In recent decades, much conceptual thinking in trophic ecology has been guided by theories of nutrient limitation and the flow of elements, such as carbon and nitrogen, within and among ecosystems. More recently, ecologists have also turned their attention to examining the value of specific dietary nutrients, in particular polyunsaturated fatty acids (PUFA), among which the omega‐3 PUFA, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) play a central role as essential components of neuronal cell membranes in many organisms. This review focuses on a new neuro‐ecological approach stemming from the biochemical (mechanistic) and physiological (functional) role of DHA in neuronal cell membranes, in particular in conjunction with G‐protein coupled receptors (GPCRs). We link the co‐evolution of these neurological functions to metabolic dependency on dietary omega‐3 PUFA. We outline ways in which deficiencies in dietary DHA supply may affect, cognition, vision, and behaviour, and ultimately, the biological fitness of consumers. We then review emerging evidence that changes in access to dietary omega‐3 PUFA may ultimately have profound impacts on trophic interactions leading to potential changes in community structure and ecosystem functioning that, in turn, may affect the supply of DHA within and across ecosystems, including the supply for human consumption.

Atopic dermatitis (AD) is an inflammatory skin disease in which epidermal barrier impairment, often owing to FLG null mutations, precedes immune hyperresponsiveness. Ichthyosis vulgaris is characterized by FLG null mutations and noninflamed dry skin. Netherton syndrome (NS), caused by SPINK5 null mutations, is characterized by generalized erythroderma with scaling and atopic manifestations. The goal of this work was to evaluate associations between specific skin disease features, such as ichthyotic and/or atopic manifestations, and the skin bacterial and fungal microbiota. Taxon diversity showed greater variation in the bacterial microbiota than in the fungal microbiota in the skin diseases. The relative abundances of Firmicutes (Staphylococcus) and Actinobacteria (Corynebacterium) were augmented in ichthyosis vulgaris, AD, and NS, whereas those of Proteobacteria/Enhydrobacter and Bacteroidetes were reduced, regardless of body site. Furthermore, proportions of Staphylococcus were correlated with transepidermal water loss and serum IgE levels. Nevertheless, the skin of patients with low to mild AD was overcolonized with Staphylococcus epidermidis and not with Staphylococcus aureus. Ascomycota were increased in both AD and NS, but from expansion of different fungal species. Finally, the expansion of pathologic bacteria in AD and NS might be supported by surrounding fungi. Thus, distinguishable bacterial and fungal skin dysbiosis in AD, NS, and ichthyosis vulgaris emphasizes disease-specific pathomechanisms.

Organisms at the base of aquatic food webs synthesize essential nutrients, such as omega‐3 polyunsaturated fatty acids (n‐3 PUFA), which are transferred to consumers at higher trophic levels. Many consumers, requiring n‐3 long‐chain (LC) PUFA, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have limited ability to biosynthesize them from the essential dietary precursor α‐linolenic acid (ALA) and thus rely on dietary provision of LC‐PUFA. We investigated LC‐PUFA metabolism in freshwater zooplankton using stable hydrogen isotopes (δ²H) of fatty acids as tracers. We conducted feeding experiments with the freshwater keystone grazer Daphnia to quantify changes in the δ²H value of body FA in response to the FA composition of their food and the δ²H value of the ambient water. The isotopic composition of LC‐PUFA changed in Daphnia, depending on the integration of ²H from ambient water during de novo synthesis or bioconversion from dietary precursors, allowing us to distinguish dietary from bioconverted EPA in body tissue. We tested the applicability of these laboratory findings in a field setting by analysing δ²H values of PUFA in primary producers and consumers in eutrophic ponds to track EPA sources of zooplankton. Multilinear regression models that included conversion of ALA to EPA correlated better with zooplankton δ²H_EPA than seston δ²H_EPA at low dietary EPA supply. This study provides evidence that zooplankton can compensate for low dietary EPA supply by activating LC‐PUFA biosynthesis and shows that herbivorous zooplankton play a crucial role in upgrading FA for higher trophic levels during low dietary EPA supply. A free Plain Language Summary can be found within the Supporting Information of this article.

Consumers feeding at the aquatic–terrestrial ecosystem interface may obtain a mixture of aquatic and terrestrial diet resources that vary in nutritional composition. However, in lake riparian spiders, the relative significance of aquatic versus terrestrial diet sources remains to be explored. We investigated the trophic transfer of lipids and polyunsaturated fatty acids (PUFA) from emergent aquatic and terrestrial insects to spiders at varying distances from the shoreline of a subalpine lake in Austria, using differences in fatty acid profiles and compound‐specific stable carbon (δ ¹³ C) and hydrogen (δ ² H) isotopes. The omega‐3 PUFA content of emergent aquatic insects was higher than that of terrestrial insects. Emergent aquatic insects contained on average 6.6 times more eicosapentaenoic acid (EPA) and 1.2 times more α‐linolenic acid (ALA) than terrestrial insects, whereas terrestrial insects contained on average 2.6 times more linoleic acid (LIN) than emergent aquatic insects. Spiders sampled directly on the lake and in upland habitats had similar EPA contents, but this EPA was derived from different diet sources, depending on the habitat. The δ ¹³ C _EPA and δ ² H _EPA values of ‘lake spiders' revealed an aquatic diet pathway (i.e. EPA of aquatic origin). In contrast, EPA of spiders collected in terrestrial habitats was depleted in both ¹³ C and ² H compared to any potential food sources, and their ALA isotopic values, suggesting that EPA was partly bioconverted from its dietary precursor ALA (i.e. internal pathway). The δ ² H values of fatty acids clearly indicated that diet sources differed depending on the spider's habitat, which was less evident from the δ ¹³ C values of the fatty acids. Our data highlight that spiders can use two distinct pathways (trophic versus metabolic) to satisfy their physiological EPA demand, depending on habitat use and dietary availability.

Rationale: Compound‐specific stable isotope analysis (CSIA) is a powerful tool for a better understanding of trophic transfer of dietary molecules in and across ecosystems. Hydrogen isotope values ( δ ² H) in consumer tissues have potential to more clearly distinguish dietary sources than ¹³ C or ¹⁵ N values within and among habitats, but have not been used at the fatty acid level for ecological purposes. Methods: Here we demonstrate a new online high‐capacity gas chromatography–isotope ratio mass spectrometry technique ( ² H‐CSIA) that offers accurate and reproducible determination of δ ² H values for a range of fatty acids from organisms of aquatic food webs. Results: We show that lipid extracts obtained from aquatic sources, such as biofilms, leaves, invertebrates, or fish muscle tissue, have distinctive δ ² H values that can be used to assess sources and trophic interactions, as well as dietary allocation and origin of fatty acids within consumer tissue. Conclusions: The new ² H‐CSIA method can be applied to evaluate sources and trophic dynamics of fatty acids in organisms ranging from food web ecology to migratory connectivity.