figure 6.1 The effect of pressure on the growth rate of piezotolerant, piezophilic and extremely piezophilic bacteria. The extremely piezophilic bacterium was isolated from the Mariana Trench and will not grow at low pressures. Redrawn from a figure in Madigan et al. (2000).
strains that only grow at extreme high pressures (such as Shewanella benthica). Microorganisms are associated not just with deep-sea pelagic and benthic habitats but also with the guts, and other parts of the bodies, of the animals which live there. Hyperthermophilic archaea associated with deep-sea hydrothermal vents also live under high pressure. They are piezotolerant rather than piezophilic, growing more rapidly at 1 atmosphere than they do at the pressures experienced in their normal habitat.
Adaptations to high pressure
Some piezophilic microorganisms and deep-sea fish maintain the fluidity of their cell membranes by increasing the proportion of
unsaturated fatty acids in the lipid component of the membrane. The structure of proteins, enzymes and nucleic acids must also be adapted so they can function at high pressure. The enzymes of deep-sea fish seem to be insensitive to pressure, rather than being adapted to function best at high pressure. Proteins which are associated with cell membranes are particularly sensitive to the effects of pressure. Different outer membrane proteins are produced by the moderately piezophilic bacterium Photobacterium profundum at high and at low pressures. The switch in production from one outer membrane protein to another is controlled by the expression of a number of pressure-regulated genes. These proteins are thought to be involved in nutrient uptake, and the outer membrane protein produced at high pressures allows a faster rate of nutrient uptake in the low-nutrient environment of the deep sea, as well as being able to maintain its function at high pressure.
Fish produce trimethylamine oxide as a nitrogenous waste product (together with ammonia or urea). Cartilaginous fish (elasmobranchs like sharks and rays) retain urea in their blood and tissues, where it assists them with their osmoregulatory problems in seawater. Trimethylamine oxide is also retained and counteracts the harmful effects of urea on proteins. It is only found in low concentrations in shallow-water teleost (bony) fish other than elasmobranchs. Deep-sea teleosts and invertebrates, however, do contain high concentrations of this compound. It may, therefore, play a role in protecting these animals against the effects of pressure and has been shown to protect proteins against pressure-induced denaturation.
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