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. The deep sea is constantly cold (2-3 °C), dark and nutrients are in short supply since it is a long way from the primary productivity of the phytoplankton in the surface waters. Deep-sea organisms have to rely on nutrients drifting down to them. In some places, though, heat and/or food are available thanks to geothermal activity or the seepage of methane (see Chapter 2, 'Hot vents and cold seeps'). Pressure is, however, a pervasive feature of the deep sea. How do the organisms which live there cope with its crushing forces?
Deep-sea organisms are not crushed by the high pressure because the pressure within their bodies is the same as it is outside, but they still need to be adapted to their high-pressure environment. Biochemical reactions are accompanied by changes in volume. If a reaction results in an increase in volume, it will be inhibited by pressure, whereas, if it is associated with a decrease in volume, it will be enhanced. Deep-sea organisms thus need to produce a different balance between their metabolic reactions than do organisms at the surface. Pressure also has effects on the structure of proteins and membranes. High pressure reduces the fluidity of membranes by squeezing their molecules together. Compression may also affect the conformation (folding) of proteins, so that it reduces the efficiency of their biological functions or stops them altogether. Ultimately, very high pressures may denature proteins.
Most animals that inhabit shallow depths are rapidly disrupted by the effects of high pressure and die. Microorganisms, being simpler, are more tolerant and may remain viable if they sink to the depths from their usual shallow habitats. Conversely, organisms that are adapted to the deep sea may not survive if they are exposed to the lower pressures at the surface and some animals will even fall to pieces as they are raised from the bottom. The ability of gases to dissolve in water is increased at high pressure. This causes problems if the pressure decreases, as gases come out of solution. This results in decompression sickness (the bends) in divers if they surface too quickly. Some deep-sea bacteria contain gas-filled spaces (vacuoles) that will expand and literally blow the organism apart if they are brought to the surface. These effects have made the study of deep-sea organisms difficult, requiring the development of devices which maintain the pressure experienced at depth as they are lifted to the surface. Marine biologists use PRATs
(Pressure-Retaining Animal Traps), and other devices, to maintain the physiological condition of deep-sea organisms during their journey to the surface for study in the laboratory.
Two main deep-sea habitats are recognised. Pelagic organisms live in the water column, while benthic organisms live on the sea floor or just beneath it. Of the 25 000 species of fish in the world, only about 15 per cent inhabit the deep sea. This is a relatively low diversity of fish, given the extent of the habitat, which may reflect the low availability of nutrients and the other problems of living there. Just about all major groups of fish have species which inhabit the deep sea, although they tend to be dominated by groups that appeared early in the evolution of fish. Special adaptations are required to meet the demands of this environment. Many different groups of invertebrates are also found in deep-sea habitats and there is a tremendous diversity of benthic invertebrates (see Chapter 2, 'The cold deep sea').
The identification of microorganisms from the deep sea is complicated by that fact that many surface-dwelling microbes will stay viable, but dormant, if they sink to the bottom of the ocean. Any sample of deep-sea sediment will thus contain representatives of organisms from the whole water column above it, as well as those that are specifically adapted to live there. Microbiologists distinguish between organisms that are piezotolerant (also called barotolerant) that survive high pressures and those which are piezophilic (baro-philic) that grow best at high pressures. Piezotolerant bacteria will not grow above 500 atmospheres and grow best at 1 atmosphere, whereas the optimum growth of piezophilic bacteria occurs at pressures above 400 atmospheres. One of the most piezophilic bacteria isolated grows best at pressures of 700-800 atmospheres. It continues to grow at up to 1035 atmospheres but will not grow at all at pressures below 350 atmospheres (Figure 6.1). The DEEPSTAR group from Japan's Marine Science and Technology centre has isolated several strains of bacteria from the Mariana Trench and other deep ocean sites. These include
piezophile piezotolerant extreme piezophile piezotolerant
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