There are many mechanisms involved, but the main are increased stability of proteins and cell membranes. For instance, archaeal membranes consist of lipids containing ether bonds, whereas most of bacterial and eucaryotic membranes contain lipids with ester bonds. Lipid membranes with majority of ether-containing lipids can stay in liquid-crystal state at a wider range of temperatures, and in helps the archaea survive in such conditions.
There are many mechanisms involved, but the main are increased stability of proteins and cell membranes. For instance, archaeal membranes consist of lipids containing ether bonds, whereas most of bacterial and eucaryotic membranes contain lipids with ester bonds. Lipid membranes with majority of ether-containing lipids can stay in liquid-crystal state at a wider range of temperatures, and in helps the archaea survive in such conditions.
Thermophilic bacteria have adaptation of all key mechanisms
1) DNA and mRNA adaptation (high GC, codone adaptation, different protein-binding properties)
Nakashima H, Fukuchi S, Nishikawa K. Compositional changes in RNA, DNA and proteins for bacterial adaptation to higher and lower temperatures. J Biochem.
2003 Apr;133(4):507-13.
Fujita M, Kanehisa M. Comparative analysis of DNA-binding proteins between
thermophilic and mesophilic bacteria. Genome Inform. 2005;16(1):174-81.
2) tRNA structure and protein synthesis mechanism adaptation
Dutta A, Chaudhuri K. Analysis of tRNA composition and folding in
psychrophilic, mesophilic and thermophilic genomes: indications for thermal
adaptation. FEMS Microbiol Lett. 2010 Apr;305(2):100-8. doi:
10.1111/j.1574-6968.2010.01922.x.
Gruber T, Köhrer C, Lung B, Shcherbakov D, Piendl W. Affinity of ribosomal
protein S8 from mesophilic and (hyper)thermophilic archaea and bacteria for 16S
rRNA correlates with the growth temperatures of the organisms. FEBS Lett. 2003
Aug 14;549(1-3):123-8.
3) enzymes
Fukuchi S, Nishikawa K. Protein surface amino acid compositions distinctively
differ between thermophilic and mesophilic bacteria. J Mol Biol. 2001 Jun
15;309(4):835-43. PubMed PMID: 11399062.
4) structural lipids and proteins
Yosuke KogaThermal Adaptation of the Archaeal and Bacterial Lipid Membranes Archaea Volume 2012 (2012), http://dx.doi.org/10.1155/2012/789652
5) Other mechanisms (secretion systems, recombination, DNA uptake, secondary methabolism).