When we think of extremophiles, organisms adapted to extreme environments, prokaryotes come to mind first. However, the unicellular red micro-alga Galdieria sulphuraria (Cyanidiales) is a eukaryote that can represent up to 90% of the biomass in extreme habitats such as hot sulfur springs with pH values of 0-4 and temperatures of up to 56 degrees C. This red alga thrives autotrophically as well as heterotrophically on more than 50 different carbon sources, including a number of rare sugars and sugar alcohols. This biochemical versatility suggests a large repertoire of metabolic enzymes, rivaled by few organisms and a potentially rich source of thermo-stable enzymes for biotechnology. The temperatures under which this organism carries out photosynthesis are at the high end of the range for this process, making G. sulphuraria a valuable model for physical studies on the photosynthetic apparatus. In addition, the gene sequences of this living fossil reveal much about the evolution of modern eukaryotes. Finally, the alga tolerates high concentrations of toxic metal ions such as cadmium, mercury, aluminum, and nickel, suggesting potential application in bioremediation. To begin to explore the unique biology of G. sulphuraria , 5270 expressed sequence tags from two different cDNA libraries have been sequenced and annotated. Particular emphasis has been placed on the reconstruction of metabolic pathways present in this organism. For example, we provide evidence for (i) a complete pathway for lipid A biosynthesis; (ii) export of triose-phosphates from rhodoplasts; (iii) and absence of eukaryotic hexokinases. Sequence data and additional information are available at http://genomics.msu.edu/galdieria.