Molecular organic tracers of biogeochemical processes in a saline meromictic lake (Ace Lake)

The chemical structures, distribution and stable carbon isotopic compositions of lipids in a sediment core taken in meromictic Ace Lake (Antarctica) were analyzed to trace past biogeochemical cycling. Biomarkers from methanogenic archaea, methanotrophic bacteria and photosynthetic green sulfur bacteria were unambiguously assigned using organic geochemical understanding and by reference to what is known about the lake's present-day ecosystem. For instance, saturated and unsaturated 2,6,10,15,19-pentamethylicosane, archaeol and sn2-hydroxyarchaeol were derived from methanogenic archaea. Carotenoid analysis revealed chlorobactene and isorenieratene derived from the green-colored and brown-colored strains of the,green sulfur bacteria (Chlorobiaceae): isotopic analyses showed that they were C-13-enriched. Phytenes appear to be derived from photoautotrophs that use the Calvin-Benson cycle, while phytane has a different source possibly within the archaea. The most C-13-depleted compounds (ca. -55 parts per thousand) identified were 4-melhyl-5acholest-8(14)-en-3 beta -ol, identified using an authentic standard, and co-occurring -4-methylsteradienes: these originate from the aerobic methanotrophic bacterium Methylosphaera hansonii. Lipids of photoautotrophic origin, steranes and alkenones, are relatively depleted (ca.-28 to -36 parts per thousand) whilst archaeal biomarkers are relatively enriched in C-13 (ca. -17 to -25 parts per thousand). The structural and carbon isotope details of sedimentary lipids thus revealed aspects of in situ biogeochemical processes such as methane generation and oxidation and phototrophic sulfide oxidation. Copyright (C) 2001 Elsetier Science Ltd.