Diagenetic fate of glycerol ethers revealed by two novel isoprenoid hydroxyphytanyl glycerol monoethers and non-isoprenoid alkyl glycerol ethers

Abstract

The diagenesis of isoprenoid glycerol ethers and non-isoprenoid mono- and dialkyl glycerol ethers (MAGEs and DAGEs) is important as they are potential markers of microbes involved in the anaerobic oxidation of methane (AOM) in cold seep ecosystems. Here we studied glycerol ethers and the respective diagenetic products in seep carbonates from the Haima cold seeps using MAGE and DAGE abundances and their carbon isotopic compositions. Two novel sn2-/sn3-O-hydroxyphytanyl glycerol monoethers hydroxylated at C-3 of the phytanyl moieties were identified, both being attributed to anaerobic methane-oxidizing archaea (ANME) of the ANME-2 group according to the high ratios of sn2-hydroxyarchaeol relative to archaeol, the extremely low δ13C values (from −132‰ to −120‰), and the simultaneously abundant crocetane. Based on the sequential degradation pathway of hydroxyarchaeols, sn2-/sn3-O-hydroxyphytanyl glycerol monoethers likely constitute hydrolysis products of di-hydroxyarchaeol since the C-3 hydroxy-phytanyl side chain is preferentially hydrolysed in the presence of H+. By contrast, the isoprenoidal 3,7,11,15-tetramethylhexadecan-1,3-diol was most likely a hydrolysis product of sn2-/sn3-O-hydroxyarchaeols, di-hydroxyarchaeol, and sn2-/sn3-O-hydroxyphytanyl glycerol monoethers, which is supported by structures with the same skeleton and the similar 13C-depletions. Moreover, a similar relationship was found for MAGEs, DAGEs and the corresponding non-isoprenoid short-chain alcohols. Besides the similar δ13C values (from –110‰ to –100‰), the non-isoprenoid short-chain alcohols, MAGEs, and DAGEs, displayed very similar chain lengths and unsaturation patterns, indicating that non-isoprenoid alcohols might be either biosynthetic intermediates or degradation products of MAGEs and DAGEs, dependent on the live status of the lipid-producing microorganisms. Our study highlights the relationship of glycerol ethers and their respective derivatives, which can be used as tracers of ancient microbes preserved in authigenic carbonates and sedimentary rocks.