So far, we have looked at organisms that can survive extreme temperatures (hot and cold) and desiccation (anhydrobiosis). There are, however, a number of other extreme environmental stresses that offer challenges and opportunities to living organisms. I will consider some of these in this chapter.

UNDER PRESSURE

There are few situations where organisms are naturally exposed to low pressure, but high pressure is a rather more common hazard than we might expect. Organisms that inhabit rocks and sediments beneath the surface of the Earth are likely to be under pressure (see Chapter 2 in the section ‘The underworld’). The study of these organisms is in its infancy. We know rather more, however, about those of the other main high-pressure environment, the deep sea (see Chapter 2, ‘The cold deep sea’). The deep sea is considered to be that volume of the oceans which is below the depth of 1000 metres. The oceans cover 71 per cent of the Earth's surface and are an average of 3800 metres deep. In volume, the deep sea comprises 75 per cent of the biosphere. This makes it the largest environment, or rather group of environments, on Earth, but it is one of the least understood. Since hydrostatic pressure increases by 1 atmosphere for every 10 metres in depth, organisms inhabiting the deep sea have to cope with the crushing forces of very high pressures – reaching up to 1100 atmospheres in its deepest parts. However, pressure is not the only problem organisms face.