Polyunsaturated fatty acids in fish tissues more closely resemble algal than terrestrial diet sources

Evaluation of lipid extraction methods for fatty acid quantification and compound-specific δ13C and δ2Hn analyses

Lipids, with fatty acids (FA) as a crucial subset, have become a focal point for diverse medical, physiological, and ecological studies. However, a comprehensive assessment of the various pre-analytical FA extraction methods published in the scientific literature remains lacking. In this study, we examined the efficacy of seven well-established sample preparation methods, specifically focusing on their effectiveness in total lipid and fatty acid extraction and their impact on compound-specific stable hydrogen (δ2H) and carbon (δ13C) isotope values. We also considered the repercussions of FA removal efficacy on residual bulk tissue δ2Hn analysis, because lipids typically have low δ2H values. Our findings showed that in most cases chloroform-based extraction methods outperformed those without chloroform. While discrepancies were not as evident for smaller organisms, such as plankton, marked variations were discernible in the extraction efficiencies for muscle and liver samples, which was also manifested in the residual bulk tissue δ2Hn results. Notably, most extraction methods had little effect on specific δ13C or δ2H isotope values of FA; instead, an emphasis should be on using an extraction method that achieves optimal baseline peak separation of the chromatograms for C and H isotope measurements.

How do small dams alter river food webs? A food quality perspective along the aquatic food web continuum

Damming of rivers poses a significant threat to freshwater ecosystems. Previous studies about the impact of damming on river ecosystems have mostly focused on large dams, with the impact of small dams largely unknown. Further, while the impacts of dams on aquatic communities have been widely studied, the effect on energy flow across river food webs remains unclear. In recent years, long-chain polyunsaturated fatty acid analysis (LC-PUFA) has emerged as a promising technique for assessing food quality and trophic interactions. In this study, LC-PUFA was applied to explore the nutritional effects of small dams on river food webs. A field investigation was conducted at upstream and downstream areas of three small dams in the headwaters of Dongjiang River, China, to evaluate the impact of small dams on the nutritional quality of basal food sources, and their consequent impacts on aquatic consumers and trophic links. Basal food sources (i.e., submerged leaves, macrophytes and periphyton) and aquatic consumers (i.e., macroinvertebrates and fish) were collected, and their fatty acid (FA) composition was measured. Our results showed that periphyton, rather than submerged leaves and macrophytes, was the primary high-quality food source for aquatic consumers, providing them with LC-PUFA, irrespective of whether sites were upstream or downstream. Damming the streams induced changes in aqueous nutrient concentrations (TP, PO4-P, DIN, and TN) from upstream to downstream of the dams, leading to significant variation in periphyton FA content. Compared with periphyton collected at downstream sites, periphyton at upstream sites contained higher LC-PUFA, but lower short-chain PUFA. Differences in periphyton LC-PUFA between the upstream and downstream areas of dams were reflected in the FA profiles of invertebrate grazers and filterers, and further transferred to fish. Furthermore, decreased periphyton nutritional quality at the downstream of the dams was one of the reasons for the simplification of stream food webs. Our results indicated that small dams negatively affected food webs, emphasizing the importance of high-quality food sources for stream ecosystems. We suggest that the trophic integrity of river food webs hinges on the dietary availability of periphyton supplying physiologically highly required nutrients for consumers and must thus not be compromised by damming of streams or other alterations.

Eutrophication and loss of riparian shading influence food quality and trophic relation in stream food webs

Eutrophication induced by excessive inputs of nutrient is one of the main stressors in aquatic ecosystems. Deforestation in riparian zones alter riparian shading, which together with eutrophication is expected to exert a complex control over stream food webs. We manipulated two levels of riparian shading (open canopy vs. shading canopy) and nutrient supply (ambient vs. nutrient addition) in three headwater streams to investigate the individual and combined effects of eutrophication and loss of riparian shading on carbon sources and nutritional quality of biofilms, and the subsequent trophic effects on macroinvertebrate grazers. Nutrient enrichment increased the autochthonous carbon (i.e., algae especially diatoms) indicated by fatty acid (FA) biomarkers within biofilms and grazers. The nutritional quality indicated by eicosapentaenoic acid (EPA) content of biofilms was increased with nutrient enrichment and more so with the combined effect of an increase in riparian shading, consequently leading to an increase in the nutritional quality, density, and biomass of grazers. In particular, the trophic linkages between biofilms and grazers were mainly influenced by EPA concentration in the biofilms, and strengthened with the combined effects of riparian shading and additional nutrients. Our study emphasizes the nutritional significance of EPA for consumers at higher trophic levels and proposes its potential as an indicator for monitoring the health of aquatic ecosystems.

