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EATING THE SUN
ОглавлениеFor 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.
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 simple, and the chemistry reflects that occurring naturally on the early Earth. Things become more interesting, however, when we ask how these two machines came to be joined together in the Z scheme. While biologists don’t yet agree on the answer, one of the more elegant hypotheses, due to Professor John Allen at Queen Mary, University of London, and detailed in Nick Lane’s excellent book, Life Ascending, is as follows.
While some bacteria employed the precursor of photosystem I, and others used the precursor of photosystem II, there may also have been bacteria that possessed the genetic coding necessary to build both photosystems. This would allow them to switch between them, depending on environmental conditions and the availability of food. This is a relatively common thing for bacteria to do today; their genes can be switched on and off, allowing them to make hay while the sun shines – or at least, in this case, to use sunshine to make sugar or ATP, depending on whether the imperative is to reproduce or simply to survive. The possibility of an ingenious evolutionary adaptation now presents itself. What if it were possible to run these two machines at once, connecting the electron circuit from photosystem II into photosystem I, which would dutifully dispose of the cascade of electrons by pushing them onto carbon dioxide to form sugar? This would confer a great advantage on the organism in question, allowing it to make both food and ATP at the same time using sunlight as an energy source. This is certainly a plausible explanation for the separate evolution and then recombination of the two photosystems, but it leaves one remaining question: where does this machinery get its electrons? Here is where the Oxygen Evolving Complex enters the story and, with it, one of the most important evolutionary steps in the history of life on Earth.
The Oxygen Evolving Complex is an odd structure: more mineral than biological. It consists of four manganese atoms and a single calcium atom, held together in a lattice of oxygen. Manganese is locked away in vast mineral deposits on the ocean floor today, but in the early history of our oceans it would have been available in seawater for organisms to use. Bacteria use manganese to protect them from UV light, in much the same way as we use melanin – manganese is easily ‘photo-oxidised’, absorbing the potentially harmful UV photon and releasing an electron in the process. This may have been one of the ways in which electrons made their way into the primitive photosystem II in early bacteria. So manganese, at least, was already an important component of living things from the earliest of times. Today, manganese performs a different task. It sits at the heart of the Oxygen Evolving Complex, whose job is to grab water molecules and hold them ready for electrons to be ripped off and used as input into photosystem II. As a result, water molecules are split apart and, just as in the electrolysis of water so beloved of Mr Bell (see here), oxygen is released as a gas.
Bacteria genes can be switched on and off, allowing them to make hay while the Sun shines – or at least to use sunshine to make sugar or ATP.
This theory is a piece of cutting-edge research. The structure of the Oxygen Evolving Complex was determined only in 2006, and it is only in the last few years that the locations of each of the 46,630 atoms in photosystem II have been mapped. There are therefore many details in this story yet to be uncovered, but the broad sweep we have outlined here is certainly a strong candidate for an explanation of how the complexity of the Z scheme arose.
There is one last quite wonderful sting in the tail of this story, however, and it is something we know for certain: oxygenic photosynthesis evolved only once.
The evidence for this rather definite statement is clear when we look down a microscope at the structures inside plants and algae that carry our photosynthesis. They are called chloroplasts, and they are all self-evidently related to each other because they are so similar. But there is more than this, because they look for all the world as though they were cyanobacteria living inside the leaves, just like those found today in the blooms on Lake Atitlan. This is because that is exactly what they are. They even maintain their own independent rings of DNA, just as free-living bacteria do today.
But how does one cell end up inside another? At some point in the history of life on Earth, a cyanobacterium cell must have been engulfed by another cell and, instead of being digested, it survived to perform a useful purpose. This process, called endosymbiosis, has happened more than once in the history of life on Earth; indeed, it is thought to have been fundamental in the evolution of complex life. Endosymbiosis allows for great leaps in the capability of living things – a merger of fully formed skills to produce a result greater than the sum of the parts. In the case of oxygenic photosynthesis, this particular example of endosymbiosis led to the evolution of two of the great kingdoms of life – the algae and the plants – by allowing machinery evolved over billions of years inside cyanobacteria to be co-opted into more complex multi-cellular organisms.
A coloured electron micrograph of a leaf of Zinnia elegans, showing chloroplasts (green), starch granules (pink), the nucleus (red), and a large vacuole (white). The large air spaces allow for gas exchange during photosynthesis.
This coloured electron micrograph shows two chloroplasts in the leaf of a pea plant (Pisum sativum). Chloroplasts convert light and carbon dioxide into carbohydrates.
