Bacterial GMGTs in East African lake sediments: Their potential as palaeotemperature indicators

Abstract

Glycerol monoalkyl glycerol tetraethers (GMGTs) are a group of membrane spanning lipids produced by some species of archaea and bacteria. They differ from the more commonly studied glycerol dialkyl glycerol tetraethers (GDGTs) in having an additional covalent carbon-carbon bond connecting the two alkyl chain. The relative abundance and distribution of bacterial branched GMGTs (brGMGTs) in surface sediments from a set of East African lakes were studied. The abundance of brGMGTs relative to the brGDGTs is positively correlated to measured mean annual air temperature (MAAT), although with a significant amount of scatter. BrGMGT abundance was not correlated to lake water pH. Seven major brGMGTs that vary in degree of methylation were identified, with m/z 1020, 1034 and 1048. Further, the mass chromatograms of the m/z 1020 and 1034 brGMGTs show an interesting distribution of peaks, which likely relates to the occurrence of distinct brGMGT isomers. This structural complexity is higher than previously observed in peats and marine sediments. Principal component analysis of the fractional abundance of bacterial tetraether lipids revealed the brGMGTs behave similarly to one another but differently from both the 5- or 6-methyl brGDGTs. This suggests the brGMGTs are produced by a common source organism and are methylated at a different position. The distribution of the seven brGMGTs showed considerable correlation with MAAT. This variability was captured in a new proxy index (the brGMGTI), which showed a strong positive linear relationship with MAAT. Lacustrine brGMGTs show potential to be applied to ancient settings to provide information about paleoclimate. © 2019 The Authors