Wonders of Life. Andrew Cohen

ago, and that this fundamental and revolutionary change happened only once. We’ll return to this quite remarkable claim later in this chapter. For now, we’ll remain in the domain of the prokaryotes, and explore a more ancient yet no less epochal leap in life’s capabilities, achieved purely by the seemingly lowly bacteria, that turned the planet green and paved the way for the eukaryotes to flourish.

EATING THE SUN

      For a plant, one of the purposes of oxygenic photosynthesis is to capture energy from the Sun. This coloured micrograph of the leaf of a Christmas rose (Helleborus niger) shows the vertical cells of chloroplasts, which perform this function.

      If you made it through school biology lessons, you will have heard of photosynthesis. Indeed, you may well be able to recite the famous chemical equation from memory:

      6CO₂ + 6H₂o → C₆H₁₂O₆ + 6O₂

      Energy from the Sun

      Photosynthesis uses carbon dioxide and water to produce sugars and oxygen in a process powered by the energy of the Sun. But the use of the term photosynthesis to describe this particular process is a colloquialism. Specifically – and this is most definitely not a pedantic distinction – the above equation refers to oxygenic photosynthesis, and this makes all the difference in the world.

      Perhaps the best way to unravel the evolutionary origins of photosynthesis, and explain the significance of the term oxygenic, is to look at it from the perspective of a plant. The purpose of photosynthesis, if you are a plant, is twofold. One is clearly visible in the famous equation: it is to make sugars, which is done by forcing electrons onto carbon dioxide. The other, which is hidden in the detail, is to capture energy from the Sun and store it in a usable form. All life on Earth stores energy in the same way, as a molecule called adenosine triphosphate, or ATP. This suggests strongly that ATP is a very ancient ‘invention’, and the details of its production and function could provide clues as to life’s origin 4 billion years ago.

      PHOTOSYNTHESIS

      Photosynthesis, therefore, has a dual job: to store energy and to make sugars. The rest of the equation – and in particular the oxygenic bit, which refers to the production of oxygen – is a largely irrelevant detail as far as a plant is concerned. This provides a clue as to how oxygenic photosynthesis evolved.

      The molecular machinery of oxygenic photosynthesis in constructed from three distinct components known as photosystem I, photosystem II, and the Oxygen Evolving Complex, linked together by two electron transport chains. This linked molecular machine is known as the Z scheme. Photosystem I takes electrons and, using energy from the Sun collected by the pigment chlorophyll, forces them onto carbon dioxide to make sugars. Photosystem II functions in a different way. It uses another form of chlorophyll and, rather than forcing its energised electrons onto carbon dioxide, it cycles them around a circuit somewhat like a battery, syphoning off a little of the Sun’s captured energy and storing it in the form of ATP.

      In order to make sugars and ATP, therefore, the plant needs sunlight, carbon dioxide and a supply of electrons. It doesn’t ‘care’ where those electrons come from. The plant may not care, but we certainly do, because plants get their electrons from water, splitting it apart in the process and releasing a waste gas (oxygen) into the atmosphere. This is the source of all the oxygen in the atmosphere of our planet, and so understanding the evolution of the Z scheme is of paramount importance if we are to understand how Earth came to be a home for complex animals like us. The story can be traced back over 3 billion years to a time when the only life on Earth were the single-celled bacteria and archaea.

      CONVERSION OF WATER TO OXYGEN AND LIGHT TO ENERGY

      This light micrograph shows cyanobacteria, or ‘blue-green algae’, which use phycocyanin to capture the energy of the Sun.

      Take a look at this picture – it’s an image of a very particular type of bacteria. Look very closely at it because you have a lot to thank this particular kind of organism for. These are cyanobacteria – lowly bacteria that sit at the very bottom of the food chain. They’re the most numerous organisms on the planet. There are more of them on Earth than there are observable stars in the Universe, and these little creatures are what enabled you – and every other complex living thing that has ever lived on the planet, from dinosaurs to daffodils – to exist.

      If you look at the picture carefully, you will see that, unlike the other monochromatic bacteria, this one is bursting with a kind of blue-green colour, which comes from a pigment known as phycocyanin – exactly the kind of pigment that would offer an organism protection from the Sun’s damaging UV radiation. But these bacteria don’t just use the pigment for protection, they use it to capture the energy of the Sun.

A BREATH OF FRESH AIR

      Today cyanobacteria are sometimes considered to be a problem. This image, although beautiful, is of a bloom of ‘blue-green algae’- or, more correctly, cyanobacteria – in Lake Atitlán in the Guatemalan Highlands. It provides a vivid example of bacteria reproducing at a ferocious rate, and, in some cases, this explosion of life can have a devastating effect on an ecosystem. Toxins produced by the bacteria can decimate water life and affect human health, so they are closely monitored by environmental agencies around the world. But we have cyanobacteria to thank for the oxygen we breathe, because it is a virtual certainty that oxygenic photosynthesis evolved in an ancient cyanobacterium.

      The way to unravel the story of the evolution of the Z scheme is to look at how each individual part may have arisen. There is evidence that an early form of photosynthesis may have emerged as far back as 3.5 billion years ago in single-celled organisms that produced enigmatic mounds known as stromatolites (see Chapter 3), although the precise date is still an area of active debate and research. Whatever the date, there is general agreement that a simple form of photosynthesis, using energy from the Sun to synthesise sugars from carbon dioxide, just as photosystem I does in plants today, is very ancient. The pigment used today is chlorophyll, a member of a family of molecules known as porphyrins. Complex though they are, porphyrins have been found on asteroids, implying that they form naturally and are likely to have been around on Earth before the origin of life. There are still bacteria alive today that have only photosystem I. They take their electrons from easy targets, such as hydrogen sulphide or iron, and don’t therefore need much else in the way of machinery.

      Over time, it is thought that some bacteria adapted this early photosynthetic machinery to perform a different task – the production of ATP. There are similarities between the two photosystems that strongly suggest a common origin and later specialisation.

      Cyanobacteria are able to reproduce rapidly, and this can have a devastating impact on an ecosystem. This satellite image of Lake Atitlán in Guatemala shows blooms of cyanobacteria, caused by polluted runoff from the surrounding land.

      The evolution of early versions of photosystems I and II in bacteria is therefore relatively well understood; their components are