Okenane, a biomarker for purple sulfur bacteria (Chromatiaceae), and other new carotenoid derivatives from the 1640 Ma Barney Creek Formation
Carbonates of the 1640 million years (Ma) old Barney Creek Formation (BCF), McArthur Basin, Australia, contain more than 22 different C 40 carotenoid derivatives including lycopane, γ-carotane, β-carotane, chlorobactane, isorenieratane, β-isorenieratane, renieratane, β-renierapurpurane, renierapurpurane and the monoaromatic carotenoid okenane. These biomarkers extend the geological record of carotenoid derivatives by more than 1000 million years. Okenane is potentially derived from the red-colored aromatic carotenoid okenone. Based on a detailed review of the ecology and physiology of all extant species that are known to contain okenone, we interpret fossil okenane as a biomarker for planktonic purple sulfur bacteria of the family Chromatiaceae. Okenane is strictly a biomarker for anoxic and sulfidic conditions in the presence of light (photic zone euxinia) and indicates an anoxic/oxic transition (temporarily) located at less than 25 m depth and, with a high probability, less than 12 m depth. For the BCF, we also interpret renierapurpurane, renieratane and β-renierapurpurane as biomarkers for Chromatiaceae with a possible contribution of cyanobacterial synechoxanthin to the renierapurpurane pool. Although isorenieratane may, in principle, be derived from actinobacteria, in the BCF these biomarkers almost certainly derive from sulfide-oxidizing phototrophic green sulfur bacteria (Chlorobiaceae). Biological precursors of γ-carotane, β-carotane and lycopane are found among numerous autotrophic and almost all phototrophic organisms in the three domains of life. In the BCF, a paucity of diagnostic eukaryotic steroids suggests that algae were rare and, therefore, that cyanobacterial carotenoids such as β-carotene, echinenone, canthaxanthin and zeaxanthin are the most likely source of observed β-carotane. γ-Carotane may be derived from cyanobacteria, Chlorobiaceae and green non-sulfur bacteria (Chloroflexi), while the most likely biological sources for lycopane in the BCF are carotenoids of the lycopene, rhodopin and spirilloxanthin series abundant in purple sulfur bacteria.