Читать книгу Quantum Evolution: Life in the Multiverse - Johnjoe McFadden - Страница 9

Our spacecraft would soon discover that all life on Earth is carbon-based, that carbon is the key ingredient of our biomolecules. We might also describe life as water-based, since water is the substrate for our cells and tissue fluids, taking an active role in most of life’s activities. Life’s other main chemical ingredients are hydrogen, oxygen and nitrogen and small quantities of minerals such as calcium, magnesium, iron and sulfur.

These are readily available in our biosphere. Water is in the sea, in rivers, streams and lakes and, of course, rains frequently down upon us. Carbon is found in both inorganic molecules like carbon dioxide (CO₂)¹, methane (CH₄) or calcium carbonate (CaCO₃) and as organic² forms such as the sugars, fats or proteins derived from the bodies of other living organisms. Hydrogen and oxygen are available in inorganic forms such as water (H₂O) or as a component of organic compounds. Similarly, nitrogen is available in inorganic nitrogen gas (N₂), ammonia (NH₃), nitrates and nitrites, and in organic compounds. Animals are unable to utilize the inorganic forms of most of these, obtaining the elements they need from organic sources – the bodies of dead plants and animals.

Life would not have progressed far on our planet if all organisms were as feeble in their synthetic capabilities as animals. Fortunately plants and microbes are much more versatile. Billions of years ago, photosynthetic bacteria³ developed the trick of extracting carbon from the carbon dioxide in the air and stringing together the carbon atoms to make simple sugars. This is not easy; in fact, photosynthesis is one of the trickiest chemical reactions we know of (we will be looking at it more closely in Chapter Five). The problem is that the carbon atoms in carbon dioxide prefer to be attached to oxygen rather than tied to each other to make complex biochemicals such as sugars or proteins. To persuade carbon atoms to form complex biochemicals, bacteria and plants need a hydrogen source (plants use water) and an energy source (sunlight). Photosynthetic organisms extract carbon dioxide from the atmosphere and add hydrogen and sunlight energy to make simple sugars. The sugars are then strung together, pulled apart and reassembled to make the cell’s complex biomolecules – proteins, fats, carbohydrates and DNA.

Plants did not invent photosynthesis but stole the idea from bacteria – quite literally. Chloroplasts, the organelles performing photosynthesis inside leaves are descendants of a bacteria called cyanobacteria. Cyanobacteria are far more ancient than plants, and performed photosynthesis on Earth at least a billion years before the arrival of plants. The ancestors of modern plants were probably symbiotic partnerships between primitive fungus-like organisms and the photosynthetic cyanobacteria, perhaps resembling today’s lichens. This partnership slowly became permanent, and all today’s trees, ferns, flowers and grasses are the descendants of this marriage of convenience.

Cyanobacteria are not the only bacteria to perform photosynthesis and probably not even the first. Like plants, cyanobacteria perform oxygenic photosynthesis – they release oxygen as a product of their photosynthesis. The oxygen comes from their hydrogen source: water. Other bacteria can utilize alternative sources of hydrogen – such as hydrogen sulfide (H₂S), ammonia or organic compounds – to fix carbon. These bacteria perform an anoxygenic photosynthesis which does not generate oxygen. This form of photosynthesis almost certainly preceded its oxygenic cousin.

Many bacteria and all animals are unable to fix atmospheric carbon dioxide, extracting it instead from alternative inorganic and organic chemical sources. Bacteria are the most versatile chemical feeders, able to extract carbon from a wide range of chemicals, which include organic compounds, carbon monoxide, calcium carbonate, methane, methanol, ether and formic acid. One group of bacteria using methane as both carbon and energy source is common in animals’ intestines, marshes and oxygen-deficient mud. But their most bizarre habitats were discovered on the sea-bed. In the summer of 1997, Chuck Fisher of Pennsylvania State University and Phil Santos from the Harbour Branch Oceanographic Institute were in a mini-submarine, Johnson Sea Link, exploring the sea-bed seven hundred metres below the Gulf of Mexico. They were examining the huge bubbles of methane hydrate forming when natural gas (methane) seeps up from the ocean floor, mixing with water and other hydrocarbons to form a dirty yellow methane ice. Scientists had suggested that methane-eating microbes might also feed on the hydrates, but what Fisher and Santos did not expect to find was a multitude of pink worms using oar-like paddles to crawl over, or burrow into, the ice. They were a new species of polychaete worms. It is unlikely that they eat methane directly, instead the worms probably graze on methane-eating bacteria colonizing the ice. It has even been suggested that the worms might build burrows to cultivate farms of these bacteria.

The next ingredient for life, nitrogen, should not be a problem since eighty per cent of the air we breathe is nitrogen gas. However, we cannot assimilate nitrogen gas from air – too unreactive – we breathe it in and right back out again. Instead we obtain our nitrogen from organic chemicals in food such as, for example, the protein in meat. Plants are able to assimilate inorganic forms of nitrogen such as nitrate (a compound of nitrogen and oxygen), but this does not solve the problem since the only non-biological source of nitrate is lightning strikes which generate temperatures high enough to burn atmospheric nitrogen and yield nitrate.

With only very limited supplies of fixed nitrogen available from lightning, life might have become severely nitrogen-limited billions of years ago. Fortunately, bacteria (including cyanobacteria) discovered how to fix nitrogen in the air to make the soluble compounds ammonia and nitrate. Nearly all biological nitrogen is derived from these nitrogen-fixing bacteria in soil and water. Leguminous plants (such as peas) form symbiotic partnerships with nitrogen-fixing bacteria, allowing them to grow in nitrogen-depleted soil.

The last ingredients of life – the minerals like calcium, sodium, magnesium, phosphorus and iron – are fairly readily available, usually as salts dissolved in water. Most organisms can readily assimilate inorganic sources of these elements, such as the sodium chloride (NaCl): the salt we sprinkle on our food.

Living organisms are extremely versatile in their ability to utilize a wide range of both organic and inorganic chemicals for the elements that make up their biomolecules. Animals need much of their biomass supplied as ready-made organic molecules. Bacteria have minimal requirements: some subsist on little more than a diet of air and rock.