In a 1974 paper in Tellus, chemist James Lovelock and biologist Lynn Margulis proposed a bold hypothesis: Earth's living organisms act together to ensure that the planet remains hospitable to life. Following a suggestion by his neighbor, novelist William Golding, Lovelock named the hypothesis after the ancient Greek goddess of the Earth, Gaia.
Anselm Feuerbach's 1875 painting
Gaia decorates a ceiling in Vienna's Academy of Fine
Arts.
Given that most organisms belong to food chains that consist of, if not predators and prey, then eaters and eaten, the notion of planetary cooperation seems far-fetched. Still, Lovelock and Margulis made a strong, scientific case. Here's the abstract of their Tellus paper:
During the time, 3.2 × 109 years, that life has been present on Earth, the physical and chemical conditions of most of the planetary surface have never varied from those most favourable for life. The geological record reads that liquid water was always present and that the pH was never far from neutral. During this same period, however, the Earth’s radiation environment underwent large changes. As the sun moved along the course set by the main sequence of stars its output will have increased at least 30% and possibly 100%. It may also have fluctuated in brightness over periods of a few million years. At the same time hydrogen was escaping to space from the Earth and so causing progressive changes in the chemical environment. This in turn through atmospheric compositional changes could have affected the Earth’s radiation balance. It may have been that these physical and chemical changes always by blind chance followed the path whose bounds are the conditions favouring the continued existence of life. This paper offers an alternative explanation that, early after life began it acquired control of the planetary environment and that this homeostasis by and for the biosphere has persisted ever since. Historic and contemporary evidence and arguments for this hypothesis will be presented.
That life has shaped Earth's environment is beyond question. Whether and how life maintains the environment is less obvious. The planet's inhabitants were evidently unable to prevent life-stifling sheets of ice spreading from polar regions during ice ages. Granted, evolution entails species adapting to the environment, but the adaptation is selfish and plays out on a local scale; it isn't cooperative on a planetary scale.
Although I'm not convinced by the Gaia hypothesis, the question of how Earth's life-sustaining environment arose is interesting and important. And thinking of Earth's biosphere as a single system is surely appropriate in some scientific contexts, not least in the search for extraterrestrial life.
Plants that water themselves
Gaia sprang into my mind earlier this month when I came across a paper in Science. In May 2011 Christopher Pöhlker of the Max Planck Institute for Chemistry in Mainz, Germany, and his colleagues went to the Amazonian Tall Tower Observatory (ATTO) 150 km northeast of Manaus, Brazil. Their goal: to gather organic aerosol particles above pristine rainforest during the rainy season. Organic aerosols cool Earth's surface by scattering sunlight. They also nucleate rain droplets. Despite their climatological importance, their origin and composition is unclear—hence Pöhlker's investigation.
The paper reports what the team found when they subjected the aerosols to three different physical and chemical assays: near-edge x-ray absorption fine structure analysis, scanning electron microscopy, and secondary ion mass spectrometry.
To the team's surprise, most of the organic aerosols contained 0.3%-20% of potassium by volume. Apparently, potassium ions serve to nucleate the formation of organic aerosols. Where does the potassium come from? Although soot from burning vegetation contains potassium, the team did not detect any fires during the collection period. They did, however, recall previous reports of plants and fungi releasing potassium inos and other inorganic species into the air. The potassium-nucleated organic aerosols are ideal for nucleating rain droplets—which suggests an intriguing possibility. To quote Pöhlker et al.:
Our findings support the hypothesis that the Amazonian rainforest ecosystem can be regarded as a biogeochemical reactor in which the formation of clouds and precipitation in the atmosphere are triggered by particles emitted from the biosphere.
I expect Pöhlker's conclusion would delight Lovelock (Margulis died last year). Whether it supports the Gaia hypothesis is unclear, at least to me. Conceivably, plants could take up potassium from the soil in the form of ions dissolved in water. If plants emit potassium ions merely as a consequence of transpiration, then their ability to regulate rainfall is a happy coincidence.
Happenstance or not, the notion of plants watering themselves is amazing.
