Molecular aspects of microbial ice nucleation

Abstract

Certain organisms nucleate the crystallization of ice. This requires a small volume of water to be induced, probably by lattice‐matching with a solid template, to form an‘ice embryo’—a region sharing at least some of the characteristics of macroscopic ice. It is of particular interest to understand the structure and function of biological structures capable of lattice‐matching (or otherwise inducing a quasi‐crystalline state). Some strains of the Gram‐negative eubacterial genera Erwinia, Pseudomonas, and Xanthomonas, and the mycobionts of certain lichens, display ice‐nucleating activity. In bacteria, the activity is conferred by a protein that contains three nested periodicities of repetition, which probably reflects a hierarchy of three motifs of structural repetition. Thus the tertiary structure of the ice‐nucleation protein is likely to be regular, consistent with the expectation of its forming a template for lattice‐matching. Even within a clonal culture, the nucleating sites formed by bacteria and lichens vary considerably in the threshold temperatures at which they display activity; this indicates wide variations in either the size of the template, or its structural regularity, or both. However, ice‐nucleating sites of lichen and bacterial origin are clearly differentiated by their sensitivities to experimental treatments.