Reinterpretation of lycopane as a biomarker of archaea based on its occurrence in authigenic sulfur-bearing carbonates

The tail-to-tail linked C 40 isoprenoid hydrocarbon lycopane is a biomarker found in a wide variety of environments and in some extremophilic archaea but is also a possibly degradation product of a carotenoid (bacterioruberin) as well as plants (various lycopenes) and green algae (lycopadiene). Although potential producers are known, the source of lycopane in water-column and sedimentary environments is commonly ambiguous. In this study, the occurrence of 13 C-depleted lycopane (δ 13 C = −114 to −106 ‰) in authigenic, sulfur-bearing carbonates from Monte Palco (Sicily) is used to re-evaluate the environmental significance of lycopane. In the Monte Palco authigenic carbonates, lycopane is accompanied by other, similarly 13 C-depleted apolar isoprenoids such as PMI, squalane, and sulfurized homologues, including a tail-to-tail linked C 35 isoprenoid, as well as typical lipids of anaerobic methanotrophic archaea (ANME) like glycerol dibiphytanyl glycerol tetraether (GDGT)-0, −2, archaeol, and sn 3-hydroxyarchaeol. The δ 13 C values of lycopane reveal linear correlations with values of PMI and squalane ( n = 7; r 2 = 0.71 and 0.82, respectively), probably reflecting a similar cellular function of the tail-to-tail linked isoprenoids in the archaeal source organisms. Lycopane is interpreted to represent a membrane intercalant, produced in response to extreme environmental conditions. The new observations resulting from the analysis of the Monte Palco authigenic carbonates – including the similarities in 13 C-depletion of PMI, squalane, and lycopane but also with other archaeal membrane lipids like archaeol, sn3 -hydroxyarchaeol, and GDGT-0 (measured as ether-cleaved biphytanes) – allowed us to re-interpret the potential source organism of lycopane in modern and ancient anoxic and euxinic basins (Cariaco Trench, Black Sea, late Cenomanian black shales of the Cape Verde Basin). These new data agree with unknown archaea as producers of lycopane as a response to environmental stress at chemoclines. The previous interpretation of lycopane as a biomarker of photoautotrophic bacteria in anoxic basins is challenged by our new findings. An alternative explanation for these occurrences is lycopane production by Marine Group II Euryarchaeota, archaea closely associated with primary producers. Lycopane production by archaea is probably more widespread than previously recognized. Future research should evaluate the potential of lycopane as a biomarker of archaeal adaptation to extreme environmental conditions. • 13 C-depleted lycopane is a biomarker of methanotrophs or methanogens in diagenetic gypsum replacement carbonate. • Correlation of δ 13 C values of lycopane and other apolar isoprenoids points to a single source. • Lycopane is apparently produced as a response of archaea to extreme conditions. • Re -evaluation of literature data suggest archaea as producers of lycopane. • Lycopane is produced by various Euryarchaeota under challenging environmental conditions.