Fractionation of the stable carbon isotope ratio of essential fatty acids in zebrafish Danio rerio and mud snails Bellamya chinensis

Exploring the use of fish as indicators of eicosapentaenoic and docosahexaenoic supply in lake ecosystems

An adequate supply of food sources with high levels (i.e., weight proportion of total fatty acids) and contents (i.e., absolute amount per mass) of long chain polyunsaturated fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are important for ecosystems. Therefore, the supply of EPA and DHA from basal food sources is a useful indicator of ecosystem health. To determine whether EPA and DHA levels and contents in fish can be used as indicators of EPA and DHA supply in lakes, five dominant species of fish and basal food sources (seston and sediment) were collected monthly from June to November from 2016 to 2021 from Lake Hachiro, Japan. Seston and Hypomesus nipponensis were collected from 12 lakes (one collection per lake) with varying seston contents in EPA and DHA. The trends of EPA and DHA in all fish species were similar to those of the basal food sources. Correlation analysis showed that the EPA levels were strongly correlated between fish and seston; moreover, the correlation coefficient increased when a 1- or 2-month moving average was applied to the basal food sources, suggesting that fish represent a time-integrated supply of EPA and DHA. EPA levels of H. nipponensis had the highest correlation coefficients with seston among all fish species. EPA levels of H. nipponensis were significantly correlated with those of seston among lakes. The results of this study suggest that H. nipponensis is a useful indicator of EPA and DHA supplies in lake ecosystems.

Compound-specific stable hydrogen isotope (δ2 H) analyses of fatty acids: A new method and perspectives for trophic and movement ecology

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 (δ² 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 δ² 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 δ² 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.

The Negative Relationship between Fouling Organisms and the Content of Eicosapentaenoic Acid and Docosahexaenoic Acid in Cultivated Pacific Oysters, Crassostrea gigas

Bivalves serve as an important aquaculture product, as they are the source of essential fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in our diet. However, their cultivation in the wild can be affected by fouling organisms that, in turn, affect their EPA and DHA content. The effects of fouling organisms on the EPA and DHA contents of cultivated bivalves have not been well documented. We examined the effects of fouling organisms on the EPA and DHA contents and condition index of cultured oysters, Crassostrea gigas, in an aquaculture system. We sampled two-year-old oysters from five sites in Shizugawa Bay, Japan, in August 2014. Most of the fouling organisms were sponges, macroalgae, and Mytilus galloprovincialis. A significant negative relationship existed between the DHA content in C. gigas and the presence of sponges and macroalgae. A lower C. gigas EPA content corresponded to a higher M. galloprovincialis fouling mass and a lower C. gigas condition index. This can be explained by dietary competition between C. gigas and M. galloprovincialis for diatoms, which were the main producer of EPA in our study sites. Our findings indicate that fouling organisms likely reduce the EPA and DHA content in cultivated oysters. Therefore, our results suggest that the current efforts to remove fouling organisms from oyster clusters is an effective strategy to enhance the content of EPA and DHA in oysters.

Fatty acid accumulation in feeding types of a natural freshwater fish population

Fatty acids are widely used to study trophic interactions in food web assemblages. Generally, it is assumed that there is a very small modification of fatty acids from one trophic step to another, making them suitable as trophic biomarkers. However, recent literature provides evidence that many fishes possess genes encoding enzymes with a role in bioconversion, thus the capability for bioconversion might be more widespread than previously assumed. Nonetheless, empirical evidence for biosynthesis occurring in natural populations remains scarce. In this study, we investigated different feeding types of perch (Perca fluviatilis) that are specialized on specific resources with different levels of highly unsaturated fatty acids (HUFAs), and analyzed the change between HUFA proportions in perch muscle tissue compared to their resources. Perch showed matching levels to their resources for EPA, but ARA and especially DHA were accumulated. Compound-specific stable isotope analyses helped us to identify the origin of HUFA carbon. Our results suggest that perch obtain a substantial amount of DHA via bioconversion when feeding on DHA-poor benthic resources. Thus, our data indicate the capability of bioconversion of HUFAs in a natural freshwater fish population.

Dominance of harmful algae, Microcystis spp. and Micrasterias hardyi, has negative consequences for bivalves in a freshwater lake

The proliferation of cyanobacteria Microcystis spp. and the invading green alga Micrasterias hardyi in Lake Biwa has been increasing. However, the available knowledge on the dietary utilization of these cyanobacterial and algal species by bivalves, which are key species in lake ecosystems, is limited. In this study, we examined the dietary quality and utilization of these species by freshwater bivalves of the Corbicula spp., which are important fishery resources, by performing feeding experiments and field investigations based on fatty acid profiles and stomach content analysis. Although a significant increase in the dry weight and condition factor of the Corbicula spp. individuals fed on diatom was observed at the end of the experiment, for the individuals fed on Microcystis aeruginosa or M. hardyi, a dry weight increase was not observed and their condition factor decreased. Moreover, the fatty acid profile of the Corbicula spp. individuals fed on M. aeruginosa or M. hardyi indicated that they did not assimilate these diets, even though filtration was confirmed during the experiments. However, the stomach contents of wild Corbicula spp. specimens, collected from six sampling sites in Lake Biwa on four sampling occasions, showed that Microcystis spp. were the most abundant dietary items in all sites and on all occasions. Moreover, M. hardyi was detected during the analysis of stomach contents; this alga was the third most abundant algal species. As shown in the feeding experiments, they do not contribute to bivalve growth, indicating that the high occupation of Microcystis spp. and M. hardyi in the consumer's stomach may inhibit effective carbon transfer. The expansion of these unsuitable dietary organisms may affect the stability of lake ecosystems.

