Halohasta litchfieldiaerepresents ∼ 44% andHalorubrum lacusprofundi∼ 10% of the hypersaline, perennially cold (≥ −20°C) Deep Lake community in Antarctica. We used proteomics and microscopy to define physiological responses of these haloarchaea to growth at high (30°C) and low (10 and 4°C) temperatures. The proteomic data indicate that both species responded to low temperature by modifying their cell envelope including protein N-glycosylation, maintaining osmotic balance and translation initiation, and modifying RNA turnover and tRNA modification. Distinctions between the two species included DNA protection and repair strategies (e.g. roles of UspA and Rad50), and metabolism of glycerol and pyruvate. ForHrr. lacusprofundi, low temperature led to the formation of polyhydroxyalkanoate-like granules, with granule formation occurring by an unknown mechanism.Hrr. lacusprofundialso formed biofilms and synthesized high levels of Hsp20 chaperones.Hht. litchfieldiaewas characterized by an active CRISPR system, and elevated levels of the core gene expression machinery, which contrasted markedly to the decreased levels ofHrr. lacusprofundi. These findings greatly expand the understanding of cellular mechanisms of cold adaptation in psychrophilic archaea, and provide insight into howHht. litchfieldiaegains dominance in Deep Lake.