Many physical quantities define the domain of life: the intangible region where life can exist. This region can be thought of as an intersection of the various domains of quantities like temperature, pH, pressure, and so on.
What's interesting about the domain of life is that extremities exist. We have life forms that can survive in conditions at the ends of the ranges of the various conditions that define the existence of life. One of these life forms is Picrophilus torridus (P. torridus) of phylum Euryarchaeota. P. torridus, first extracted from moderately high-temperature hydrothermal areas in solfataric fields in Hokkaido, Japan, are known to grow in regions of pH around 0 and temperatures between 47 and 65 degrees Celsius.
Thermoacidophile P. torridus was discovered by Dr. Christa Schleper and her colleagues in 1995 among the wet layers of coal in the acidic and geothermally heated fields of the solfataras. P. torridus is an aerobic life form, hence discovered in the oxygenous upper layers of coal. The acid that gives the solfataric fields their acidic behavior is sulfuric acid, produced from the oxidation of hydrogen sulfide.
The fact that P. torridus, together with Picrophilus oshimae, could grow in these highly acidic and hot environments makes it one of the most thermoacidophilic organisms known to exist. Most organisms cannot grow in these conditions — low intracellular pH values denature protein structures, leading to cell death through reactions between peptide linkages and hydronium ions. High temperatures weaken the hydrogen bonding that gives proteins their structure and functionality.
We've established that acidic and high-temperature environments are destructive to proteins. But how does P. torridus keep its proteins functional in such an environment? Little is known about the complete mechanisms by which these organisms combat their environment. However, P. torridus maintains an intracellular pH of about 4-5 pH units above the extracellular pH.
pH homeostasis — the cell's regulation of cytoplasmic concentration of hydronium ions — is aided by active uptake of sodium and potassium ions. This creates a chemiosmotic potential that extracellular hydronium ions must overcome to enter the cell, effectively keeping the acidity outside.
This hypothesis is supported by the evident use of secondary cation transporters, especially potassium ions, in the sequenced genome of P. torridus. The genome also contains genes that encode for enzymes that degrade organic acids — another pH correction mechanism.
We've explored how P. torridus thrives in low pH regions, but how does it combat high temperatures? The genome of P. torridus offers insights. These microorganisms have the smallest genome among all aerobic non-parasitic microorganisms, with one of the highest coding densities compared to other extremophiles like Thermoplasma acidophilum.
The densely packed genome might account for why these organisms survive high temperatures in their environment. Perhaps the compact structure creates stronger hydrogen bonds that hold protein components together, with shorter bond lengths that can withstand higher temperatures.
The existence of P. torridus is not by mere coincidence. This microorganism evolved under the selective pressure of both low pH and high temperatures. The key features that resulted from these pressures are the very small genome size and its high coding density. This can be deduced by comparing P. torridus with closely related extremophiles like Sulfolobus solfataricus and Thermoplasma volcanii.
The genome contains a large proportion of genes encoding protein transporters, particularly gradient-driven secondary transporters, suggesting adaptation to use the pH difference between the inner cell membrane and extracellular regions. We can conceptualize the phenotype of P. torridus by studying the selective pressures that led to its evolution: low pH and high temperature.
P. torridus is an aerobic, non-parasitic, and thermoacidophilic microorganism that presents one of the most extreme examples of life, stretching the boundaries where life was once thought impossible.