The importance of omega-3 polyunsaturated fatty acids as high-quality food in freshwater ecosystems with implications of global change

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.

Microalgal polyunsaturated fatty acids: Hotspots and production techniques

Algae play a crucial role in the earth’s primary productivity by producing not only oxygen but also a variety of high-value nutrients. One such nutrient is polyunsaturated fatty acids (PUFAs), which are accumulated in many algae and can be consumed by animals through the food chain and eventually by humans. Omega-3 and omega-6 PUFAs are essential nutrients for human and animal health. However, compared with plants and aquatic sourced PUFA, the production of PUFA-rich oil from microalgae is still in the early stages of exploration. This study has collected recent reports on algae-based PUFA production and analyzed related research hotspots and directions, including algae cultivation, lipids extraction, lipids purification, and PUFA enrichment processes. The entire technological process for the extraction, purification and enrichment of PUFA oils from algae is systemically summarized in this review, providing important guidance and technical reference for scientific research and industrialization of algae-based PUFA production.

Genome-wide comparative identification and analysis of membrane-FADS-like superfamily genes in freshwater economic fishes

Membrane fatty acid desaturase (FADS)-like superfamily proteins (FADSs) are essential for the synthesis of unsaturated fatty acids (UFAs). Recently, studies on FADS in fishes have mostly focused on marine species, and a comprehensive analysis of the FADS superfamily, including the FADS, stearoyl-CoA desaturase (SCD), and sphingolipid delta 4-desaturase (DEGS) families, in freshwater economic fishes is urgently required. To this end, we conducted a thorough analysis of the number, gene/protein structure, chromosomal location, gene linkage map, phylogeny, and expression of the FADS superfamily. We identified 156 FADSs genes in the genome of 27 representative species. Notably, FADS1 and SCD5 were lost in most freshwater fish and other teleosts. All FADSs proteins contain 4 transmembrane helices and 2-3 amphipathic α-helices. FADSs in the same family are often linked on the same chromosome; moreover, FADS and SCD or DEGS are frequently collocated on the same chromosome. In addition, FADS, SCD, and DEGS family proteins share similar evolutionary patterns. Interestingly, FADS6, as a member of the FADS family, exhibits a similar gene structure and chromosome location to that of SCD family members, which may be the transitional form of FADS and SCD. This study shed light on the type, structure, and phylogenetic relationship of FADSs in freshwater fishes, offering a new perspective into the functional mechanism analysis of FADSs.

Export of dietary lipids via emergent insects from eutrophic fishponds

Fishponds, despite being globally abundant, have mainly been considered as food production sites and have received little scientific attention in terms of their ecological contributions to the surrounding terrestrial environment. Emergent insects from fishponds may be important contributors of lipids and essential fatty acids to terrestrial ecosystems. In this field study, we investigated nine eutrophic fishponds in Austria from June to September 2020 to examine how Chlorophyll-a concentrations affect the biomass of emergent insect taxa (i.e., quantity of dietary subsidies; n = 108) and their total lipid and long-chain polyunsaturated fatty acid content (LC-PUFA, i.e., quality of dietary subsidies; n = 94). Chironomidae and Chaoboridae were the most abundant emergent insect taxa, followed by Trichoptera, Ephemeroptera, and Odonata. A total of 1068 kg of emergent insect dry mass were exported from these ponds (65.3 hectares). Chironomidae alone exported 103 kg of total lipids and 9.4 kg of omega-3 PUFA. Increasing Chl-a concentrations were associated with decreasing biomass export and a decrease in total lipid and LC-PUFA export via emergent Chironomidae. The PUFA composition of emergent insect taxa differed significantly from dietary algae, suggesting selective PUFA retention by insects. The export of insect biomass from these eutrophic carp ponds was higher than that previously reported from oligotrophic lakes. However, lower biomass and diversity are exported from the fishponds compared to managed ponds. Nonetheless, our data suggest that fishponds provide crucial ecosystem services to terrestrial consumers by contributing essential dietary nutrients to consumer diets via emergent insects.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10750-022-05040-2.

