Involved in the lipid biosynthesis. Catalyzes the formation of unsaturated archaetidylserine from CDP-unsaturated archaeol and L-serine. Activity with ester-linked substrate analogs containing straight aliphatic chains (typical bacterial substrates) is two to three times higher than that with the corresponding ether-type substrate (typical archaeal substrates). Both enantiomers of CDP-unsaturated archaeols with ether-linked geranylgeranyl chains and CDP-saturated archaeol with ether-linked phytanyl chains are similarly active. The enzyme also accepts D-serine, although activity is only about third of that with L-serine. CDP-2,3-bis-O-(geranylgeranyl)-sn-glycerol + L-serine = archaetidylserine + CMP + H(+) CDP-2,3-bis-O-(phytanyl)-sn-glycerol + L-serine = 2,3-bis-O-phytanyl-sn-glycero-3-phospho-L-serine + CMP + H(+) Activated by Mn(2+) ions. Optimum pH is between 8 and 8.5. Optimum temperature is 60 degrees Celsius. Membrane lipid metabolism; glycerophospholipid metabolism. Belongs to the CDP-alcohol phosphatidyltransferase class-I family.