PSYCHROPHILES: CHARACTERISTICS, TYPES AND EXAMPLES - BIOLOGY - 2024

The psychrophilic are a subtype of extremophiles characterized by low withstand temperatures, usually between -20 ° C and 10 ° C, and permanently occupy cold habitats. These organisms are usually bacteria or archaea, however there are metazoans such as lichens, algae, fungi, nematodes and even insects and vertebrate animals.

Cold environments dominate the Earth's biosphere and are colonized by abundant and diverse microorganisms that play potentially critical roles in global biogeochemical cycles.

Lichen Xanthoria elegans is a well-known psychrophile that can photosynthesize at temperatures as low as -24 ° C. Photograph taken in Alberta, Canada. Source: Jason Hollinger via

In addition to withstanding low temperatures, psychrophilic organisms must also be adapted to other extreme conditions, such as high pressures, high concentrations of salts, and high ultraviolet radiation.

Characteristics of psychrophilic organisms

Habitats

The main habitats of psychrophilic organisms are:

-Polar marine environments.

-Bank or sea ice.

-Polar terrestrial environments.

-Lakes of high altitude and latitude.

-Subglacial lakes.

-Cold alpine regions.

-Surfaces of glaciers.

-Polar deserts.

-Deep ocean.

Adaptations

Psychrophiles are protected from freezing by various adaptations. One of them is the flexibility of their cell membranes, which they achieve by including a high content of short and unsaturated fatty acids in the structures of their lipid membranes.

The effect of the incorporation of these fatty acids is to decrease the melting point, increasing at the same time its fluidity and resistance.

Another important adaptation of psychrophiles is the synthesis of antifreeze proteins. These proteins keep body water in a liquid state and protect DNA when temperatures drop below the freezing point of water. They also prevent ice formation or recrystallization from occurring.

Types of psychrophiles and examples

Unicellular organisms

The diversity of unicellular psychrophiles is very large, among these we can mention members of most bacterial lineages: Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, Cianobacteria, Firmicutes, Gemmatimonadetes, OP10 and Planctomycetes.

Furthermore, Proteobacteria and Verrucomicrobia have been detected in the Arctic, Antarctic and in Alpine cryocones. They have also been detected in Greenland, Canada, Tibet, and the Himalayas.

Among the psychrophilic cyanobacteria we find Leptolvngbva, Phormidium and Nostoc. Other common genera are the unicellular Aphanothece, Chroococcus, and Charnaesiphon, and the filamentous Oscillatoria, Microcoleus, Schizothrix, Anabaena, Calothrix, Crinalium, and Plectonerna.

Multicellular organisms

Among the psychrophilic insects we can name the genus Diamesa from the Himalayas (Nepal), which remains active until it reaches a temperature of -16 ° C.

Also found is the wingless mosquito, Belgica Antarctica, 2–6 mm long, endemic to Antarctica. This is the only insect on the continent and also the only exclusively terrestrial animal.

Figure 2. The apteric mosquito Belgica antarctica, an endemic insect of Antarctica. Source: Tasteofcrayons, from Wikimedia Commons

Vertebrate animals can also be psychrophiles. Some examples include a small number of frogs, turtles, and a snake that use extracellular water freezing (water outside cells) as a survival strategy to protect their cells during the winter.

The Antarctic nematode Panagrolaimus davidi can survive intracellular water freezing and subsequently regrow and reproduce.

Also the fish of the Channichthyidae family - which live in the cold waters of Antarctica and southern South America - use antifreeze proteins to protect their cells against complete freezing.

Growth temperatures and psychrophilic organisms

The maximum growth temperature (T _max) of an organism is the highest that it can tolerate. While the optimal temperature (T _opt) for growth is the one in which the organism grows faster.

All organisms that survive and thrive in low-temperature environments are generally considered to be psychrophiles. However, as we know, the term psychrophilic should only be applied to those organisms whose T _max is 20 ° C (that is, they cannot survive at higher temperatures).

Microorganisms have been isolated from very cold areas, which can grow in laboratory conditions at temperatures above 20 ° C, which indicates that although they are adapted to low temperatures, they should not be considered psychrophiles. These microorganisms are called "mesotolerant", that is, they tolerate medium temperatures.

Methanococcoides burtonii

Sphingopyxis alaskensis

Sphingopyxis alaskensis is a bacterium isolated from the marine waters of the northern hemisphere, where temperatures of 4 - 10 ° C prevail. On the other hand, haloarchaeas, which are archaea that inhabit waters highly saturated with salt, grow at -20 ° C temperature.

Despite having high populations in their natural habitats, none of these microorganisms could be cultivated in the laboratory at less than 4 ° C.

In turn, S. alaskensis has a T _max of 45 ° C and haloarchaea can grow at temperatures above 30 ° C, so they could not be considered psychrophilic. However, their populations are well adapted and are very abundant in extremely cold areas.

From the above, we can assume that there are other limiting environmental factors that influence the survival of these organisms in their natural habitats, and temperature is not the factor with the greatest weight.

Biotechnological applications

The enzymes of psychrophilic organisms are characterized by high activity at low and moderate temperatures. In addition, these enzymes have poor thermal stability.

Due to these characteristics, the enzymes of psychrophilic organisms are very attractive to be applied in various processes in the food industry, medicine, molecular biology, in the pharmaceutical industry, among others.

References

1. Cavicchioli, R. (2015). On the concept of a psychrophile. The ISME Journal, 10 (4), 793–795. doi: 10.1038 / ismej.2015.160
2. Krembs, C. and Deming, JW (2008). The role of exopolymers in microbial adaptation to sea ice. In: Margesin, R., Schirmer, F., Marx, J.-C. and Gerday, C. reds) Psychrophiles: from Biodiversity to Biotechnology. Springer-Verlag, Berlin, Germany, pp. 247-264.
3. Kohshima, S. (1984). A novel cold-tolerant insect found in a Himalayan glacier. Nature, 310 (5974), 225-227. doi: 10.1038 / 310225a0
4. Margesin, R. (editor). (2017). Psychrophiles: from Biodiversity to Biotechnology. Second edition. Springer Verlag, Heidelberg, Germany. pp. 685.
5. Miteva, V. (2008). Bacteria in snow and ice. In: Margesin, R. and Schirmer, F. (eds) Psychrophiles: from Biodiversity to Biotechnology. Springer Verlag, Heidelberg, Germany, pp. 31-50.
6. Price, PB (2000). A habitat for psychrophiles in deep Antarctic ice. Proceedings of the National Academy of Sciences of the United States of America 97, 1247-1251.