A piezophile (also called a barophile) is a microorganism which thrives at high pressures and constitutes a sub-group of extremophiles (microorganisms that live in conditions of extreme temperature, acidity, alkalinity or chemical concentration). They inhabit the deep sediments of oceanic plains and abyssal trenches where temperatures range from 2 to 4°C. Between 0 and 2000 metres, baro-tolerant psychrophile species prevail (these species are subservient to the low temperatures of the deep and are capable of developing in pressure conditions ranging from atmospheric pressure to the extreme pressures of the sampling sites). Beyond 6500 m is the habitat zone of psychropiezophiles (microorganism subservient to pressure conditions). Between these two depths, microorganisms show variable behaviour in response to pressure, baro-tolerant for some, strictly barophile at certain temperatures for others.
Thermococcus barotolerans strain, isolated from a rock sample from Snake Pit on the Mid-Atlantic ridge
Depth: 3550 meters - Expedition MAR93
Deep sea hydrothermal vents provide microorganisms with extreme conditions of temperature, pressure and fluid composition. Several studies made from samples taken on hydrothermal vents have already provided proof of hyperthermophile microorganism isolate cultures (optimum growth beyond 80°C) at high pressures. However, the majority of strains living in depths of around 2000 meters, do not show any signs of barophilia in pressure conditions in situ.
The baro-hyperthermophiles known to-date stem from depths beyond 2000 meters. The strain Thermococcus barotolerans was isolated from a rock sample taken from the Snake Pit site on the Mid-Atlantic Ridge at a depth of 3550 meters during the MAR93 expedition.
This strain, whose maximum temperature for growth at atmospheric pressure is 95°C is capable of developing at between 95 and 100°C as soon as it is re-pressurized at 15-17 Mpa. Its optimum growth temperature at atmospheric pressure is 85°C both at atmospheric pressure and at 44 Mpa. However, its growth rate doubles at high pressure compared to atmospheric pressure conditions. This strain is thereby barotolerant below 95°C and strictly barophile between 95 and 100°C.
The Serpentine expedition aimed at studying the deepest hydrothermal vents known to date making it a unique opportunity to collect rock, fluid and animal samples from depths never visited before. In particular, the deepest site known anywhere in the world Ashadze (Mid-Atlantic Ridge), which is of major interest in pursuit of this research work and the search for new piezo-hyperthemophiles.
The samples are taken aboard at atmospheric pressure and conditioned according to their tolerance to oxygen and are then replaced in bottles pressurized at the equivalent value to that of the sample site. They are then stored at 4°C in view of future culture in the lab.
The samples will then again be depressurized and used to inoculate selective culture mediums for different groups of microorganisms and grown in different temperature and pressure conditions. The microbial diversity present at the beginning of culture is analyzed. At the end of the experiment, one or several of the new strains will be isolated and described.
What may be the practical interest of these microorganisms?
Given the extreme development conditions, it is proven that their enzymes are thermostable and apt to function in reactors under pressure. It is therefore tempting to use them in industrial processes which require at some steps high temperature and pressure conditions notably to obtain high-value products derived from fine chemistry.