Provided herein are genetically engineered archaea. A genetically engineered archaea includes a heterologous polynucleotide that has a promoter operably linked to a coding region, where the coding region encodes a polypeptide having optimal activity below the optimum growth temperature (T.sub.opt) of the genetically engineered archaeon. Also provided herein are methods for using genetically engineered archaea and cell-free extracts of such genetically engineered archaea. In one embodiment, the methods include culturing a genetically engineered archaeon at a temperature that is at least 20.degree. C. below the T.sub.opt of the genetically engineered archaeon, such that the activity in the genetically engineered archaeon of a polypeptide encoded by the coding region is increased compared to the activity in the genetically engineered archaeon of the polypeptide during growth at a second temperature that is at or near the T.sub.opt of the genetically engineered archaeon.
 The present invention was made with government support under Grant No. DE-PS02-06ER64304 and Grant No. DE-AR0000081, each awarded by the Department of Energy, and under Grant No. BES-0617272, awarded by the National Science Foundation. The Government has certain rights in this invention.