Consumption of marine-derived nutrients from invasive Chinook salmon (Oncorhynchus tshawytscha) transfer ω-3 highly unsaturated fatty acids to invasive resident rainbow trout (O. mykiss)

Marine-derived nutrients (MDN) contained in gametes (mature eggs and sperm), carcasses and metabolic wastes from anadromous migratory salmon can transfer energy and materials to fresh water, thereby affecting the structure and function of stream ecosystems. This is crucial among ecosystems where humans have mediated biological invasions by propagating non-native species. Previous studies have demonstrated that consumption of MDN from salmon can benefit both native and invasive resident fishes. Yet, a more detailed understanding of the transfer of biomolecules with important physiological functions such as ω-3 highly unsaturated fatty acids (HUFAs) have received less attention among researchers. Here we demonstrate that consumption of MDN contained in invasive Chinook salmon eggs transfers ω-3 HUFAs (e.g., EPA and DHA) to resident invasive rainbow trout in a river food web. We conducted a field study in river sections previously identified as spawning areas for Chinook salmon in the Cisnes River, Patagonia. Rainbow trout were sampled around salmon spawning areas before, during, and after the salmon spawning season. Additionally, we collected tissue from different food web resources and components of different origin (e.g., primary producers, aquatic and terrestrial items) from the Cisnes River system. Analyses of stomach contents of trout were performed in conjunction with analyses of both lipid content and fatty acid profiles of trout tissue and food web components. Chinook salmon eggs showed higher content of ω-3 HUFAs, especially EPA (31.08 ± 23.08 mg g DW^-1) and DHA (27.50 ± 14.11 mg g DW^-1) than either freshwater or terrestrial components (0-6.10 mg g DW^-1 both EPA and DHA). We detected marked shifts in the fatty acid profile (~six-fold increase in EPA and DHA) of trout following consumption of Chinook salmon eggs. Our findings suggest that MDN via consumption of salmon eggs by resident rainbow trout may positively influence resident trout and likely contribute to gauge synergistic interactions between invaders on receiving ecosystems of Patagonia.

Intercontinental analysis of temperate steppe stream food webs reveals consistent autochthonous support of fishes

Quantifying the trophic basis of production for freshwater metazoa at broad spatial scales is key to understanding ecosystem function and has been a research priority for decades. However, previous lotic food web studies have been limited by geographic coverage or methodological constraints. We used compound-specific stable carbon isotope analysis of amino acids (AAs) to estimate basal resource contributions to fish consumers in streams spanning grassland, montane and semi-arid ecoregions of the temperate steppe biome on two continents. Across a range of stream sizes and light regimes, we found consistent trophic importance of aquatic resources. Essential AAs of heterotrophic microbial origin generally provided secondary support for fishes, while terrestrial carbon did not seem to provide significant, direct support. These findings provide strong evidence for the dominant contribution of carbon to higher-order consumers by aquatic autochthonous resources (primarily) and heterotrophic microbial communities (secondarily) in temperate steppe streams.

Linking omega-3 polyunsaturated fatty acids in natural diet with brain size of wild consumers

Omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) are key structural lipids and their dietary intake is essential for brain development of virtually all vertebrates. The importance of n-3 LC-PUFA has been demonstrated in clinical and laboratory studies, but little is known about how differences in the availability of n-3 LC-PUFA in natural prey influence brain development of wild consumers. Consumers foraging at the interface of aquatic and terrestrial food webs can differ substantially in their intake of n-3 LC-PUFA, which may lead to differences in brain development, yet this hypothesis remains to be tested. Here we use the previously demonstrated shift towards higher reliance on n-3 LC-PUFA deprived terrestrial prey of native brown trout Salmo trutta living in sympatry with invasive brook trout Salvelinus fontinalis to explore this hypothesis. We found that the content of n-3 LC-PUFA in muscle tissues of brown trout decreased with increasing consumption of n-3 LC-PUFA deprived terrestrial prey. Brain volume was positively related to the content of the n-3 LC-PUFA, docosahexaenoic acid, in muscle tissues of brown trout. Our study thus suggests that increased reliance on diets low in n-3 LC-PUFA, such as terrestrial subsidies, can have a significant negative impact on brain development of wild trout. Our findings provide the first evidence of how brains of wild vertebrate consumers response to scarcity of n-3 LC-PUFA content in natural prey.

