Gradients in intact polar diacylglycerolipids across the Mediterranean Sea are related to phosphate availability

Abstract. Intact polar membrane lipids compose a significant fraction of cellular material in plankton and their synthesis imposes a substantial constraint on planktonic nutrient requirements. As a part of the Biogeochemistry from the Oligotrophic to the Ultraoligotrophic Mediterranean (BOUM) cruise we examined the distribution of several classes of intact polar diacylglycerolipids (IP-DAGs) across the Mediterranean, and found that phospholipid concentration as a percent of total lipids correlated with phosphate concentration. In addition, the ratios of non-phosphorus lipids to phospholipids – sulfoquinovosyldiacylglycerol (SQDG) to phosphatidylglycerol (PG), and betaine lipids to phosphatidylcholine (PC) – were also found to increase from west to east across the Mediterranean. Additionally, microcosm incubations from across the Mediterranean were amended with phosphate and ammonium, and in the course of several days nutrient amendments elicited a shift in the ratios of IP-DAGs. These experiments were used to assess the relative contribution of community shifts and physiological response to the observed change in IP-DAGs across the Mediterranean. The ratio of SQDG to chlorophyll-a was also explored as an indicator of phytoplankton response to nitrogen availability. This study is the first to demonstrate the dynamic response of membrane lipid composition to changes in nutrients in a natural, mixed planktonic community.