A series of novel non-isoprenoid macrocyclic glycerol diethers (McDGs) has been identified in authigenic carbonates formed from marine evaporites in Spain (Lorca Basin) and Egypt (Gemsa). They contain a C32 diabolic acid-derived alkyl moiety (i.e. 15,16-dimethyltriacontane), with some components bearing additional methyl branching at C-7 and/or C-24, and show a similar distribution at each sampling site. These McGDs are suggested to have been synthesized by some extremophilic (thermophilic and/or halophilic) bacteria. Compound specific δ13C data indicate distinct carbon cycling by bacteria and archaea at both sites.