Kitabı oku: «Quantum Evolution: Life in the Multiverse», sayfa 3

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 an element or compound) compound of sulfur, hydrogen sulfide, is a foul smelling and poisonous gas (the smelly gas generated by the stink-bombs so beloved of schoolchildren). Sulfuric acid is one of the most corrosive acids. Yet sulfur is essential for life. Proteins and fats are particularly rich in sulfur. Our bodies contain about two-hundred grams of sulfur. The source of all this sulfur is bacteria, able to eat or breathe both sulfur and its noxious compounds.

We have already met hydrogen sulfide-eating bacteria at the depths of the ocean; but this metabolic capability is widespread. Sulfur-eating bacteria, such as Thiobacillus, live in soil and in fresh and saltwater; and use sulfur and hydrogen sulfide as an energy source to fix carbon dioxide and generate biomass. Other sulfur bacteria, such as Desulfobacter, live in brackish water and animal intestines and use oxidized forms of sulfur (sulfate) as we use oxygen – to breathe (they exhale hydrogen sulfide). Other bacteria such as Chromatium use hydrogen sulfide as a hydrogen source for photosynthesis, depositing the leftover sulfur granules within their cells. The combination of differing survival skills of various sulfur bacteria allows it to be cycled through the entire ecosystem of the Earth. On a smaller scale, miniature sulfur cycles take place within some warm fresh-water lakes fed by sulfur-rich steams. In the sulphuretas of Libya and Japan, it is cycled between its oxidized and reduced forms and in the process, elemental sulfur accumulates in the lake and is harvested commercially. So, far from being an instrument of infernal torture, pools of brimstone can be healthy and productive environments for many microbes.

Many of the bacteria that metabolize sulfur also generate sulfuric acid as a by-product. Although science fiction films such as Alien (1979) and its many sequels featured monsters with acid for blood, it is microbes, rather than monsters, which are most tolerant to acid. Acidity is measured in units of pH: pH7 is neutral, below pH7 is acid and from pH7–pH14 is alkaline. Our cells function within a fairly narrow pH range, from about pH7.5 to 8.5: very slightly alkaline. Blood contains a bicarbonate-based buffering system that maintains its pH within this range. These stores can however be depleted during illness, such as severe diarrhoea, resulting in the drifting of body fluids outside their normal pH range, causing metabolic acidosis or alkalosis. The consequences can be disastrous, leading to tissue damage, shock and death.

Other animals are much more tolerant of acid. Acid rain from the burning of fossil fuels has caused the acidification of many lakes in Europe and the USA. Some fish can survive in lakes with water as low as pH4 but if it becomes more acidic, all fish die. Yet these acid lakes still harbour many invertebrates and microbes. Algae, fungi and bacteria are able to tolerate the highest levels of acidity, down to about pH0. We have met some of these microbes already – the sulfur-oxidizing bacteria found in hydrothermal vent systems which excrete hot sulfuric acid. Many of these bacteria can grow at concentrations and temperatures of sulfuric acid that would dissolve metals. Even our own bodies harbour acid-tolerant microbes. Our stomach contents have a pH of 1–2. The acid not only helps to digest our food but kills microbial pathogens such as salmonella, which normally have to be ingested in huge numbers (generally more than a million) to cause disease. However, a few microbes do survive within our stomach’s acidity, most notably the spiral bacterium Helicobacter pylori, colonize the stomach lining and cause ulcers.

A remarkable feature of acid-tolerant microbes is that the insides of their cells are not particularly acidic – about pH6. Acidity is a measure of hydrogen ion concentration. It is a logarithmic scale so that pH zero has one million times the concentration of hydrogen ions as pH6. Somehow the bacteria are able to maintain a million-fold concentration difference of protons (remember that a hydrogen ion, H⁺, is just a proton) across their cell membranes. It is not entirely clear how the bacteria achieve this feat; presumably either by excluding protons from their cells or by possessing a very efficient proton pump to pump them out.

The extreme alkaline end of the pH scale (10–14) is also harmful to most animals and plants. Strong alkaline solutions such as caustic soda dissolve cell membranes and destroy cells. Many plants and microbes are however fairly tolerant of soils that may have pH values up to about 10. Environments more alkaline than pH10 are rare on this planet. The only stable systems are soda lakes fed by bicarbonate-rich natural springs. The pH of these lakes may be as high as 11.5, yet they are often rich in microbial life.

