Psychrophilic (cold-adapted) microorganisms make a major contribution to Earth's biomass and perform critical roles in global biogeochemical cycles. The vast extent and environmental diversity of Earth's cold biosphere has selected for equally diverse microbial assemblages that can include archaea, bacteria, eucarya, and viruses. Underpinning the important ecological roles of psychrophiles are exquisite mechanisms of physiological adaptation. Evolution has also selected for cold-active traits at the level of molecular adaptation, and enzymes from psychrophiles are characterized by specific structural, functional, and stability properties. These characteristics of enzymes from psychrophiles not only manifest in efficient low-temperature activity, but also result in a flexible protein structure that enables biocatalysis in nonaqueous solvents. In this review, we examine the ecology of Antarctic psychrophiles, physiological adaptation of psychrophiles, and properties of cold-adapted proteins, and we provide a view of how these characteristics inform studies of astrobiology.