Dependence on epiphytic bacteria for freezing protection in an Antarctic moss, Bryum argenteum

Mosses are the dominant flora of Antarctica, but their mechanisms of survival in the face of extreme low temperatures are poorly understood. A variety of Bryum argenteum from 77° S was previously shown to have strong ice-pitting activity, a sign of the presence of ice-binding proteins (IBPs) that mitigate freezing damage. Here, using samples that had been stored at -25(o) C for 10 years, it is shown that much if not all of the activity is due to bacterial ice-binding proteins secreted on the leaves of the moss. Sequencing of the leaf metagenome revealed the presence of hundreds of genes from a variety of bacteria (mostly Actinobacteria and Bacteroidetes) that encode a domain (DUF3494) that is associated with ice binding. The frequency of occurrence of this domain is one to two orders of magnitude higher than it is in representative mesophilic bacterial metagenomes. Genes encoding 42 bacterial IBPs with N-terminal secretion signals were assembled. There appears to be a commensal relationship in which the moss provides sustenance to the bacteria in return for freezing protection.