Mono- and bicyclic squalene derivatives as potential proxies for anaerobic photosynthesis in lacustrine sulfur-rich sediments

Two novel triterpenoid biomarkers have been isolated from a recent anoxic sulfur-rich sediment (Lake Cadagno, Switzerland) and identified by NMR as 11,15-dihydro-Δ 13,10(14)-cyclosqualene 1 and bis-(6,11-cyclofarnesa-2,7(14)-diene) 2. Both compounds have a novel hydrocarbon skeleton not known thus far in living organisms and formed by cyclization of squalene, likely through an enzymatic process. Regarding their origin, the 13C-depleted values of their stable carbon isotopic composition suggest a derivation either from purple sulfur photosynthetic bacteria or from anaerobic heterotrophic organisms feeding on the 13C-depleted biomass fixed by photosynthetic bacteria. Therefore, these novel triterpenoids may represent potential proxies for photic zone anoxia in lacustrine settings.