Bacterial Ice Nucleation

Abstract

Publisher Summary This chapter investigates the ability of certain biological systems to initiate physical processes in metastable systems. The best-characterized biological initiators are bacterial ice nuclei that trigger crystallization of ice from supercooled water. The ice-nucleating bacteria are members of plant epiphytic communities. Three mechanisms of ice nucleation are (1) homogeneous ice nucleation, (2) heterogeneous ice nucleation, and (3) secondary ice nucleation. Nucleation activity is calculated from the frequency of freezing observed in multiple, small volumes (often droplets) of a sample diluted in water or a weak buffer so that, at the measurement temperature, some but not all of the replicate volumes freeze. In every microorganism, the Ina + phenotype is the result of expression of a single ice-nucleation ( ina ) gene, to yield a single ice-nucleation (Ina) protein. The genetic basis of the Ina + phenotype, sizes, and locations of the nucleation sites, and the source of the heterogeneity of nucleation-threshold temperatures exhibited by clonal populations of bacteria are discussed. Ice-nucleating bacteria are the chief initiators of frost damage to many economically important crop plants. This non-intuitive effect is the result of the ability of many air-exposed plant parts to supercool to temperatures of -6°C or lower before nucleation sites intrinsic to the plant material become active. The chapter concludes with the applications of bacterial ice nucleation.