Novel Extended Tetraether Lipids Found in a High-CO₂ Geyser

The growing research into the archaeal lipidome has uncovered a remarkable structural diversity in isoprenoidal glycerol dialkyl glycerol tetraethers (iGDGTs) and revealed complex membrane adaptations, especially in extreme environments. We performed a comprehensive analysis of the lipidome from the subsurface aquifer of the CO₂-rich, cold-water Geyser Andernach (Germany), using ultra-high-resolution mass spectrometry. We detected iGDGT-0, presumably derived from the dominant community member Candidatus Altiarchaeum, providing supporting evidence for its ability to synthesise tetraethers, as previously predicted from metagenomic data. Beyond the typical iGDGT-0 and acyclic glycerol trialkyl glycerol tetraether (iGTGT-0), we discovered novel structural derivatives, here referred to as extended iGDGTs and iGTGTs, characterised by the asymmetrical addition of up to two isoprenoid units to only one of their hydrocarbon side chains, analogous to those found in extended archaeols. The apparent absence of GDGT ring synthase A and B genes in the corresponding metagenome-assembled genome raises the possibility that the producing archaea may utilise extended iGDGTs as a membrane adaptation to cope with the nutrient-depleted conditions of the geyser environment, highlighting the adaptive flexibility of archaea to extreme physicochemical conditions.