Assessment of Compound-Specific Fatty Acid δ13C and δ2H Values to Track Fish Mobility in a Small Sub-alpine Catchment

Methods for identifying origin, movement, and foraging areas of animals are essential for understanding ecosystem connectivity, nutrient flows, and other ecological processes. Telemetric methods can provide detailed spatial coverage but are limited to a minimum body size of specimen for tagging. In recent years, stable isotopes have been increasingly used to track animal migration by linking landscape isotope patterns into movement (isoscapes). However, compared to telemetric methods, the spatial resolution of bulk stable isotopes is low. Here, we examined a novel approach by evaluating the use of compound-specific hydrogen and carbon stable isotopes of fatty acids (δ²H_FA and δ¹³C_FA) from fish liver, muscle, brain, and eye tissues for identifying site specificity in a 254 km² sub-alpine river catchment. We analyzed 208 fish (European bullhead, rainbow trout, and brown trout) collected in 2016 and 2018 at 15 different sites. δ¹³C_FA values of these fish tissues correlated more among each other than those of δ²H_FA values. Both δ²H_FA and δ¹³C_FA values showed tissue-dependent isotopic fractionation, while fish taxa had only small effects. The highest site specificity was for δ¹³C_DHA values, while the δ²H isotopic difference between linoleic acid and alpha-linolenic acid resulted in the highest site specificity. Using linear discrimination analysis of FA isotope values, over 90% of fish could be assigned to their location of origin; however, the accuracy dropped to about 56% when isotope data from 2016 were used to predict the sites for samples collected in 2018, suggesting temporal shifts in site specificity of δ²H_FA and δ¹³C_FA. However, the predictive power of δ²H_FA and δ¹³C_FA over this time interval was still higher than site specificity of bulk tissue isotopes for a single time point. In summary, compound-specific isotope analysis of fatty acids may become a highly effective tool for assessing fine and large-scale movement and foraging areas of animals.

Dietary protein:lipid ratio modulates somatic growth and expression of genes involved in somatic growth, lipid metabolism and food intake in Pejerrey fry (Odontesthes bonariensis)

Pejerrey is a freshwater fish from South America with high potential for aquaculture. This study was designed to determine the effects of different dietary protein:lipid ratio on growth rate and the expression of growth, lipid metabolism and feeding-related genes of this species during early developmental stages. Pejerrey fry were fed for 60 days with four experimental diets containing low (400 g Kg^-1) or high (500 g Kg^-1) protein (LP or HP, respectively) and low (120 g Kg^-1) or high (200 g Kg^-1) lipid (LL or HL, respectively), in the combinations: LP-LL; LP-HL; HP-LL and HP-HL. Measurements of growth, lipid and fatty acid content of fry, expression of genes from the endocrine axis (gh, ghrs, igfs), fatty acid metabolism (∆6-desaturase), and food intake behavior (nucb2/nesfatin-1) were collected. Fry fed with diets LP-LL and HP-LL showed the highest growth rate and growth hormone (gh) mRNA expression levels. The gene expression of ∆6-desaturase was high in head of fry fed with diet LP-HL. The mRNA expression of nucb2/nesfatin-1 and gh followed the same patterns in head, and the inverse pattern in body. In conclusion, diets with LL ensure a higher growth of pejerrey fry compared to those that contain HL, without altering the final lipid amount nor the fatty acid profile on fry. In LL groups, the expression of genes from the GH-IGF axis is associated with the observed promotion of somatic growth. The expression of nucb2/nesfatin-1 indicates an effect of this peptide not related to food intake regulation, e.g., a negative regulatory role on GH expression, that would warrant future research.

Longitudinal variation in the nutritional quality of basal food sources and its effect on invertebrates and fish in subalpine rivers

