Isoprenoid glycerol dialkanol diethers: A series of novel archaeal lipids in marine sediments

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.