Antarctica, which may remain active down to about – 12°C.
HOW HOT IS TOO HOT?
To explore the upper extremes of temperature, our spacecraft must leave the Antarctic to travel to the baking deserts. Life is surprisingly abundant in many desert regions. Burrowing mammals survive by engineering air-conditioning systems to keep their tunnels and hence their bodies relatively cool during the day and restrict their hunting to the cool (often very cold) night hours. Their prey – small snakes, reptiles and arthropods – may tolerate temperatures up to 50°C. Temperatures as high as 60°C have been recorded in foraging ants as they race across the hot desert sands of the Sahara. But these are transient temperatures and cannot be tolerated for long by any animal. Plants including cacti and a number of desert grasses can tolerate quite high temperatures, but do not grow above about 45°C. Mosses and lichens may survive and grow at temperatures up to about 50°C. No plant or animal is known to be able to thrive above this temperature. This appears to be the upper limit for multicellular life on our planet.
It has long been known that bacteria are capable of growth at higher temperatures. Thermophilic (heat-loving) bacteria, growing at temperatures as high as 65–70°C, have been isolated from a number of hot habitats. Lichen and other microbes penetrate the surface of desert rocks. Even the sand is inhabited by microbes. The surface of sand drifts is often crusty from the presence of a tangled mesh of photosynthetic microbes and lichen that live in its top millimetre. These microbial mats can be quite productive, with roughly the same density of chlorophyll as a plant leaf, but are limited by the availability of moisture and must await the arrival of dew, fog or a rare shower before they are able to set their photosynthesis machinery into action. Thermophilic microbes can also be found in more mundane environments such as compost heaps, slag heaps (that reach temperatures as high as 60–70°C), and domestic hot-water systems.
It was generally thought that temperatures higher than about 75°C were incompatible with life. This view changed dramatically when, in the late 1960s, Thomas Brock, a microbiologist from the University of Wisconsin, was walking in Yellowstone National Park. The park, famous for its hot volcanic springs, lies within a volcanic crater where rain-water seeping through the surface rocks meets the hot magma below. The superheated water and steam erupt as geysers and springs, feeding hot volcanic pools. These pools are deadly to most plants and animals. The bones of buffalo or elk and even the occasional tourist are occasionally washed up on their shoreline. Yet, in 1964, Brock noticed that the surfaces of many hot springs were covered with a pale-pink gelatinous scum, not dissimilar from the bacterial scum clinging to the inside of bathroom taps. He and his wife Louise returned the following year and isolated algae and bacteria from the hot scum. One of the bacteria, Thermus aquaticus, isolated from a hot volcanic spring called Mushroom Pool, was found to thrive at temperatures as high as 80°C. These hyperthermophilic bacteria are now of considerable interest to industry as a source of heat-stable enzymes.
The most extraordinary hot-water habitat was discovered in 1977 when the geologist John Corliss of Oregon State University and John Edmond of the Massachusetts Institute of Technology boarded the submarine Alvin. The two scientists and a pilot climbed inside a two-metre diameter titanium sphere, built to withstand the massive pressures at the depths of the ocean floor. The vessel was dropped into the Pacific, two hundred and eighty kilometres north of the Galapagos Islands to search for hot springs associated with the mid-oceanic ridges, where the continents were being pushed apart by molten magma welling up from cracks in the earth’s crust. The craft (descending at a leisurely rate of 30 metres a minute) took about ninety minutes to reach the ocean floor, two and a half kilometres below the surface.
The crew stared through Plexiglas portholes to see a bleak terrain of black basaltic rock cut by faults and fissures. For thirty minutes they surveyed this monotonous sterile landscape seeing nothing unusual until a pair of large purple sea anemones drifted in front of their searchlights. The crew chased their prey over the crest of a ridge and were astonished to find themselves in the midst of a fabulous oasis of life. Sea anemones and snake-like pink fish with bulging eyes moved through shimmering warm waters, whilst crabs and miniature lobsters crawled amongst fields of giant clams and reefs of mussels. For the remaining five hours the crew took photographs and measurements and hastily collected as many of the animals as they could catch in Alvin’s specimen basket, before ascending to the surface.
Alvin made fifteen dives to the underwater oasis in 1977 and collected a mass of data, photographs and specimens. Since then several other expeditions have descended to discover more about the geology and biology of these unique habitats. As the team suspected, the hydrothermal vents form when seawater seeps into cracks a mile or two deep. The water is heated by hot magma to temperatures above 400°C (high pressure prevents the water from boiling), mixed with hydrogen sulfide and spewed out of the seafloor through lava-encrusted chimneys, known as black smokers. The animals inhabiting the vent live in the cooler waters that surround the hot springs. One of the most curious creatures is the giant tubeworm, which forms dense pink forests around the vents. The worms grow to several metres long but have no digestive system: no mouth or gut. Instead they depend on symbiotic bacteria that live within their tissue and utilize hydrogen sulfide as an energy source to make organic compounds such as sugars, which nourish the worms. Bacteria are present not only as symbiots but are prevalent in the surrounding cold waters and the hot walls of the black smokers. Massive temperature gradients are found within the walls of the smokers and, within the cooler zones, thermophilic bacteria flourish. The record is currently held by a bacterium named Methanopyrus, plucked out of a black smoker by Alvin, which can grow at temperatures as high as 112°C.
LIFE IN THE DARK
It is often stated that all life on Earth depends ultimately on the energy from sunlight. Plants need sunlight, animals eat plants and some animals eat other animals. But the oceanic trenches discovered by Alvin are thousands of metres below the ocean surface, far beneath the depths that light can penetrate. These ecosystems thrive in the dark by capturing chemical energy from the hot vents. The bacteria that form the basis of these deep ocean food chains are called lithotrophs, literally rock-eaters. Like plants, they extract carbon dioxide from seawater and string the atoms together to make sugars; but, unlike plants, they use minerals (principally hydrogen sulfide) spewed out of the volcanic vents as a source of energy. The bacteria eat hydrogen sulfide; everything else eats the bacteria.
Christian Lascu and Serban Sarbu discovered another lightless ecosystem in a limestone cave in southern Romania. The cave appears to have been isolated from the surface for five million years; yet Lascu and Sarbu found transparent crabs, blind spiders and water scorpions crawling through its dark, damp interior. Microbial mats that cover the surface of a ground-water lake and the limestone walls of the cave, nourish the whole ecosystem. The bacteria appear to be able to extract carbon from limestone (calcium carbonate), using energy derived from the oxidation of hydrogen sulfide dissolved in the ground water.
FIRE AND BRIMSTONE
The Christian Hell is an inhospitable place: ‘and he shall be tormented with fire and brimstone’ (Revelations, 14:11). The most vociferous hellfire preachers conjure up images of fiery mountains, scorching deserts and bubbling pools of brimstone to roast the souls of mortals deserving eternal damnation. Yet, harsh though such environments might appear, they would in fact provide quite comfortable habitats for many (perfectly virtuous) living creatures.
Brimstone is an archaic name for sulfur, which is found in meteorites, hot springs and sprayed out of active volcanoes. It is often visible as pale yellow streaks decorating volcanic slopes. The element itself is relatively harmless. It gets its infernal reputation from its ability to float on water and burn, releasing poisonous fumes of sulfur dioxide. Many of its other compounds are also noxious. The reduced (reduction is the opposite of oxidation and often involves the addition of hydrogen atoms to
