Intact polar lipid export in the temperate western North Atlantic and Sargasso Sea

Abstract

Intact polar diacylglycerolipids (IP-DAGs), abundant membrane components in phytoplankton and bacterioplankton, comprised 5.9±0.5% of suspended particulate organic carbon (POC) in the euphotic zone at two stations in the western North Atlantic Ocean. The IP-DAG assemblage included 9 phospholipid, glycolipid and betaine lipid classes that are routinely found in marine samples. We also report the pelagic occurrence of two recently discovered IP-DAGs, phosphatidyl-S,S-dimethylpropanethiol (PDPT) and a betaine-like lipid, both originally identified in Emiliania huxleyi. At the mesotrophic station north of the Gulf Stream, IP-DAGs comprised 0.62±0.38% of sinking POC collected in sediment traps, and at the oligotrophic station in the Sargasso Sea they comprised 0.31±0.05% of sinking POC.Phospholipids and glycolipids with polyunsaturated fatty acid (PUFA) moieties were relatively abundant in sinking particles at 150m compared with suspended particles in the euphotic zone at the mesotrophic station north of the Gulf Stream. The sinking particles were probably associated with the demise of a dinoflagellate or diatom bloom that resulted in a maximum total IP-DAG export flux of 1.8μmol/m2/day. At the oligotrophic station in the Sargasso Sea, sinking particles included microalgal glycolipids, phospholipids and betaine lipids with PUFAs; picocyanobacterial glycolipids with saturated and monounsaturated FAs; and bacterial phospholipids with saturated and monounsaturated FAs. The microalgal and bacterial IP-DAGs were relatively abundant in sinking particles compared with suspended particles. The IP-DAG export flux at the station in the Sargasso Sea was similar in three sediment traps deployed on consecutive days, with an average of 0.46±0.08μmol/m2/day.A diel cycle was observed in the IP-DAG euphotic zone inventory in the Sargasso Sea, with higher concentration in samples taken at dusk than at dawn, and we propose a model using this dynamic to calculate IP-DAG residence time. Export efficiency was calculated from the residence time and the ratio of IP-DAG export flux to euphotic zone inventory, with values ranging from 0.02% for PDPT to 6.9% for monogalactosyldiacylglycerol (MGDG) with PUFAs. The greater export flux and export efficiency of IP-DAGs with PUFAs contrasts with previous results for total FAs in sinking particles, which showed preferential export of saturated and monounsaturated FAs. The difference likely resulted from IP-DAGs with PUFAs being membrane components of rapidly sinking organic matter from microalgae. Alternatively, PUFA moieties cause steric hindrance of microbial lipase activity, potentially decreasing their hydrolysis rate and increasing the relative concentration of IP-DAGs with PUFAs in sinking particles.