High concentrations of salt are toxic to most living organisms; as attested by salt-curing to prevent microbial growth and preserve meat and fish. When cells are suspended in salt, their internal water is sucked out of their cells by osmosis, which dehydrates and eventually kills cells. There are however many natural saline environments on Earth. The sea, with a salt concentration of about three per cent, is toxic to most land animals and plants but is of course haven to marine creatures. The Dead Sea is a twenty-eight per cent solution of salts, nearly ten times the salinity of sea water. Yet the Dead Sea is far from dead. Although no fish swim in its waters, it contains algae and a rich microbial flora. One of these microbes, called Halobacterium, produces a purple pigment, bacteriorhodopsin that is able to harvest light energy and is the only non-chlorophyll based natural light-harvesting system that we know of. Halobacteria are so salt tolerant that they can survive intact inside salt crystals. Salt-loving bacteria employ two principal mechanisms to survive the osmotic pressures of their saline habitats. The first is simply to accumulate lots of salt (usually potassium chloride) within their own cells. The second strategy is to synthesize large quantities of small organic molecules (like glycerol) inside their cells, which counteract the pull of the external salt.

The Gulf War left devastation in the Persian Gulf. Burning oil wells belched noxious black smoke and leaked millions of tons of crude oil into the surrounding land. It was an environmental disaster that many predicted would take centuries to mend. Yet only a few years later, wild flowers returned to the oil well sites. The key to the rapid recovery was the presence of oil-eating microbes in the soil. Many microbes can tolerate or even feed on chemicals poisonous to plants and animals. The soils surrounding the oil wells were probably already rich in these microbes before but thrived in the oil-polluted soil left by the war. The microbes fed on the crude oil, degrading it into less non-toxic chemicals. Microbes are able to feed on a wide range of chemicals poisonous to many other creatures, such as benzene, toluene, cyclohexane and kerosene.

LIFE WITHOUT AIR

Living and breathing are, for humans, inextricably linked. We talk of ‘the breath of life’. Animals need oxygen to live. Yet there are many living organisms for which the breath of life is poisonous. Microbes, known as anaerobes, do not breathe air: indeed many are instantly killed on exposure to oxygen. This sensitivity makes anaerobes difficult to study and so their prevalence has not been appreciated until fairly recently. It may come as a surprise to you to know that more than eighty per cent of your faeces is made up of anaerobic bacteria. Very little air penetrates our lower bowels, making it an ideal environment for the proliferation of these microbes. The vast majority of these gut bacteria are entirely harmless colonizers of our intestinal tract or even beneficial; but they may occasionally cause problems (mostly abscesses and ulcers) particularly if introduced into the rest of the body by wounds or surgery. Anaerobic microbes are also widespread in the environment. They are found in the soil and in fresh and seawater, particularly in the air-depleted sediments at the bottom of lakes, rivers, seas and oceans.

So how do anaerobes live without oxygen? It’s easy – it’s living with oxygen that is the difficult feat. Toxic to all living organisms, oxygen is highly reactive – reacting with tissue to generate even more unpleasant chemicals such as hydrogen peroxide (used to bleach hair) and molecules known as free radicals. Air-breathing organisms have an armoury of protective enzymes to remove and destroy these toxic chemicals. Strict anaerobes lack these protective enzymes and are killed by oxygen.

So why do we go to so much trouble to breathe air when one of its chief components, oxygen, is so toxic? We use it to burn our food in the process known as respiration. Chapter 5 will examine respiration more closely, but briefly: electrons are harvested from our food and rolled down a kind of energy cascade to oxygen. The difference in energy (high-energy electrons from food to low-energy electrons in oxygen) is captured, providing energy for the cell. Oxygen-based respiration is a very efficient means of extracting maximum energy from food and has thus superseded the anaerobic metabolism in most higher organisms.

Some anaerobes burn their food in respiration, but not with oxygen. Many minerals (such as sulfate or nitrate) serve as low-energy electron dumps for their respiratory cascade. You may well have noticed that the sand alongside estuary waters is often black and smelly. The bad smell is hydrogen sulfide and the blackness is due to the presence of iron sulfide, both products of anaerobic bacteria’s respiration in these estuarine waters. In fact, a wide range of minerals can be utilized by bacteria for respiration; some bacteria can even breathe iron.

Still other anaerobes do not actually respire at all but derive their energy from chopping up their food molecules into small pieces, usually into simple acids or alcohol, in a process known as fermentation. Fermented foods and beverages like wine, beer, sauerkraut, cheese and even coffee, depend upon the actions of these busy microbes. Anaerobes are of enormous ecological importance since they are often responsible for the final decay of organic matter. A giant compost heap would have long since enveloped the whole planet were it not for these bacteria. Cows and other ruminants have learned to harness the powers of these microbes. Their stomachs house an internal compost heap of plant material decomposing through the activity of billions of fermentative bacteria. One of the by-products of their fermentation is the greenhouse gas, methane. The huge quantities of the gas flatulently emitted by (the bacteria inside) domesticated ruminants is thought to contribute signifycantly to the greenhouse effect.

Far from being the breath of life, life carries on very well in oxygen’s complete absence. This must of course be so, since (as covered further in Chapter 4) life emerged on this planet in an atmosphere completely devoid of oxygen. It was only after photosynthetic plants and microbes began to pour oxygen into the earth’s atmosphere that aerobic life became possible on Earth.