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Understanding the stable hydrogen isotope (δ2H) composition and fractionation in lipid biomolecules of primary producers, such as terrestrial and aquatic plants, is crucial for deciphering past environmental conditions, as well as applying compound-specific stable isotope analysis for the study of metabolic and ecological processes. We conducted a new tracer experiment to explore the δ2H composition of algal fatty acid biomarkers, focusing on freshwater algae, which form the base of aquatic food webs. We selected a range of algal species widely found in freshwater ecosystems and cultivated them under controlled conditions. First, we added 2H2O to ambient water as a tracer to investigate the net hydrogen isotope fractionation during algal lipid synthesis at isotopic equilibrium, which is particularly informative for paleo-geochemical studies. Then, we conducted kinetic experiments to quantify the time needed for algal fatty acids to achieve isotopic steady-state conditions in response to the change in ambient water δ2H values. Our findings revealed substantial variability in hydrogen isotope fractionation among different algal taxa and various fatty acids. Based on taxa, different fatty acids exhibited faster integration of water hydrogen than others, but they were not necessarily in the order of the biosynthetic pathway. This experiment underscores the complexity of hydrogen isotope fractionation and the requirement for controlled laboratory studies to properly apply compound-specific stable H isotope analysis techniques in ecological and paleo-environmental studies. [ABSTRACT FROM AUTHOR]

Traditionally, trophic ecology research on aquatic ecosystems has focused more on the quantity of dietary energy flow within food webs rather than food quality and its effects on organisms at various trophic levels. Recent studies emphasize that food quality is central to consumer growth and reproduction, and the importance of food quality for aquatic ecosystems has become increasingly well recognized. It is timely to synthesise these findings and identify potential future research directions. We conducted a systematic review of omega‐3 polyunsaturated fatty acids (ω3‐PUFAs) as a crucial component of high‐quality food sources in freshwater ecosystems to evaluate their impact on a variety of consumers, and explore the effects of global change on these high‐quality food sources and their transfer to higher trophic consumers within and across ecosystems. In freshwater ecosystems, algae rich in ω3 long‐chain PUFAs, such as diatoms, dinoflagellates and cryptophytes, represent important high‐quality food sources for consumers, whereas cyanobacteria, green algae, terrestrial vascular plants and macrophytes low in ω3 long‐chain PUFAs are low‐quality food sources. High‐quality ω3‐PUFA‐containing food sources usually lead to increased growth and reproduction of aquatic consumers, e.g. benthic invertebrates, zooplankton and fish, and also provide ω3 long‐chain PUFAs to riparian terrestrial consumers via emergent aquatic insects. Consumers feeding on high‐quality ω3‐PUFA‐containing foods in turn represent high‐quality food for their own predators. However, the ω3‐PUFA content of food sources is sensitive to global environmental changes. Warming, eutrophication, increased light intensity (e.g. from loss of riparian shading), and pollutants potentially inhibit the synthesis of algal ω3‐PUFAs while at the same time promoting the growth of lower‐quality foods, such as cyanobacteria and green algae. These factors combined could lead to a significant reduction in the availability of ω3‐PUFAs for consumers and constrain their overall fitness. Although the effect of individual environmental factors on high‐quality ω3‐PUFA‐containing food sources has been investigated, multiple environmental factors (e.g. climate change, human activities, pollution) will act in combination and any synergistic effects on aquatic food webs remain unclear. Identifying the sources and fate of ω3‐PUFAs within and across ecosystems could represent an important approach to understand the impact of multiple environmental factors on trophic relationships and the implications for populations of freshwater and riparian consumers. Maintaining the availability of high‐quality ω3‐PUFA‐containing food sources may also be key to mitigating freshwater biodiversity loss due to global change. [ABSTRACT FROM AUTHOR]

Abstract: Polyunsaturated fatty acids (PUFA), especially long-chain (i.e., [greater than or equal to]20 carbons) polyunsaturated fatty acids (LC-PUFA), are fundamental to the health and survival of marine and terrestrial organisms. [...]