Figures, Box, and Tables
Fig. 1: Consumers differ in their capacity to synthesize key fatty acids from precursors. Three of the major gaps in synthesis capacity (A) include: 1) conversion of saturated fatty acids (SFA), which may be derived from carbohydrates in diet, to the monounsaturated fatty acid (MUFA) oleic acid (18:1n-9; OA) and the omega-6 (n-6) polyunsaturated fatty acid (PUFA) linoleic acid (18:2n-6; LIN), 2) conversion of the n-6 PUFA LIN and arachidonic acid (20:4n-6; ARA) to the omega-3 PUFA alpha-linolenic acid (18:3n-3; ALA) and eicosapentaenoic acid (20:5n-3; EPA), respectively, and 3) conversion of the short-chain n-3 PUFA ALA into the long-chain n-3 PUFA EPA and docosahexaenoic acid (22:6n-3; DHA). Within primary producers (A), vascular terrestrial plants are only capable of synthesizing fatty acids up to ALA whereas different species of algae and non-vascular terrestrial plants (e.g., mosses and liverworts) are also able to produce EPA and/or DHA. Consumers (B-E) have evolved synthesis capabilities that differ based upon the availability of key fatty acids in their diets. While some consumers (B) are capable of synthesizing both short-chain and long-chain n-3 and n-6 PUFA from SFA and OA, others (C-D) require short-chain n-3 PUFA from diet, and still others (E) must receive all key fatty acids directly from diet. Some animals, such as soil nematodes (B), consume PUFA-deficient resources like bacteria and organic matter and derive only SFA and OA from their diet, which they use as precursors to synthesize LA, ARA, ALA, EPA, and DHA. Others, such as finches and other terrestrial birds (C), consume resources like seeds that contain only SFA, OA, and LIN, as well as resources like terrestrial insects that also contain ALA. They must therefore convert dietary LIN into ARA and dietary ALA into EPA and DHA. Still others, like Daphnia and other aquatic invertebrates (D), consume some resources that contain both short-chain and long-chain PUFA, and are incapable of converting EPA to DHA, but are also capable of synthesizing EPA from DHA, such as from the Cryptophyte alga Cryptomonas, through the process of beta-oxidation (BETA). Finally, some animals, like tuna (E) and other carnivorous marine fishes, consume resources that contain the full set of key fatty acids, including both EPA and DHA, and are unable to perform any of the major synthesis steps in (A).