Preservation of archaeal core lipids in siliceous hot spring deposits: An experimental study

Abstract

Siliceous hot spring systems have been present on our planet since at least 3.5 Ga, often hosting diverse communities of Archaea. Isoprenoid core lipids are important biomarkers for these microorganisms, but these lipids have only been reported from modern and subrecent siliceous sinter settings, perhaps indicating a bias against their long-term preservation in such environments. We conducted closed-system maturation experiments with the archaeal core lipids archaeol and isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) at 250 °C and 300 bar for 14 days. Compounds were heated with artificial sea water, with and without silica, as single compounds, and as part of a lipid mixture from a microbial mat. The products were analyzed using gas chromatography-mass spectrometry, scanning electron microscopy, and X-ray diffraction. This allowed to assess the preservation potential of these biomarkers during simulated siliceous sinter formation and subsequent diagenesis. Amorphous silica gel transformed into small microspheres (∼10 µm) when heated with single organic compounds, but formed more irregularly shaped rounded grains when heated with the lipid mixture. The presence of silica led to an enhanced degradation of archaeol, most likely caused by hydroxyl radicals deriving from silica-fluid interactions. IsoGDGTs, in contrast, were less affected by silica and showed an enhanced stability when heated as part of the lipid mixture. The results indicate that (i) the silica matrix, (ii) the molecular structure, and (iii) the presence of other organic compounds play a critical role for the preservation of archaeal core lipids in siliceous hot spring systems.