| Summary: | Lipids are key compounds in the transfer of energy and nutrients in marine food webs. Specifically, neutral lipids are the main reservoir of dietary energy in most marine animals and are often used as indicators of nutritional condition. In contrast, polar lipids, including acetone-mobile polar lipids and phospholipids, are key structural components of cell membranes. Here, we explored the fatty acid (FA) composition across the neutral and polar lipid fractions in benthic organisms belonging to five major phyla (Arthropoda, Cnidaria, Echinodermata, Mollusca, and Porifera), which were collected from a deep-water region in the Northwest Atlantic. Overall, the FA composition of acetone-mobile polar lipids (e.g., glycolipids) and phospholipids was more similar to each other and characterized by higher proportions of saturated FA (e.g., 16:0) compared to that of neutral lipids. This result reflected the different functions (structural vs storage) of these lipid fractions. Furthermore, independently of the taxon, monounsaturated FA (e.g., 16:1 omega 7, 18:1 omega 9) were dominant in neutral lipids, suggesting their preferential utilization for energy production. Whereas several species, including the gastropod Buccinum sp., retained the essential polyunsaturated FA (i.e., omega 3 and omega 6 FA) in their neutral lipids, others, such as the crustacean Pasiphaea tarda and the echinoderm Phormosoma placenta incorporated them into their polar lipids, pointing to differences in the metabolism of essential FA across species. These findings highlight the value of quantifying differences among lipid fractions. Last, the relatively high content of essential FA in neutral lipids of most species suggested a dietary surplus indicating a healthy, productive food web.
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