Catalyzes the addition of molecular CO(2) and H(2)O to ribulose 1,5-bisphosphate (RuBP), generating two molecules of 3-phosphoglycerate (3-PGA) (PubMed:9988755, PubMed:10512715). Functions in an archaeal AMP degradation pathway, together with AMP phosphorylase and R15P isomerase (PubMed:17303759). 2 (2R)-3-phosphoglycerate + 2 H(+) = CO2 + D-ribulose 1,5-bisphosphate + H2O D-ribulose 1,5-bisphosphate + O2 = (2R)-3-phosphoglycerate + 2-phosphoglycolate + 2 H(+) Binds 1 Mg(2+) ion per subunit. Optimum temperature is 90 degrees Celsius (PubMed:9988755). Highly thermostable, has a half-life of 220 minutes at 90 degrees Celsius (PubMed:9988755). Homodecamer, consisting of five dimer units which form a ring-like pentagonal structure. This arrangement is essential for its high thermostability (PubMed:12070156). In contrast to form I RuBisCO, the form III RuBisCO is composed solely of large subunits (PubMed:9988755). Up-regulated by nucleosides (at protein level). Cells lacking this gene show a slight decrease in growth as compared to the wild-type when they are grown in nutrient-rich medium. Because the Archaea possessing a type III RuBisCO are all anaerobic, it is most likely that only the carboxylase activity of RuBisCO, and not the competitive oxygenase activity (by which RuBP reacts with O(2) to form one molecule of 3-phosphoglycerate and one molecule of 2-phosphoglycolate), is biologically relevant in these strains. Belongs to the RuBisCO large chain family. Type III subfamily.