Adaptations for Life in Extreme Environments

Much life exists in locations of extreme conditions on Earth having very high or very low temperatures, high pressure, extremely alkaline or acidic conditions, high radiation and lacking sufficient energy resources (oligotrophic). There are cellular adaptations required in order to thrive in such environments. Some organisms even require these extreme conditions to survive. The cullular mechanisms are highly susceptible to changes in temperature, pressure, pH and salinity. Studying how life survives in such conditions will also be useful for the search for life on other planets.

The organisms living in extreme conditions are known as extremophiles. They have been found 6.7 km below the Earth’s crust, 10 km inside the ocean at pressures of 110 MPa, at 0 pH and 12.8 pH, and in hydrothermal vents at 122°C to frozen seawater at -20°C. They have evolved strong mechanisms to maintain homeostasis. Extremophiles are classified on the basis of the conditions in which they grow:

Thermophiles and hyperthermophiles grow at extremely high temperatures. Proteins form thermophiles exhibit higher proportions of hydrophobic residues or ion pairs. They exhibit increased global stability of their native, active states compared to their unfolded, inactive states (ΔGu). Psychrophiles grow at low temperatures. Their proteins have evolved to maintain dynamic motion even in the cold. They have more overall negative charge, smalled amino acids and more surface hydrophobic residues. Acidophiles and alkaliphiles have adapted to acidic or basic environments respectively. Piezophiles grow under high pressure conditions through the modulation of membrane fluidity. High pressure has a similar effect on lipid membranes as decreased temperature. Piezophilic microorganisms compensate for this effect by fluidizing their membranes further by increasing the proportion on unsaturated fatty acids within their membranes. Deep sea organisms increase their levels of trimethylamine-N-oxide with increasing depth to compensate for the cellular effects of high pressure. Halophiles require NaCl for growth. They must maintain high levels of intracellular ions like K+ and have proteins exhibiting a strong preference for surface-exposed acidic residues, particularly aspartate. Moreover, these organisms are usually polyextremophiles, that is, they have adapted to live in habitats having various extreme factors at the same time.

Extremophilic organisms require extreme conditions to survive whereas extremotolerant organisms tolerate these conditions and grow optimally under normal conditions. Most extremophiles are microorganisms (bacteria and archaea) but some are eukaryotes like algae and fungi as well.

Archaea: This group constitutes the majority of extremophiles. They are some of the most hyperthermophilic organisms (eg. Methanopyrus kandleri) and acidophilic organisms (eg. Picrophilus) known. Several lineages diversified from moderately acidic environments into more acidic environments, owing to acquisition of metabolic pathways and enzymes that allowed cells to integrate oxygen into their energy metabolism and maintain cytoplasmic osmotic balance and an an ability to synthesize more rigid isoprenoid lipid structures that decrease the potential for proton permeation into the cytoplasm. It has also been suggested that acidophiles and their aerobic sulphur oxidation generated acidic hot spring environments, allowing these cells to evolve alongside the progressive acidification of their habitats.

Bacteria: In this group, cyanobacteria are best adapted for extreme conditions like Antarctic ice and hot springs. They can also tolerate high metallic concentrations and xerophilic conditions (less availability of water). They are rarely found in areas having a pH lower than 5 or 6.

Eukaryotes: Apart from hyperthermophilic conditions, eukaryotes adapt well to other extreme habitats. For example, the microscopic invertebrate tardigrade can survive temperatures as low as -272°C, as high at 151°C, a pressure of up to 6000 atm and X-ray exposure in its hibernation state.

The adaptations of extremophiles provide new insights to the fundamental nature of biological processes, like the biochemical limits to macromolecular stability and the genetic instructions required for macromolecules stable in extreme conditions. Their enzymes remain active even under extreme temperature and solvent conditions, making them a subject of interest for biotechnological purposes. Hyperthermophiles also lie close to the ‘universal ancestor’ of all organisms on Earth, making them critical in evolutionary studies related to the origin of life and astrobiology.