Free fatty acids, tri-, di- and monoacylglycerol production and depth-related cycling in the Northeast Atlantic

Abstract

We present the characterization and vertical distribution of suspended particulate lipids containing C, H and O which have the potential to sequester carbon from the upper ocean when associated with sinking particles. Lipids have been shown to be valuable in a host of environments to provide insights into the sources and processing of organic materials in the oceans. Here we present, direct-infusion, high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with bulk lipid measures for marine lipid characterization. We present the water column distribution of free fatty acids, tri-, di- and monoacylglycerols from the surface layer to abyssopelagic depths (4800m) for samples collected in the Northeast Atlantic at the Porcupine Abyssal Plain sustained observatory (PAP-SO) (49.0°N, 16.5°W). Triacylglycerols (TG) with even carbon number (TG) and odd carbon number (oddTG, reflecting bacterial origin), were analyzed, while free fatty acids were analyzed as unsaturated (UFA), branched (BrFA) and saturated (SAFA) fatty acids. The surface productive layer (euphotic zone) was characterized with the highest incidence of lipids that are not reported in the Nature Lipidomics Gateway database, especially lipids that are highly unsaturated (acyl chain unsaturation was on average 3.8 for TG, oddTG, UFA and diacylglycerols (DG)). Additionally, we observed high lipid degradation at epipelagic depths. Fatty acid markers indicate that diatoms and dinoflagellates were important contributors to the lipid pool. Depth-resolved lipid change includes decreased lipid abundance and molecular diversity together with substantial loss of unsaturation with increasing depth. The major lipid change occurs at upper mesopelagic depths. Unlike other observed lipids, the abundance of SAFA remained essentially constant down the water column whereas the number of SAFAs and their contribution to total lipids increased with depth. Thus, we demonstrate that lipid saturation affects the export of carbon from the atmosphere to the deep ocean.