There is growing recognition of the importance of food quality over quantity for aquatic consumers. In streams and rivers, most previous studies considered this primarily in terms of the quality of terrestrial leaf litter and importance of microbial conditioning. However, many recent studies suggest that algae are a more nutritional food source for riverine consumers than leaf litter. To date, few studies have quantified longitudinal shifts in the nutritional quality of basal food resources in river ecosystems and how these may affect consumers. We conducted a field investigation in a subalpine river ecosystem in Austria to investigate longitudinal variations in diet quality of basal food sources (submerged leaves and periphyton) and diet source dependence of stream consumers (invertebrate grazers, shredders, filterers and predators, and fish). Fatty acid (FA) profiles of basal food sources and their consumers were measured. Our results indicate systematic differences between the FA profiles of terrestrial leaves and aquatic biota, that is periphyton, invertebrates and fish. Submerged leaves contained very low proportions of long-chain polyunsaturated fatty acids (LC-PUFAs), which were conversely rich in aquatic biota. While the FA composition of submerged leaves remained similar among sites, the LC-PUFAs of periphyton increased longitudinally, which was associated with increasing nutrients from upstream to downstream. Longitudinal variations in periphyton LC-PUFAs were reflected in the LC-PUFAs of invertebrate grazers and shredders, and further tracked by invertebrate predators and fish. However, brown trout Salmo trutta contained a large proportion of docosahexaenoic acid (DHA, 22:6ω3), a LC-PUFA almost entirely missing in basal sources and invertebrates. The fish accumulated eicosapentaenoic acid (EPA, 20:5ω3) from invertebrate prey and may use this FA to synthesize DHA. Our results provide a nutritional perspective for river food web studies, emphasizing the importance of algal resources to consumer somatic growth and the need to account for the longitudinal shifts in the quality of these basal resources.

Cross-Ecosystem Linkages: Transfer of Polyunsaturated Fatty Acids From Streams to Riparian Spiders via Emergent Insects

Polyunsaturated fatty acids (PUFAs) are essential resources unequally distributed throughout landscapes. Certain PUFAs, such as eicosapentaenoic acid (EPA), are common in aquatic but scarce in terrestrial ecosystems. In environments with low PUFA availability, meeting nutritional needs requires either adaptations in metabolism to PUFA-poor resources or selective foraging for PUFA-rich resources. Amphibiotic organisms that emerge from aquatic ecosystems represent important resources that can be exploited by predators in adjacent terrestrial habitats. Here, we traced PUFA transfer from streams to terrestrial ecosystems, considering benthic algae as the initial PUFA source, through emergent aquatic insects to riparian spiders. We combined carbon stable isotope and fatty acid analyses to follow food web linkages across the ecosystem boundary and investigated the influence of spider lifestyle (web building vs. ground dwelling), season, and ecosystem degradation on PUFA relations. Our data revealed that riparian spiders consumed considerable amounts of aquatic-derived resources. EPA represented on average 15 % of the total fatty acids in riparian spiders. Season had a strong influence on spider PUFA profiles, with highest EPA contents in spring. Isotope data revealed that web-building spiders contain more aquatic-derived carbon than ground dwelling spiders in spring, although both spider types had similarly high EPA levels. Comparing a natural with an anthropogenically degraded fluvial system revealed higher stearidonic acid (SDA) contents and Σω3/Σω6 ratios in spiders collected along the more natural river in spring but no difference in spider EPA content between systems. PUFA profiles of riparian spiders where distinct from other terrestrial organism and more closely resembled that of emergent aquatic insects (higher Σω3/Σω6 ratio). We show here that the extent to which riparian spiders draw on aquatic PUFA subsidies can vary seasonally and depends on the spider’s lifestyle, highlighting the complexity of aquatic-terrestrial linkages.

The evolutionary ecology of fatty-acid variation: Implications for consumer adaptation and diversification

The nutritional diversity of resources can affect the adaptive evolution of consumer metabolism and consumer diversification. The omega-3 long-chain polyunsaturated fatty acids eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) have a high potential to affect consumer fitness, through their widespread effects on reproduction, growth and survival. However, few studies consider the evolution of fatty acid metabolism within an ecological context. In this review, we first document the extensive diversity in both primary producer and consumer fatty acid distributions amongst major ecosystems, between habitats and amongst species within habitats. We highlight some of the key nutritional contrasts that can shape behavioural and/or metabolic adaptation in consumers, discussing how consumers can evolve in response to the spatial, seasonal and community-level variation of resource quality. We propose a hierarchical trait-based approach for studying the evolution of consumers' metabolic networks and review the evolutionary genetic mechanisms underpinning consumer adaptation to EPA and DHA distributions. In doing so, we consider how the metabolic traits of consumers are hierarchically structured, from cell membrane function to maternal investment, and have strongly environment-dependent expression. Finally, we conclude with an outlook on how studying the metabolic adaptation of consumers within the context of nutritional landscapes can open up new opportunities for understanding evolutionary diversification.

Omega-3 PUFA profoundly affect neural, physiological, and behavioural competences - implications for systemic changes in trophic interactions

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.