Growing Gaia needs iron

The name of the Greek goddess Gaia has become much more familiar since James Lovelock used the term for his hypothesis about the integrity of the earth as a self-regulating and self-sustaining organism. Gaia was perhaps an unfortunate name since the theory was proposed as a serious scientific hypothesis. The healthy debate that has ensued over the past two decades has been a considerable stimulus to both earth sciences and to the conservation movement. It is, therefore, timely to have a scientific analysis of Gaia, the life-inhabited part of the earth, as a physiological system.

Tyler Volk took a sabbatical year in the mountains of New Mexico to be close to Gaia and to put together this thorough analysis of the physiological systems and complex cycles that sustain life on planet earth. It is a book about the interactions of all life with the soil, the atmosphere and the oceans, and it is a most useful and informative treatment of the way in which our planet functions. It is a timely volume because it is only relatively recently that enough measurements have been made and analysed and computer models generated for us to begin to understand the chemical and biological cycles and the material flows that maintain life on earth. The author is well qualified to write about Gaia since his own research has involved work with Nasa's closed environmental systems and on earth's carbon cycle. The scientific details are combined with many striking analogies and anecdotes to enable the reader to understand extremely complicated systems. For example there are frequent links to the biological cycles of the author's year in New Mexico, which are cleverly woven into the theme.

The first chapter treats the carbon cycle based on observations about the seasonal rise and fall in levels of atmospheric carbon dioxide observed at Mauna Loa in Hawaii. It also touches on the greenhouse effect caused by the man-made increase of CO2. The second chapter treats nitrogen, the gas that makes up 70 per cent of the atmosphere. In this chapter we are introduced to a much referred marine micro-organism, Thioploca, a filamentous bacterium, that forms large colonies in the ocean off the coast of Chile. The important role that Thioploca plays in Gaia is as a denitrifier. The nitrogen cycle functions because of the roles and balance of nitrogen fixers and denitrifiers.

Having established the scientific base for the carbon and nitrogen cycles, upon which life depends, the next chapter deals with the circulation of these and other elements throughout Gaia by means of ocean and air currents. The toposphere, hydrosphere, pedosphere (soil) and biota are all components of Gaia. The next chapter attempts to break Gaia down into parts. Are they the biomes, or the taxonomic hierarchy of different living kingdoms or biochemical guilds of organisms that perform similar chemical functions such as photosynthesis or decomposition? The author prefers to think of the parts of Gaia as life, soil, atmosphere and ocean, but concludes that Gaia is a multiple system. This is further demonstrated by an analysis of the six elements easily remembered by the acronym CHNOPS. What is striking and supportive of Gaia is the fact that the proportions of carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur are remarkably constant across the entire diversity of life. The fifth chapter is a masterful description of the process of photosynthesis and of respiration, the processes that maintain life on earth. If you did not understand the make-up of the chlorophyll molecule in biology classes, read the explanation and study the diagrams given here. The central role of the enzyme rubisco, the master molecule maker, is thoroughly explained.

All the processes such as photosynthesis and the cycles of elements such as carbon, nitrogen and sulphur are driven by energy and so the sixth chapter treats this important aspect of Gaia. Central to this theme is that of embodied energy, that is the portion of solar energy that is converted into chemical energy in the structures of all photosynthetic organisms and that is used as a fuel for the metabolism of all other non-photosynthetic organisms. The fate of the energy that flows from the sun to the planet is analysed and it is shown that only a small amount of the total becomes embodied. Much more, some 25 per cent of the Earth's absorbed energy drives the water cycle, changing water from its liquid to vapour form through evaporation and transpiration. The mix of elements is amazingly uniform across the spectrum of different organisms. In case we humans feel too superior, an informative diagram compares the relative abundances of various elements between humans and a grape leaf. The similarity is striking, but it is given to show the way in which life feeds on its own productions by making crucial elements available. The next chapter continues to treat the cycling of the elements in the ocean depth, especially phosphorus, calcium and nitrogen. The emphasis is on the mechanisms that maintain the steady state of the dissolved nutrients in ocean water. Here again there is interplay between biological and non-biological causes.

I was fascinated to read about the duet between nitrogen and phosphorus. As samples are taken deeper and deeper in the ocean, phosphorus content rises and nitrogen density follows in parallel the same curve but at close to seven times the density of phosphorus. Iron is a limiting factor for the growth of phytoplankton in the ocean, as was shown by an experiment where iron was spread in a patch of the Pacific Ocean and the plankton concentrations immediately rose. It is shown how wind-blown dust is crucial for the delivery of iron to sustain oceanic phytoplankton. The experiment indicated that a trace element like iron can be a keystone element of Gaia.

The iron experiment also caused a concentration of sulphur gas from the biochemical guild of dimethyl sulphide (DMS) emitters. DMS is a gas that has a huge effect on climate since when oxidised on contact with air it forms tiny particles of sulphate aerosols. These aerosols seed cloud droplets. This is why clouds form in abundance over oceans and so help to maintain a cooler climate. Without the action of these DMS- emitting organisms in the ocean the earth would be from 150 to 250F warmer. Here we see another of the delicate control mechanisms of Gaia.

The final chapter treats the history of Gaia over time and so we have a description of the gradual evolution of Gaia from the Archean Age, 3.5 billion years ago, to the present. On the final two pages Gaia is allowed to speak and the human race is chided for what it is doing to Gaia. Gaia and environmental action are at last linked. My only caution is if we believe too much in the self-correcting ability of Gaia it could lead to complacency about the environmental destruction and species loss that is taking place.

Sir Ghillean Prance is director, Royal Botanic Gardens, Kew.