Not quite perfect


5 July 2013

Earth is a bounteous place, home to a diverse collection of species. It seems like a perfect crucible for life, but is it? Toby Tyrrell thinks it could be better.

If you've thought about why there's life on our planet at all, you have probably encountered James Lovelock's Gaia hypothesis. This proposes that the biota - the collection of life on Earth - helped shape the Earth environment to make it and keep it especially hospitable for life.

Early in my career, as I studied nutrient cycles in the ocean and their effect on phytoplankton growth, I did not at first see the links to Gaia. But a research fellowship from NERC provided a welcome opportunity to think more deeply about the wider implications of my work.

I had long been intrigued by the Redfield ratio. This is the observation, first made by Alfred Redfield in the 1930s, of a puzzling similarity between the ratio of nitrogen and phosphorus found in plankton with that dissolved in seawater as nitrate and phosphate: in both cases it's roughly 16:1. When deep waters upwell to the surface they are rich in nutrients, and proliferating phytoplankton take up nitrate and phosphate in the ratio of ~16:1 until one or the other, or both, become exhausted.

There's an even greater mystery here, which is why nitrogen limitation occurs at all.

Redfield found that they nearly always run out together - suggesting that the levels of these nutrients in the sea are perfectly matched to the needs of phytoplankton growth. As he put it: "That two compounds of such great importance in the synthesis of living matter are so exactly balanced in the marine environment is a unique fact and one which calls for some explanation, if it is not to be regarded as a mere coincidence."

I developed a simple computer model of nitrogen and phosphorus cycling which reproduced the match, stabilising at a nitrogen:phosphorus ratio for seawater similar to that of phytoplankton. This model (others have since obtained similar results) showed that the similarity is not a coincidence but rather is brought about by a complex negative feedback in the nitrogen cycle, which effectively restores the seawater ratio to 16:1 if it changes away from that value. Redfield's puzzle had been solved.

That wasn't the end of the story for me though. This match might seem ideal for the plankton, but really they would be better off facing one scarce nutrient rather than two. It also dawned on me that there's an even greater mystery here, which is why nitrogen limitation occurs at all.

Death or incapacitation through nitrogen starvation is widespread, not just at sea but also on land where herbivores such as caterpillars often struggle to get enough nitrogen from the leaves they eat. It's also a problem in agriculture, with prodigious quantities of nitrogenous fertilizers having to be applied worldwide to maximise crop yields.

Chlorosis in leaf

Yellowing of leaves from lack of chlorophyll can be due to insufficient nutrients.

Yet nitrogen is exceptionally abundant in the environment, it makes up 78 per cent of air, as dinitrogen (N2). N2 is also much more plentiful in seawater than other dissolved forms of nitrogen. The problem is that only organisms possessing the enzyme nitrogenase (organisms known as nitrogen-fixers) can actually use N2, and there aren't very many of them. This is obviously a less than ideal arrangement for most living things. It is also unnecessary. Nitrogen starvation wouldn't happen if just a small fraction of the nitrogen locked up in N2 was available in other forms that can be used by all organisms; yet biological processes taking place in the sea keep nearly all that nitrogen as N2. If you think about what is best for life on Earth and what that life can theoretically accomplish, nitrogen starvation is wholly preventable.

This realisation led me to wonder what other aspects of the Earth environment might be less than perfect for life. What about temperature? We know that ice forming inside cells causes them to burst and that icy landscapes, although exquisite to the eye, are relatively devoid of life. We can also see that ice ages - the predominant climate state of the last few million years - are rather unfortunate for life as a whole. Much more land was covered by ice sheets, permafrost and tundra, all biologically impoverished habitats, during the ice ages, while the area of productive shelf seas was only about a quarter of what it is today. Global surveys of fossil pollen, leaves and other plant remains clearly show that vegetation and soil carbon more than doubled when the last ice age came to an end, primarily due to a great increase in the area covered by forests.

Although the cycle of ice ages and interglacials is beyond life's control, the average temperature of our planet - and hence the coldness of the ice ages - is primarily determined by the amount of CO2 in the atmosphere. As this is potentially under biological control it looks like another example of a less than perfect outcome of the interactions between life on Earth and its environment.

Look further and you find still more examples. The scarcity of light at ground level in rainforests inhibits growth of all but the most shade-tolerant plants. There's only really enough light for most plants at canopy height, often 20 to 40 metres up, or below temporary gaps in the canopy. The intensity of direct sunlight does not increase the higher you go, so having the bulk of photosynthesis taking place at such heights brings no great advantage to the forest as a whole. Rather the contrary, trees are forced to invest large amounts of resources in building tall enough trunks to have the chance of a place in the sun. This arrangement is hard to understand if you expect the environment to be arranged for biological convenience, but is easily understood as an outcome of plants competing for resources.

During the course of my career, these - and other - conclusions about Earth's habitability led me to examine the whole Gaia hypothesis with a much more critical eye. The Earth is indeed a wondrous place, and home to many millions of species including ourselves - but in my view it's by no means perfect.

Toby Tyrrell is Professor in Earth System Science at the National Oceanography Centre Southampton.

His book On Gaia: A critical investigation of the relationship between life and Earth, (2013) was published by Princeton University Press on 4 July. ISBN: 978-0691121581