The quite dizzying conclusion is that, because everything that carries out oxygenic photosynthesis today does so in precisely the same way, we owe the beauty of life on Earth – with its hues, colours and seemingly limitless diversity – to a cyanobacterium whose ancestors, somehow, found their way inside another cell. The descendants of that cell are still present on Earth today, inside every leaf, every blade of grass and every algal bloom, and they have filled our atmosphere with oxygen.
A coloured electron micrograph of the inside of a chloroplast’s thylakoid membrane, containing the green pigment chlorophyll.
As levels of atmospheric oxygen rose, the Earth began to rust. Evidence of this rusting can be seen at Rockham beach, North Devon, where deposits of iron oxide appear as orange patches.
For almost half of Earth’s history, one of the most important ingredients for complex life was absent from the Earth’s atmosphere. Oxygen is an unstable, reactive gas that must be constantly replenished. The first rush of oxygen released from water by the cyanobacteria did what oxygen does best, reacting with the myriad elements present on Earth’s primordial surface to form oxides. In our planet’s infancy, large amounts of iron could be found in the oceans and, to a lesser extent, on land. Left over from the Earth’s formation, this dissolved iron remained stable for billions of years, but as the levels of atmospheric oxygen began to rise, a very familiar reaction began to take place. The Earth began to rust. Today, across the planet the evidence of this global rusting can be found in deposits of iron oxides known as banded iron formations.
Oxygenic photosynthesis doesn’t automatically fill the atmosphere with oxygen, however. It is necessary, but not sufficient, because both rusting and respiration act to undo all the good works of the plants, algae and cyanobacteria, and remove oxygen from the atmosphere. While photosynthesis takes carbon dioxide out of the atmosphere and turns it into organic matter, aerobic respiration takes organic matter and burns it using oxygen, releasing carbon dioxide and water. These processes will naturally reach a balance, which is why oxygen levels today have been stable at around 21 per cent for many millions of years. In order to change oxygen levels, something has to happen. It is known that oxygen levels first increased on Earth around 2.4 billion years ago, a time when many of the great banded iron formations were laid down. This rise may have been triggered by the complete oxidation of the Earth’s iron and other elements, which until that time acted as a sink, removing the photosynthetic oxygen from the atmosphere as quickly as the bacteria could release it. This is one plausible scenario, although there is not widespread agreement on the reason for this ‘Great Oxidation Event’, and it is still a very active area of research. Whatever the reason, the oxygenation of the atmosphere made possible by the evolution of oxygenic photosynthesis was critically important for the emergence of complex animals.
VARIATION OF ATMOSPHERIC OXYGEN CONCENTRATION OVER THE LAST 3.5 BILLION YEARS
Aerobic respiration, in which energy is released from organic matter, makes the existence of food chains possible because it is so efficient. Releasing energy from food using oxygen is around 40 per cent efficient, while oxidising food using iron or sulphur is only around 10 per cent efficient. This means that animals can eat plants, and in turn get eaten by a tower of predators that can still extract enough energy to flourish. It is almost certainly no accident that the Cambrian explosion – the rapid diversification of life resulting in the emergence of virtually everything we would regard today as complex – followed (on geological timescales) a rapid increase in atmospheric oxygen levels.
BRINGING IT ALL BACK HOME
The story of the emergence of today’s Earth is complex. That we understand not only the broad sweep of the narrative, but also the fine detail of at least some of the chapters, is one of the great achievements of science, and the presence of some uncertainties in the story of the emergence and development of a 4-billion-year-old biosphere is surely unsurprising. We have seen that water is a prerequisite for life on Earth, and most likely for life anywhere in the Universe. Likewise, an oxygen atmosphere, while not necessary for microbes, is a vital component of a complex ecosystem able to support large predators and prey, and probably therefore intelligent civilisations. As oxygen atmospheres are inherently unstable, oxygenic photosynthesis on a global scale is necessary to maintain high levels of this life-giving gas. And we know that this evolved only once on Earth. But there is one final ingredient that is more elusive and certainly beyond life’s control: time. It is a certainty that the evolution of complex life requires an ecosystem that is stable over many millions of years. But how many millions? This question will occur again and again throughout this book. Why did life emerge so soon after the birth of our planet, only half a billion years after its formation? And how did the first life blossom into the magnificent complexity we see on Earth today? A good place to start is to look at the evolutionary history of a single animal, and see how precisely we can trace its origins back into the deep past.
