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

The chemical structures, distribution and stable carbon isotopic compositio ns of lipids in a sediment core taken in meromictic Ace Lake (Antarctica) w ere analyzed to trace past biogeochemical cycling. Biomarkers from methanog enic archaea, methanotrophic bacteria and photosynthetic green sulfur bacte ria 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, archae ol and sn2-hydroxyarchaeol were derived from methanogenic archaea. Caroteno id analysis revealed chlorobactene and isorenieratene derived from the gree n-colored and brown-colored strains of the,green sulfur bacteria (Chlorobia ceae): isotopic analyses showed that they were C-13-enriched. Phytenes appe ar to be derived from photoautotrophs that use the Calvin-Benson cycle, whi le phytane has a different source possibly within the archaea. The most C-1 3-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 methano trophic bacterium Methylosphaera hansonii. Lipids of photoautotrophic origi n, 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 o f sedimentary lipids thus revealed aspects of in situ biogeochemical proces ses such as methane generation and oxidation and phototrophic sulfide oxida tion. Copyright (C) 2001 Elsetier Science Ltd.