Phospholipids as a component of the oceanic phosphorus cycle

Abstract

We characterize the distribution of oceanic phosphorus-containing lipids (PL) in the Northeast Atlantic by Iatroscan thin layer chromatography and high resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). Phospholipids are a small but significant fraction of oceanic particulate organic carbon (POC) (1.5%). We describe the distribution of 1862 PL compounds in total, of which only ~27% have elemental compositions that match those found in the Nature Lipidomics Gateway database (e.g., phosphatidylglycerol (PG), phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidic acid (PA), phosphatidyl serine (PS), and phosphatidylinositol (PI)). The highest phospholipid concentration is found in the epipelagic, which reflects primary production in that depth horizon. Depth-related PL removal was the strongest for PL signals that match database-reported (known) lipids and was lower for saturated non-database (novel) matched PL. The transformation of known PL is marked by depth-related increase in saturation with PA that is assumed to be generated as the earliest transformation product of PL. Novel unsaturated P-lipids likely originate from both PL transformation processes and in-situ biological production at the surface layer. Novel PL are dominated by unsaturated compounds for which unsaturation increased between the epipelagic (average molecular double bond equivalents, DBE = 5) and the abyssopelagic (average DBE = 6.7) zones. Additionally, those compounds increase in both average molecular weight and contribution to all lipid content with increasing depth, likely from cross-linking of unsaturated compounds. Our data indicate that novel PL are selectively preserved with depth and therefore are P and C carriers to the deep Atlantic. We demonstrate that a full appreciation of phosphorus cycling requires additional data on phospholipid composition and especially the ecological role and depth-related molecular change of these compounds.