Involved in the biosynthesis of the stress protectant 2-O-alpha-D-mannosyl glycerate (MG) which is produced in response to growth at supraoptimal temperature and salinity, and protects several enzymes against inactivation by temperature, freeze-drying and osmotic stress. Catalyzes the condensation of alpha-GDP-D-mannose (GDP-Man) with D-glycerate to produce alpha-mannosyl-D-glycerate. It is specific for GDP-Man, but it can also use alpha-GDP-D-glucose (GDP-Glc), beta-GDP-D-fructose, alpha-UDP-D-mannose and alpha-UDP-D-glucose as sugar donors. It is specific for D-glycerate, but it can also use D-lactate and glycolate as sugar acceptors. This reaction occurs with a net retention of anomeric configuration; the newly formed glycosidic linkage has the same alpha configuration as the sugar donor. (R)-glycerate + GDP-alpha-D-mannose = (2R)-2-O-(alpha-D-mannosyl)-glycerate + GDP + H(+) Divalent cations such as magnesium, calcium and to a lesser extent manganese, nickel and cobalt. Inhibited by GDP. kcat is 6.5 sec(-1) for mannosylglycerate synthase activity with D-glycerate as sugar acceptor (in the presence of calcium ions at 65 degrees Celsius) (PubMed:15951819). kcat is 6.1 sec(-1) for mannosylglycerate synthase activity with GDP-Man as sugar donor (in the presence of calcium ions at 65 degrees Celsius) (PubMed:15951819). kcat is 1.9 sec(-1) for mannosylglycerate synthase activity with D-glycerate (PubMed:21288903). kcat is 1.1 sec(-1) for mannosylglycerate synthase activity with GDP-Glc as sugar donor (in the presence of calcium ions at 65 degrees Celsius) (PubMed:15951819). kcat is 1.02 sec(-1) for mannosylglycerate synthase activity with GDP-Man and D-glycerate as sugar donor and acceptor, respectively (in the presence of calcium ions at 25 degrees Celsius) (PubMed:15951819). kcat is 0.63 sec(-1) for mannosylglycerate synthase activity with GDP-Glc as sugar donor (in the presence of calcium ions at 25 degrees Celsius) (PubMed:15951819). Optimum pH is between 5.5 and 7. Optimum temperature is between 85 and 90 degrees Celsius. It is stable at high temperatures, however at 90 degrees Celsius the stability of the enzyme is only moderate and 50% of the activity is lost after 30 minutes incubation. At 65 degrees Celsius, the specific enzymatic activity is 5-fold lower than the maximum value. Homotetramer. Dimer of dimers. The biosynthesis of mannosylglycerate in R.marinus can proceed via an other alternative pathway, in wich a mannosyl-3-phosphoglycerate synthase catalyzes the conversion of GDP mannose and D-3-phosphoglycerate into a phosphorylated intermediate, which is subsequently converted to mannosylglycerate by the action of a mannosyl-3-phosphoglycerate phosphatase. A noticeable feature distinguishing the two enzymatic systems involved in the synthesis of MG is their dependence on salt; while the mannosylglycerate synthase reaction is salt-independent, addition of NaCl or KCl is required to achieve full activity of the mannosyl-3-phosphoglycerate synthase/phosphatase system (PubMed:10585410). Belongs to the glycosyltransferase 78 family.