Bacterial and eukaryotic intact polar lipids in the eastern subtropical South Pacific: Water-column distribution, planktonic sources, and fatty acid composition

Fatty acids are generally the most abundant lipid molecules in plankton, and thus play a central role in the cycling of organic matter in the upper ocean. These fatty acids are primarily derived from intact polar diacylglycerolipids (IP-DAGs), which compose cell membranes in plankton. The molecular diversity of IP-DAGs in the upper ocean remains to be fully characterized, and the advent of high performance liquid chromatography/electrospray ionization–mass spectrometry (HPLC/ESI-MS) approaches have now provided the opportunity to readily analyze IP-DAGs from marine planktonic communities. We used HPLC/ESI-MS to determine the concentrations of three classes of phospholipids (phosphatidlyglycerol (PG), phosphatidylethanolamine (PE), and phosphatidylcholine (PC)), three classes of betaine lipids (diacylglyceryl trimethylhomoserine (DGTS), diacylglyceryl hydroxymethyl-trimethyl-β-alanine (DGTA), and diacylglyceryl carboxyhydroxymethylcholine (DGCC)), and three classes of glycolipids (monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), and sulfoquinovosyldiacylglycerol (SQDG)) in plankton filtered (>0.2 μm) from seawater collected within the euphotic zone of the eastern South Pacific. The distributions of these IP-DAGs along the cruise transect provided important new insights on their tentative planktonic sources. Complementary data from our cruise, a principle components analysis of our IP-DAG concentrations, observed fatty acid compositions of IP-DAG classes and published IP-DAG distributions in pure cultures of plankton suggest that heterotrophic bacteria were the dominant sources of PG and PE, while MGDG and SQDG originated primarily from Prochlorophytes. The origins of the other classes of IP-DAGs were less clear, although it is likely that PC, DGTS, DGTA, and DGCC were derived primarily from eukaryotic phytoplankton. The molecular distributions of fatty acids attached to the different classes of IP-DAGs were generally distinct from one another, and suggest that reported distributions of total fatty acids (as analyzed by gas chromatography) in the literature should be regarded as homogenized mixtures of distinct molecular pools of fatty acids.