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
