Substantial contribution of <i>in situ</i> produced bacterial lipids to the sedimentary lipidome

The sedimentary lipid pool is comprised of a myriad of individual components. Due to their importance for organic carbon sequestration and their application in paleoclimatic and geobiological reconstructions, its composition has been studied for many decades with targeted approaches but an overall view on its composition is still lacking. In part this uncertainty relates to the different sources of sedimentary lipids, they can be both delivered from the overlying water column by sedimentation, but also produced <i>in situ</i> by sediment dwelling organisms. Another uncertainty relates to the differing degree of preservation, both between lipid groups and relative to other organic matters. Here we conduct an untargeted analysis of the sedimentary lipidome in the Black Sea using ultra high-pressure liquid chromatography coupled with high-resolution tandem mass spectrometry (UHPLC-HRMS²). Besides commonly reported phytoplankton-derived fossil lipids, a diverse and abundant set of sphingolipids was discovered, accounting for ~20% of the sedimentary lipidome. We hypothesize that these sphingolipids are produced <i>in situ</i> by sedimentary anaerobic bacteria, which likely use sphingolipids instead of phospholipids, probably because phospholipids are preferentially utilized in the uppermost layers of the anoxic sediments. Our results suggest that while phytoplankton-derived lipids contribute 50%-60% of the sedimentary lipidome, the importance of bacterial lipids, particularly <i>in situ</i> produced sphingolipids, may have been overlooked.