Biosynthesis of long-chain omega-3 fatty acids by the nestlings of a generalist seabird.

Docosahexaenoic acid (DHA), a long-chain omega-3 fatty acid (n3-LCPUFA), is an integral component of vertebrate brains. Vertebrates maintain their DHA levels through biosynthesis using alpha-linolenic acid (ALA; omega-3 precursor) or by consuming preformed DHA and other n3-LCPUFAs which abound in the natural diets of marine predators. Yet, numerous marine predators, including generalist seabirds, now exploit anthropogenic resources potentially deficient in n3-LCPUFAs. Whether they can offset such deficiency by bioconverting ALA into DHA remains unknown. Here, we tested whether chicks of the ring-billed gull (Larus delawarensis), a generalist seabird thriving in cities, can biosynthesize n3-LCPUFAs, including DHA, from ALA. We brought into captivity 12 hatchlings from an urban colony and 12 from a natural colony. Nine hatchlings per colony were gavaged 490 μl of ALA-rich flaxseed oil daily for 3 days. The control groups (N=3 urban hatchlings, 3 natural hatchlings) received an omega-3-free caloric equivalent in place of the ALA supplement. All chicks received an omega-3-free diet throughout captivity (72 h). We also attempted to follow ALA's potential bioconversion into n3-LCPUFAs using an oral 13C1-enriched ALA tracer. Unfortunately, compound-specific isotope analyses of brain and liver tissue failed to detect any 13C enrichment. Nevertheless, the flaxseed oil supplementation study provided evidence of some ALA bioconversion. Compared with controls, supplemented chicks from both colonies accumulated more of all ALA derivates in their tissues except for DHA. We demonstrate for the first time that a seabird shows incomplete omega-3 bioconversion abilities, leaving them potentially vulnerable to deficiencies associated with urban foraging and shifting marine ecosystems.