Tracking Intact Phospholipids and Triacylglycerides in Bering Sea Euphausiids during Two Pulsed Feeding Experiments via Tandem LC-MS

In the eastern Bering Sea and Chukchi Sea, Thysanoessa raschii are the most abundant krill species and a keystone trophic component that serves as both an important grazer and link to upper levels consumers including whales. Krill experience large variation in food resources annually and store multiple lipid classes for both reproduction and growth. Two shipboard feeding experiments tested the lipid retention in adult T. raschii and examined the fluctuation of specific lipid biomarkers under food-limited conditions. Phospholipids represent the major structural and storage lipids; their retention as intact phospholipids (IPL), as well as other glycerides (i.e. diacyl- and triacylglycerides; DG and TG), were followed over 19- and 31-day experiments using RPLC ESI-MS/MS on an LTQ Orbitrap XL. Identification and quantification of the suite of phospholipids and associated fatty acids with each experiment was performed with Lipid Search software. IPL’s comprised the majority of intact lipids present, most of which had phosphatidylcholine (PC) headgroups; smaller contributions were made by phosphatidylethanolamine (PE) and phosphatidylserine (PS)-contaning IPL’s. Fatty acids were largely represented by seven compounds – C14:0n, C16:0n, C16:1(n-7), C18:1(n-7), C18:1(n-9), C20:5(n-3), C22:6(n-3) – and were typically present as mixed acyl groups within each intact lipid class. Concentrations (μmole/g wet weight) of IPL and glyceride lipids showed a decrease of 21% and 26%, respectively, from initial values, suggesting that both are mobilized in times of food scarcity and during overwintering. Structures containing 16:1 decreased most for IPL’s, reflecting the absence of the 16:1(n-7) dietary algal fatty acid. This powerful set of analytical and software tools allows determination of the suite of intact lipids within euphausiids to provide a more comprehensive picture of krill structural and storage lipids and their retention during times of varied food availability.