Eating the Sun: How Plants Power the Planet

  • Oliver Morton
Fourth Estate: 2007. 384 pp. £25 0007163649 000717179X | ISBN: 0-007-16364-9
The ability of green plants to harness the Sun's energy is the basis of almost all life on Earth. Credit: F. LANTING/FLPA

All the greatest monsters are green. The Incredible Hulk had to turn green before going on the rampage and the Eagle comic featured a supremely evil green being called the Mekon, who was opposed in almost every issue by the chisel-jawed space hero, Dan Dare. One explanation for this odd association of colour with character is that green belongs to the vegetable kingdom. Humanoids have no right to have chloroplasts in their tissues — and if they do have them, well, they are probably not quite right. In the plant world, green is a heroic tint. It's a measure of the presence of chlorophyll and a sign that an organism captures energy from the Sun to convert it into organic matter. This is the basis of almost all life on the planet, and is arguably the single most important biochemical pathway there is.

Oliver MortonFootnote 1 has written a biography of this organic greenery. He takes us on a grand tour from molecules to biosphere, and a very impressive journey it is. He tackles the difficulties of explaining how photosynthesis works, and teases out the exciting story of how electrons hop from one molecule to another along their complex pathways. If you thought that photosynthesis simply 'splits' carbon dioxide into its constituent elements in order to build organic molecules and release oxygen, you will soon be disabused. The oxygen is derived from water molecules, as part of the elaborate atomic trade-offs that power cells.

These complexities might have benefited from a few more diagrams to help the less biochemically literate, although to the uninitiated, looking at folded proteins can be as confusing as contemplating a plate of tagliatelle. And Morton's writing is exemplary in its clarity, drawing an analogy where it will help, and grasping conceptual nettles where it won't.

Morton goes on to outline the 3.5-billion-year-plus history of photosynthesis on Earth. It is by now a familiar fact that we owe our oxygenated atmosphere to the early and relentless activity of photosynthetic cyanobacteria and, subsequently, of algae. These small organisms converted the seas into something that could be colonized by respiring animals by harnessing the Sun's energy over three-quarters of geological time. There is simply no escaping a kind of modified Gaia outlook here: life, nutrient cycles and rock weathering are all locked together in one inescapable dance.

Less familiar to most readers will be some of the crucial episodes during this long history. There were times when Earth is thought to have almost completely frozen over, for example. The Precambrian was not one slow story of advance towards the emergence of animal life. During the mid-Proterozoic, there was a period dubbed the 'boring billion' when nothing much happened in the way of biological innovation, at least according to many palaeontologists — life simply ticked over. It is not certain whether this stasis was linked to a shortage of available nitrogen or to the absence of animals to provide a fillip to innovation.

Whatever the reason, when animals did appear after about 1 billion years ago, the pace of evolution sped up dramatically. Morton is very good on what is needed to turn an alga into a land plant, and then to prop up that plant so that it can bathe in air and light to make a tree. The greening of the ancient continents was the final triumph of the chloroplast.

Eating the Sun proceeds smoothly to an account of climate history. Plants have influenced, and been gripped by, climatic fluctuations that have produced alternations of a 'greenhouse' and an 'icehouse' world. The climatic changes that seem inevitable to us now have, in all likelihood, been paralleled at one time or another in geological history. What is unprecedented is the burning of so much photosynthetically fixed carbon that had been sealed away in rocks. What time had sequestered so securely is now being blasted into the atmosphere by thousands of power stations.

The book describes with admirable dispassion the different hypotheses detailing how the biological and human world might cope with this challenge. It is good to have a proper acknowledgement of the difficulties of predictive climate modelling, where there are so many unknowns — particularly in biotic responses to increased carbon dioxide availability. Theoretical mitigation possibilities — such as seeding some parts of the ocean with iron to stimulate plankton blooms — are carefully considered and then given short shrift. The problem is that we cannot simply wait around for the science to improve just so that we might better appreciate the particular flavour of our doom. Unfortunately, quite a few of the precautions that should have been taken against this have already been surpassed.

If we want to see just how bad things might become, we can contemplate the dreadful object lesson of the biological world after the mass extinction at the end of the Permian period, when a sick and enfeebled biosphere took millions of years to recover — indeed, nothing as bad followed even the extinction of the dinosaurs. Morton briefly reviews the alternative sources for the energy consumption to which humankind is so addicted. The problem with such sensible words is that the reader still feels that no rational course will ever be adopted until the very tragedies that we seek to avert have come to pass. Of several recent accounts of what might happen to climate in the next decades, Morton's is among the most balanced, but I am still left crossing my fingers and recycling a few plastic bags. As T. S. Eliot remarked, humankind cannot bear very much reality.

Morton's account of the ubiquitous importance of photosynthesis is an original viewpoint for looking at the world. It is written with verve and an eye for detail. His breadth of scholarship could leave other science writers green — with envy.