the alpha-helix. The alpha-helix is held in the helical configuration by hydrogen bonds between the CO groups and the NH groups four amino acids apart. DNA, the carrier of genetic information, has the form of a double helix that is held together by hydrogen bonds between the base pairs. Nucleic acid and protein interact to form nucleosomes which are joined into a flexible chain. Hydrogen bond is a weak bond, broken by heat. The three­ dimensional structure of macromolecules is therefore altered by high temperatures resulting in the loss of functional properties. We may ask why a macromolecular structure is not based on strong covalent bonds. The answer is that cellular function is dependent on the flexibility of a macromolecule. For example, an enzyme (protein) molecule must be flexible for folding into a shape into which its substrate can fit in precise orientation. Denaturation of proteins and melting of the membranes are two major reasons why 45°C is a sort of temperature limit for the majority of living beings. Life in Boiling Water It was a great surprise to discover that a variety of single-celled organisms, with diameter close to or less than a micron, actually -RE-S-O-N-A-N-C-E--I-~--Pt-em-oo--r--20-0-5----------~~-----------------------------~- GENERAL I ARTICLE Figure 2. A boiling pool in Yelllowstone National Park, USA. From [1J, with permis­ sion. require temperatures above 40-45 °C for growth. During 1965 to 1975, the American microbiologist Thomas Brock and his asso­ ciates [1] discovered microbial species thriving in hot springs in the Yellowstone National Park, USA (Figure 2). To determine if the bacteria were actually growing in hot springs or had been disseminated from elsewhere, the investigators immersed mi­ croscope slides in the boiling pool; exposing one side of the slide to germicidal ultraviolet radiation at regular intervals. The rationale being that any organism that had become attached to the slide would be killed and not be able to reproduce. The colony growth on the non-irradiated, but not on the control (irradiated) side (Figure 3), proved that microbial growth was not only occurring at temperatures close to 90°C, it was occurring at rapid rates. The generation time in different hot springs varies from 2 to 6 hours. In Yellowstone, water boils at 92.5°C. This work demonstrated that life can exist at close to the boiling point of water. An Important Generalization in Biology In India, hot springs (·agnikunds·) are found in Brock investigated several types of thermal habitats. One of Kashmir, Himachal Pradesh, these was the self-heating piles of coal-waste in the vicinity of Uttaranchal, GUjarat, w.est Ben­ coal mines. Although the bulk of coal is removed, the refuse gal and in other places, but their always contains coal fragments and other organic material. Brock microbial diversity has not been discovered that this is the habitat of Thermoplasma acidophilum. studied. -26-------------------------------~~------------------------------ RESONANCE I September 2005 GENERAL I ARTICLE So far, the coal-refuse pile is the only habitat where this organ­ Figure 3. (left) Bacterial ism has been found. Thermoplasma has no cell wall, yet can grow growth on microscope slide at temperatures of 61°C, and at a pH close to 1 (the acidity of immersed in a boiling pool. (right) No growth occurred O.1M hydrochloric acid). Its discovery refuted the belief that on the side of the slide cell-wall provides the chief protection against heat. These dis­ which was exposed to ger­ coveries led to an important generalization in biology: No mat­ micidal UV light at intervals. ter how harsh the environment, if there is liquid water, there is From {1J, with permission. life. Or to put it differently, there is no life without liquid water. Amazing Creatures in the Vicinity of Deep-Sea Hydrother­ mal VentslBlack Smokers Although man had conquered the highest mountain peak, the outer space and the moon, the greatest depths of the oceans had remained impenetrable because of the enormous hydrostatic pressure due to the column of water above. Only USA, France and Japan have built submersibles capable of descending to the ocean floor. It carries a pilot and two scientists, and a single dive takes nearly 4 hours. The submersible is fitted with strobe lights, portholes, and robotic arms for collecting samples. Finally, in 1977, deep sea diving to depths of some 1.6 miles (or 2.5 km) at locations along the thick line shown in Figure 4 could be achieved. The line represents mid-oceanic ridges where tectonic plates that form earth's crust separate, creating fissures on the sea floor. Sea water penetrates into the fissures and interacts with the hot, -RE-S-O-N-A-N-C-E--I-~--p~-m-~--r--20-0-5----------~------------------------------u- GENERAL I ARTICLE Figure 4. The line repre­ volcanic crust. The thermally-expanded, mineral-enriched wa­ sents mid-oceanic ridges ter with temperature of 350°C or more rises up and exits from where gigantic plates that chimney-like structures on the ocean floor.