Impact of silica on archaeal isoprenoid hydrocarbons and bacterial n-tricosenes under experimental hydrothermal conditions

Archaea-derived isoprenoid hydrocarbons appear to have a limited preservation potential in deposits from hydrothermal silica-rich environments (e.g. hot springs). Here we conducted closed-system maturation experiments at 250 °C and 300 bar for 14 days to simulate the diagenetic alteration of specific archaeal isoprenoid hydrocarbons (crocetane, phytane, and pentamethylicosane (PMI)) and bacterial linear hydrocarbons ( n -tricosenes, mainly n -tricos-10( Z )-ene). The experiments were performed both with and without silica, using a microbial mat lipid extract containing these hydrocarbon biomarkers. Products were analyzed using gas chromatography–mass spectrometry (GC–MS), and compound-specific δ 13 C measurements were used to verify precursor-product relationships. n -Tricos-10( Z )-ene was mainly converted to n -tricos-10( E )-ene and n -tricosane, whereas archaeal 2,6,10,15,19-pentamethylicosenes (PMI∆) partially hydrogenated to saturated PMI, both in the absence and in the presence of silica. Additionally, phytenes, phytadienes, PMI∆, and octahydrosqualene (C 20 , C 25 , and C 30 ) underwent partial transformation into isoprenoid thiophenes (isoTPs), likely via reactions with elemental sulfur present in the microbial mat extract. Semi-quantitative results indicate that the archaea-derived crocetane, phytane, and PMI skeletons are more susceptible to degradation in the presence of silica than n -tricosane and n -tricosene derivatives. This enhanced breakdown is possibly driven by hydroxyl radicals generated from silica-fluid interactions, offering a mechanism for the scarcity of archaeal isoprenoid hydrocarbons in ancient hydrothermal silica-rich deposits. Conversely, the detection of isoTPs in the presence of silica implies that sulfurization could improve the preservation of some archaeal isoprenoid carbon skeletons in sulfur-rich settings.