Life is ubiquitous on and near the Earth's surface. Every conceivable niche is exploited by organisms with appropriate lifestyles. Under the Antarctic ice cap, in the middle of the hottest and driest deserts, in volcanic vents in the depths of the oceans, even in the clouds, there are organisms. The polar seas swarm with archaea; bacteria live and reproduce in cumulus clouds, which have life-times in the order of several days24. Prokaryotes live in hot rocks deep in the Earth's crust. Each organism, no matter where it is found, is equipped to survive in its particular environment. Each is part of a local ecosystem.
24 The discovery that there are about 1500 bacteria per millilitre of cloud meltwater is recent. The bacterial types have not, at the time of writing, been identified. It has been known for some time that bacteria can be transmitted from place to place on the Earth's surface via the upper atmosphere, but the discovery that bacteria actually live in clouds - at sub-zero temperatures in high-intensity ultraviolet radiation - is remarkable. What do they live on? Organic matter thrown up into the clouds in sea-born droplets is one possibility. And do they affect the weather, e.g. by affecting cloud formation and rainfall? Do they account for the fact that clouds are more opaque to sunlight than their water contents entitle them to be? The questions attendant on this - as on any striking new discovery - are legion.
Most television wildlife programmes make the point that the world teems with life. But television programmes often concentrate on macroscopic rather than microscopic life, the minority not the majority, so they do not tell the viewers how extreme some environments are. No plant or animal can survive for long at temperatures of more than 50oC and no protist at temperatures of more than 60oC, or at pressures greater than a very few atmospheres. But certain archaea live in volcanic vents at pressures of hundreds of atmospheres and temperatures well in excess of 100oC (for example, Pyrolobus fumarii lives at 113oC), and many of them enjoy conditions so acidic that most other organisms would die within seconds. There are numerous examples of apparently lethal environments in which life flourishes. Thiobacillus ferrooxidans thrives on the iron in reinforced concrete; it breeds furiously in modern architectural masterpieces and makes bridges and tunnels collapse.
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