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There is no generally accepted value for the lower temperature limit for life on Earth. We present empirical evidence that free-living microbial cells cooling in the presence of external ice will undergo freeze-induced desiccation and a glass transition (vitrification) at a temperature between −10°C and −26°C. In contrast to intracellular freezing, vitrification does not result in death and cells may survive very low temperatures once vitrified. The high internal viscosity following vitrification means that diffusion of oxygen and metabolites is slowed to such an extent that cellular metabolism ceases. The temperature range for intracellular vitrification makes this a process of fundamental ecological significance for free-living microbes. It is only where extracellular ice is not present that cells can continue to metabolise below these temperatures, and water droplets in clouds provide an important example of such a habitat. In multicellular organisms the cells are isolated from ice in the environment, and the major factor dictating how they respond to low temperature is the physical state of the extracellular fluid. Where this fluid freezes, then the cells will dehydrate and vitrify in a manner analogous to free-living microbes. Where the extracellular fluid undercools then cells can continue to metabolise, albeit slowly, to temperatures below the vitrification temperature of free-living microbes. Evidence suggests that these cells do also eventually vitrify, but at lower temperatures that may be below −50°C. Since cells must return to a fluid state to resume metabolism and complete their life cycle, and ice is almost universally present in environments at sub-zero temperatures, we propose that the vitrification temperature represents a general lower thermal limit to life on Earth, though its precise value differs between unicellular (typically above −20°C) and multicellular organisms (typically below −20°C). Few multicellular organisms can, however, complete their life cycle at temperatures below ∼−2°C.

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Large coherent eddies have been observed in turbulent shear layers and seem to play an important role in their growth, mixing and noise production. Winant & Browand (1974) have observed that the pairing of large eddies is central to the question of shear-layer development, and they model the pairing process with discrete line vortices. It is shown here that the growth of the layer and amalgamation of large eddies are not adequately treated by isolated-line-vortex models, which are proved to be essentially non-evolutionary. A more detailed model of the shear layer, itself consisting of vortex elements, is shown to provide the definition required to observe the evolution and coalescence of the large eddies. The observed development of this model shear layer is consistent with many features of experimentally observed flows.