© Ione Padilla Aramburuzabala
Cold-adapted microorganisms are inhabitants of the Earth long time ago. The Earth is about 4.6 billion years old and the fossilized remains of cells in ancient rocks that we find nowadays are dated to be 3.5-3.86 years old. Due to the extreme environmental conditions during the history of Earth, such as long glacial periods, microbial life has evolved specific adaptations in order to survive these hostile environments. Today, Earth is primarily a cold planet, since 90% of the ocean’s waters are at 5ºC or lower including the deep ocean, frozen soils, terrestrial glaciers, perennially ice-covered lakes, and polar sea ice and ice sheets . These cold environments are part of the cryosphere, which is the portion of the Earth where water exists in the solid phase as snow or ice, and are indeed called cryoenvironments which are generally defined as environments that exist continuously and predominantly at subzero temperatures. Cryoenvironments consist mainly of permafrost, glaciers, sea ice, cold lakes and ponds, and subzero saline springs.
Generally, cold-adapted microbes are termed psychrophiles (cold-loving). Psychrophiles are defined as organisms having an optimal temperature for growth at about 15ºC or lower, a maximal temperature for growth at about 20º C, and a minimal temperature for growth at 0ºC or below. The temperature limit for microbial reproduction is considered to be -12ºC and that for metabolism -20ºC. However, psychrophiles were reported to perform metabolic activities at -20ºC under particular conditions. However, Fellet G. and Gerday C. claim that the widely used definition proposed by Morita is not entirely right and they give three strong reasons for it. On the one hand, the temperature limits in this Morita´s definition were randomly chosen so that they do not correspond to real studied temperature limits. On the other hand, it is not possible to apply this definition to eukaryotes. In addition, it is not feasible the use growth rates in order to define the optimum growth temperature. There are other more precise definitions ranging from moderate psychrophiles (with a minimum and maximum growth temperature at or below 0ºC and 25ºC respectively), to psychro-active (organisms growing at or below -1ºC) passing through the terms eurypsychrophile (they can grow from -10ºC up to 30ºC) and stenopsychrophile (do not tolerate temperatures higher than 25ºC). When the knowledge of cold adapted microorganisms will have increased, a more precise definition of it will arise.
Organisms respond to environmental changes through adaptations, which improves their performance and its reproductive fitness. These adaptations, which occur over a range of timescales, have the aim of maximizing the match between the environment and the organism. For the immediate challenges there are the homoestatic mechanisms which are immediate and reversible; if the challenges continue through a longer period, from several days to years, developmental plasticity comes into action, where the organism tries to optimize its life-course strategy for maximum fitness, aided by environmental cues. This way, the organism uses current environmental conditions in order to be prepared for the future environment. Lastly, if the challenge continues for generations natural selection occurs. In the case of cold-adapted microorganisms the change must be at genetic level and long-lasting.
Not only do the psychrophiles suffer from a lack of liquid water, they also have to cope with desiccation, low nutrients or high osmolarity. However the main constraint is the low temperatures which can influence the response of such microorganisms either directly (growth rate, cold-adapted enzymes and proteins, conformational changes of proteins, unique cell membrane constituents) or indirectly (solubility of solute molecules, ion transport and diffusion, osmotic effects on membranes, surface tension, and density).
The aim of this review is to summarize the available literature about microbial adaptations to survive subzero temperatures. A lot of research has been done at above zero temperatures, which might be due to its biotechnological applications, yet there is not much at below zero temperature. Having said this, let us explore life on the edge: adaptations of psychrophiles to cryoenvironments.
- BIODIVERSITY OF PSYCHROPHILES IN CRYOENVIRONMENTS
- ADAPTATIONS OF PSYCHROPHILES TO CRYOENVIRONMENTS
3.1. Growth rate
3.2. Membrane fluidity
3.2.1. Cold sensing signaling and membrane stability at low temperatures
3.2.2. Thermal changes in lipid fatty acyl composition of membrane lipids
- Branched-chain fatty acids
- Fatty acid chain length
- Membrane proteins
- Sterols, hopanoids, and carotenoids
3.3. Antifreeze proteins and cryoprotectants
3.3.1. Freeze avoidance and freeze tolerance
3.3.2. Ice-binding proteins
3.3.3. Antifreeze proteins
3.3.4. Ice-nucleating proteins
3.3.5. Extracellular polysaccharide substances
3.4. Cold-shock response
3.5. Cold adapted proteins and enzymes
3.5.1. Protein synsthesis
3.5.2. Low temperature challenge
3.5.3. Activity of cold-adapted enzymes
3.5.4. Flexibility of cold-adapted enzymes