Carbon isotopic composition of isoprenoid tetraether in surface sediments of Lake Qinghai and surrounding soils

Isoprenoid GDGTs (iGDGTs), along with their constituent biphytanyl moieties, are biomarkers for archaea. In order to obtain more information on identifying the carbon source and potential carbon assimilation pathway of archaea in surface sediments of Lake Qinghai and the surrounding soils, the stable carbon isotopic composition of iGDGT-derived biphytanes as well as its relationship with δ13C values of total organic carbon (TOC) and dissolved inorganic carbon (DIC) was investigated. The δ13C values of iGDGT-derived biphytanes ranged from −42.7‰ to −20.3‰. For example, the values for acyclic biphytanes (BP-0), the most abundant, varied from −25.0‰ to −22.1‰ in offshore sediments, −30.5‰ to −25.4‰ in surrounding soils and −42.7‰ to −32.0‰ in nearshore sediments. The wider variation than that of δ13CTOC (−26.9‰ to −25.1‰) in offshore sediments and that in surrounding soils indicated that δ13C values of BP-0 may be a sensitive indicator for examining the depositional environments between terrestrial and lacustrine systems. With average δ13C values of −23.6‰, crenarchaeol-derived biphytanes (BP-cren) in offshore sediments were enriched in 13C (ca. 3.6‰) relative to TOC. In addition, the carbon isotopic fractionation between BP-cren and DIC was −21.3‰, consistent with that reported for marine Thaumarchaeota. The enrichment in 13C relative to TOC and the similar carbon isotopic fractionation indicated that Thaumarchaeota in offshore sediments of Qinghai Lake are autotrophic. With average δ13C values of −29.2‰, BP-cren in surrounding soils was depleted by 0.9‰ to 12.8‰ in 13C relative to DIC. The quite large scatter in carbon isotopic fractionation between BP-cren and DIC indicated that the biomass of soil Thaumarchaeota originated from inorganic and organic carbon sources, suggesting a mixotrophic lifestyle.