Coping with Our Cold Planet

Abstract

Of all the natural stress conditions on our planet and in our solar system, cold is arguably the most widespread, at least from the perspective of mesophilic and thermophilic organisms. For instance, 90% of the Earth's oceans have a temperature of 5°C or less. When terrestrial habitats are included, over 80% of the Earth's biosphere is permanently cold (149). Among terrestrial environments, 85% of Alaska, 55% of Russia and Canada, 20% of China, and the majority of Antarctica are permanently cold (136). Furthermore, six of the other eight planets of our solar system are permanently cold, and hence understanding life's adaptations to cold environments on our planet should be useful in the search for and understanding of life on other planets. Indeed, the current debate about whether life exists in Lake Vostok or on other planets is rooted in understanding what we know about microbial life in cold environments (83; C. Zimmer, New York Times, 6 July 2007).Since water is the major constituent of life, the manner in which its physical properties depend on temperature is fundamental to understanding the mechanisms that govern cold tolerance and acclimation. A number of Earth's microorganisms have the ability to cope with low temperatures, which makes them among Earth's most successful colonizers (62,63). In this review we summarize how physical properties of water at cold temperatures affect the physiology of microorganisms and we focus on the molecular mechanisms revealed by recent biochemical and genetic studies that shed light on microbial adaptations to cold.Effects of cold on physical properties of water and on biochemical reactions. Properties of water have a profound influence on the physical and chemical processes that are essential for life (51). Water is the only chemical compound that occurs naturally in solid, liquid, and vapor phases and the only naturally occurring inorganic liquid on Earth (51). The water molecule consists of two hydrogen atoms and one oxygen atom. The HOH angle is 104.5 o , and the intramolecular OH distance is 0.957 Å (37). A strong, directional attraction occurs between an oxygen atom and a hydrogen atom belonging to a different water molecule, i.e., a hydrogen bond (168). At temperatures below 4°C, hydrogen bonds force energy and volume, as well as entropy and volume, to be negatively correlated, while the correlations are positive for most liquids. The negative correlation between entropy and volume is a consequence of the formation of an open hydrogen-bound network in which a decrease in orientation entropy is accompanied by an increase in volume (37). This tendency of water molecules to attract each other strongly through hydrogen bonds is responsible for the unusual properties of water (37).In a subzero environment, water can be supercooled or frozen (52). Supercooling is not accompanied by changes in the concentrations of water-soluble substances, except in rare cases where such substances might be present at near-saturation concentrations at normal temperatures and precipitate ...

Structural flexibility of the heme cavity in the cold‐adapted truncated hemoglobin from the Antarctic marine bacterium Pseudoalteromonas haloplanktis TAC125

“…The ability of an organism to survive and grow in the cold depends on a number of adaptive strategies, finalized to maintaining vital cellular functions at low temperatures [49]. In addition to adaptations at the cellular level, to preserve their functions proteins must maintain a balance between conformational stability and flexibility under the selected environmental conditions.…”

Protection ofBacillus subtilisagainst Cold Stress via Compatible-Solute Acquisition

“…Most of the Earth's biosphere is cold (53), and a large number of Bacteria and Archaea have specifically adapted the functioning of their proteins, their systems for protein folding, their biosynthetic machinery, their genetic apparatus, and their entire physiology to a life in permanently cold environments (26,55,58). B. subtilis does not belong to this group of psychrophilic microorganisms; it is a typical mesophile that is temporarily exposed, either suddenly (5) or on a more continued basis (15), to low temperature surroundings in its various habitats.…”

Stepwise Adaptations to Low Temperature as Revealed by Multiple Mutants of Psychrophilic α-Amylase from Antarctic Bacterium

“…Among the numerous adverse effects of low temperature on the cell (1,5,6), the major constraint is exerted on enzyme activity, which is exponentially reduced by temperature decreases. In psychrophiles, this constraint is alleviated by the synthesis of cold-active enzymes to maintain metabolic fluxes compatible with life.…”