We report a new series of archaeal lipids, widespread in marine sediments and tentatively assigned as isoprenoid glycerol dialkanol diethers (GDDs). They are structural analogs of isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs), with one glycerol unit missing and with each biphytanyl moiety possessing a terminal OH group. Identification is based on molecular formulae determined from accurate mass measurement and interpretation of mass spectral fragmentation patterns. Acetylation of GDD-0 confirmed the presence of three OH groups, and ether cleavage and reduction of the products afforded two biphytanyl chains. Tests of different protocols for both extraction and acid hydrolysis indicate that GDDs were not formed during sample preparation. The co-existence of GDDs and GDGTs in 12 selected marine sediment samples of varying origin showed that the ring distribution in these two ether lipid pools is related and implies that the two compound classes share a common biological source. The presence of isoprenoid GDDs is possibly linked to the occurrence of biphytane diols in marine sediments, where both GDD and diol can be either biosynthetic intermediates or degradation products, and supports the recently proposed fossil lipid recycling by benthic archaea (Takano, Y., Chikaraishi, Y., Ogawa, O.N., Nomaki, H., Morono, Y., Inagaki, F., Kitazato, H., Hinrichs, K.-U., Ohkouchi, N., 2010. Sedimentary membrane lipids recycled by deep-sea benthic archaea. Nature Geoscience 3, 858–861). GDD core lipid was detected in a culture of Methanothermococcus thermolithotrophicus, suggestive of a potential biological function for these compounds, but no intact polar lipids (IPLs) containing GDDs as core lipids have been detected to date.Highlights► Isoprenoid glycerol dialkanol diethers (GDDs) are widespread in marine sediments. ► They co-occur with corresponding GDGTs. ► Recycling of fossil GDGTs by benthic archaea may produce GDDs. ► GDDs were also found in an archaeal culture, suggestive of a biological origin.

Two types of intact branched glycerol dialkyl glycerol tetraethers (GDGTs) were detected in peat bog samples from Bullenmoor, Northern Germany. Glucuronosyl and glucosyl branched GDGTs comprise on average ca. 4% of the microbial intact polar lipids in the anoxic, acidic peat layer ca. 20 cm below the surface of the bog, suggesting an important ecological role for the source microorganisms. No corresponding phospholipids were detected. Notably, glycosidic branched GDGTs are 5–10 times less abundant than their intact isoprenoid counterparts derived from Archaea, while branched GDGT core lipids exceed their isoprenoid analogues by about an order of magnitude. These contrasting relationships may reflect lower standing stocks of the biomass of producers of branched GDGTs, combined with higher population growth rates relative to soil Archaea. Search strategies for the microbial producers of these conspicuous orphan lipids should benefit from the discovery of their intact polar precursors.