Selective preservation among bacterial alkyl glycerol ether lipid structures during long term oxic and anoxic incubation

Bacterial mono- and dialkyl glycerol ether lipids (MAGEs and DAGEs) are potential powerful lipid biomarkers whose respective diagenetic fate still needs to be precisely assessed. To do so, the lipid content of cultures of an anaerobic bacterium synthesizing both MAGEs and DAGEs was analyzed at different growth stages and after long-term incubation under aerobic or anaerobic conditions. The proportions of the different lipid classes (fatty acids, MAGEs and DAGEs), as well as their structural distributions, did not significantly vary during anaerobic growth under optimal conditions until the stationary phase. Long-term exposure to oxygen (up to 480 days) induced the preferential degradation of fatty acids and MAGEs relative to DAGEs and of sn-1 relative to sn-2 positional isomers of MAGEs. The degradation of fatty acids and MAGEs appeared slower under anaerobic conditions and no specific structures appeared preferentially degraded. All DAGE structures were very well preserved whatever the redox conditions and the time of incubation considered (480 days and 13 years under oxic and anoxic conditions, respectively). The results highlight the excellent diagenetic stability of DAGEs and the much lower potential of preservation of MAGEs, warranting caution for an eventual use of the latter compounds as tracers of environmental or microbial community changes.