Next, we studied the effect of Mg 2+ ions on the phase behavior of GDGTs (Figure ). Both the phosphate-containing lipids P2 and P1Glc exhibited strong lamellar ordering at all temperatures in presence of MgCl2. P2 exhibited sharp Bragg peaks at all temperatures with lamellar d-spacing of 4.95 nm at 10 °C which monotonically decreased to 3.93 nm at 90 °C. P1Glc exhibited Bragg peaks corresponding to a mixture of two lamellar phases having lamellar dspacing of 4.67 nm and 4.49 nm at 10 °C which monotonically decreased to 3.99 nm and 3.95 nm respectively at 90 °C. On the other hand, the phosphoglycerol-containing lipids PG2 and PG1Glc exhibited hexagonal phases in the presence of MgCl2. In the case of PG2, a weak and broad hexagonal phase peak appeared at 50 °C and it became more prominent at higher temperatures. For PG1Glc, a poorly resolved broad 1 st HII peak appeared at 10 °C which increased in prominence with temperature, and the 2 nd and 3 rd HII peaks could be unambiguously assigned only above 50 °C. We reason that upon binding of Mg 2+ to the PG headgroup phosphates, they become dehydrated and bridge together. Consequently, the volume of the headgroup region becomes smaller in comparison to the hydrocarbon region and the molecules organize into a hexagonal phase. Interestingly, we also observed Mg 2+ -induced HII phase formation in the monopolar diphytanoyl archaeal lipid DPhPG (which has a negative spontaneous curvature) at all temperatures (Figure ), which supports the idea that bridging of PGheadgroups by Mg 2+ ions is the primary driver of HII phase formation.