In general, living beings move in environments whose environmental variables, such as temperature, pH or salinity, remain within relatively small ranges. However, there are exceptions to this generality: they are the organisms called extremophiles, which adapt to a life in which some of these variables is extreme. Thus, there are organisms adapted to extraordinarily acidic or alkaline environments, to highly saline environments or to extreme temperatures.
Each extremophile organism has a series of adaptations that allow it, facilitate it, or sometimes force it to live in these extreme environments. Depending on the environment they receive a different name, and the organisms that live in extraordinarily cold environments, such as a glacier or the ice of Antarctica, are called psychrophiles (from the Greek ψυχρος, ‘psyjros’, cold, and φιλíα, ‘filía’, affection). ; cold lovers).
Ways of living in extreme cold
In Antarctica, the temperature is extremely cold throughout the year. To maintain life in such a cold environment, living beings need strategies that prevent the water that makes up their cells from freezing.
In psychrophilic organisms there are, therefore, adaptations to cold through physiological mechanisms; based mainly on a specific set of proteins that produces significant metabolic differences with respect to mesophilic species —those that live in more benign temperatures—. These proteins are grouped into four types.
Cold shock proteins are expressed immediately as soon as the temperature drops. They are small proteins, involved in the transcription and folding of other proteins. They allow to maintain metabolic activity in low temperature conditions.
Cold acclimation proteins are expressed during prolonged growth exposed to low temperatures. Their function is related to the stabilization of other proteins and the maintenance of their expression, so that they can maintain growth and metabolism.
Antifreeze proteins are responsible for preventing cell material from freezing. The interior of a cell is basically water, and its internal components can easily act as nuclei that promote freezing. These proteins prevent ice nucleation by inhibiting new water molecules from adding to ice crystals. The synthesis of antifreeze proteins is one of the most important strategies to withstand subzero temperatures; it is shared by bacteria, fish, insects, fungi, plants, and most of the living beings that inhabit this type of environment.
The fourth type of proteins involved are called ice-binding proteins , and although their existence and structure are known, their function is still unclear. The most likely hypothesis is that they act as inhibitors of crystallization in the membranes, so that they maintain their fluidity, which is essential for living beings to remain alive.
Fragilariopsis cylindrus , the Antarctic diatom
Under the ice in some areas of Antarctica, liquid water is preserved; the ice cap acts as a shield and under it water can be kept at a few degrees below zero or even a few degrees plus. Even so, the cold remains extreme and permanent in these waters and, for more than 4 months of the year, the light is practically zero .
Diatom algae , such as the species Fragilariopsis cylindrus , live in that dark, frigid environment.
The ability of these Antarctic diatoms to survive prolonged darkness depends on their metabolic capacity. In addition to the adaptations of a psychrophilic organism, the enzymes responsible for the metabolic process become more abundant in the dark. In that dark environment, carbon fixation stops — there is no photosynthesis —, but they continue to live, carrying out an alternative metabolism that allows them to survive in the long term.
On the other hand, by keeping the metabolic processes in the dark phase, F. cylindrus preserves the functionality of the photosynthetic apparatus, ensuring a rapid recovery when, months later, dawn finally arrives and the long daytime period of several months.
Polaromonas vacuolata , the most extreme psychrophilic bacteria
Polaromonas vacuolata is an obligate aerobic bacterium, that is, it can only live in environments where there is no oxygen. It lives under the sea ice, off the coast of the Palmer Peninsula, in Antarctica. It is the first species of beta-proteobacteria that has been isolated from marine habitats.
It is also the bacterium with the highest psychrophilic capacity of which we are aware. It is capable of living and maintaining its metabolism between 0 and 10 °C, with optimum growth in 4 °C . Mesophilic bacteria usually have their optimum between 35 and 38 °C.
Genomic analysis of this bacterium revealed the presence of genes involved in adaptation to high salinity and cold conditions. Apart from the usual proteins, this bacterium produces small gaseous vesicles and cell adhesion proteins.
It has a photoheterotrophic lifestyle: it uses sunlight as an energy source, like plants, but its carbon source is not atmospheric CO₂, but organic matter.
Lichens active at -17 °C
Although the majority of psychrophilic living beings thrive at positive temperatures or a few degrees below zero, and with colder temperatures they enter a state of dormancy or die, some lichens make an extraordinary exception.
Lichens are very particular organisms; they are the result of the symbiotic union between a fungus , which provides the support, an algae or cyanobacteria that carry out photosynthesis feeding the organism and recently it has been known that a yeast also collaborates, which stabilizes the whole.
The native lichens of Antarctica have a high tolerance to freezing stress and a great capacity to recover their metabolic activity after a period of severe and prolonged cold. Its optimum growth temperature is
between 0 and 15 °C; however, they can keep their metabolism active at temperatures as low as -17 °C , and even at a lower temperature, with -20 °C, they still retain part of their metabolic activity, such as the exchange of CO₂.
References:
Bej, A. K. et al. 2009. Polar Microbiology: The Ecology, Biodiversity and Bioremediation Potential of Microorganisms in Extremely Cold Environments. CRC Press.
Hwang, K. et al. 2021. Complete genome of Polaromonas vacuolata KCTC 22033T isolated from beneath Antarctic Sea ice. Marine Genomics, 55, 100790. DOI: 10.1016/j.margen.2020.100790
Kappen, L. et al. 1996. Cold resistance and metabolic activity of lichens below 0°C. Advances in Space Research, 18(12), 119-128. DOI: 10.1016/0273-1177(96)00007-5
Kennedy, F. et al. 2019. Dark metabolism: a molecular insight into how the Antarctic sea-ice diatom Fragilariopsis cylindrus survives long-term darkness. New Phytologist, 223(2), 675-691. DOI: 10.1111/nph.15843
Spribille, T. et al. 2016. Basidiomycete yeasts in the cortex of ascomycete macrolichens. Science, 353(6298), 488. DOI: 10.1126/science.aaf8287
