Origin, formation and environmental significance of des-A-arborenes in the sediments of an East African crater lake

Non-hopanoid pentacyclic triterpenoids occur widely in lake sediments, but their biological sources and diagenetic pathways are not fully resolved. We tentatively identified a number of des-A-arborene isomers occurring in relatively high abundance in the 25,000 year (25 kyr) sedimentary record of Lake Chala, a deep crater lake in tropical East Africa. The mono-, di- and tri-unsaturated des-A-arborenes are transformation products of isoarborinol/arborinone. These precursors have an aquatic source and are most likely biosynthesized by algae or aerobic bacteria in the epilimnion. The relatively depleted δ¹³C values (on average −32.3‰ ± 1.3‰) of des-A-arbor-9(11)-ene are consistent with an aquatic source (algae or aerobic bacteria). In general, isoarborinol and its microbially induced transformation products are found in present and ancient tropical lacustrine settings (typically crater lakes) with permanently anoxic bottom waters and sediments. Based on molecular mechanics calculations of des-A-arborenes, it is clear that these transformation products are not in thermodynamic equilibrium, strongly indicating that their formation is microbially mediated. Subtle temporal and spatial differences in the microbial community might therefore not only dictate the variable relative contributions of different des-A-arborenes found in the sediments of Lake Chala over the last 25 kyr, but also explain the distribution of arborane derivatives in comparable crater-lake settings elsewhere.