The potential of fatty acid isotopes to trace trophic transfer in aquatic food-webs

Compound-specific isotope analyses (CSIA) of fatty acids (FA) constitute a promising tool for tracing energy flows in food-webs. However, past applications of FA-specific carbon isotope analyses have been restricted to a relatively coarse food-source separation and mainly quantified dietary contributions from different habitats. Our aim was to evaluate the potential of FA-CSIA to provide high-resolution data on within-system energy flows using algae and zooplankton as model organisms. First, we investigated the power of FA-CSIA to distinguish among four different algae groups, namely cyanobacteria, chlorophytes, haptophytes and diatoms. We found substantial within-group variation but also demonstrated that δ¹³C of several FA (e.g. 18:3ω3 or 18:4ω3) differed among taxa, resulting in group-specific isotopic fingerprints. Second, we assessed changes in FA isotope ratios with trophic transfer. Isotope fractionation was highly variable in daphnids and rotifers exposed to different food sources. Only δ¹³C of nutritionally valuable poly-unsaturated FA remained relatively constant, highlighting their potential as dietary tracers. The variability in fractionation was partly driven by the identity of food sources. Such systematic effects likely reflect the impact of dietary quality on consumers' metabolism and suggest that FA isotopes could be useful nutritional indicators in the field. Overall, our results reveal that the variability of FA isotope ratios provides a substantial challenge, but that FA-CSIA nevertheless have several promising applications in food-web ecology. This article is part of the theme issue 'The next horizons for lipids as 'trophic biomarkers': evidence and significance of consumer modification of dietary fatty acids'.

Origin of Carbon and Essential Fatty Acids in Higher Trophic Level Fish in Headwater Stream Food Webs

Dietary carbon sources in headwater stream food webs are divided into allochthonous and autochthonous organic matters. We hypothesized that: 1) the dietary allochthonous contribution for fish in headwater stream food webs positively relate with canopy cover; and 2) essential fatty acids originate from autochthonous organic matter regardless of canopy covers, because essential fatty acids, such as 20:5ω3 and 22:6ω3, are normally absent in allochthonous organic matters. We investigated predatory fish Salvelinus leucomaenis stomach contents in four headwater stream systems, which are located in subarctic region in northern Japan. In addition, stable carbon and nitrogen isotope ratios, fatty acid profile, and stable carbon isotope ratios of essential fatty acids were analyzed. Bulk stable carbon analysis showed the major contribution of autochthonous sources to assimilated carbon in S. leucomaenis. Surface baits in the stomach had intermediate stable carbon isotope ratios between autochthonous and allochthonous organic matter, indicating aquatic carbon was partly assimilated by surface baits. Stable carbon isotope ratios of essential fatty acids showed a positive relationship between autochthonous sources and S. leucomaenis across four study sites. This study demonstrated that the main supplier of dietary carbon and essential fatty acids was autochthonous organic matter even in headwater stream ecosystems under high canopy cover.

Effect of oil spill stress on fatty acid stable carbon isotope composition of Ulva pertusa

Petroleum is one of the most important pollutants in the marine ecosystem, and oil spills have a long-term effect on the marine environment. After an oil spill, petroleum continues to diffuse, flowing to the intertidal zone where it accumulates. Ulva pertusa is the main macroalgae species in the intertidal zone of the ocean, and petroleum is toxic to marine macroalgae. Fatty acids, which are involved in various physiological activities, including metabolism, are indispensable substances in organisms. Thus, an organism's fatty acid composition reflects its physiological state. Because fatty acids are carbon-rich compounds, their stable carbon isotope composition can be measured to assess the state of the organism. In this study, the effects of the water accommodated fraction (WAF) of different concentrations of 180# fuel oil and 0# diesel oil on the chlorophyll a and fatty acid carbon stable isotope composition (δ¹³C_FAs) of U. pertusa were assessed. The fatty acid carbon stable isotope value varied among the different petroleum treatments, and high WAF concentrations significantly inhibited the synthesis of unsaturated fatty acids. Thus, exposure to petroleum products affected the synthesis of fatty acids in U. pertusa and also caused carbon stable isotope fractionation during the fatty acid synthesis process. The results indicate that the saturated fatty acid δ¹³C_16:0 and the unsaturated fatty acid δ¹³C_18:1 can be used as biomarkers to evaluate the degree of petroleum-induced stress in U. pertusa and that carbon stable isotope analysis of U. pertusa can be used as an environmental risk assessment tool in the intertidal zone.

The application of compound-specific isotope analysis of fatty acids for traceability of sea cucumber (Apostichopus japonicus) in the coastal areas of China

BACKGROUND

Geographical origin traceability is an important issue for controlling the quality of seafood and safeguarding the interest of consumers. In the present study, a new method of compound-specific isotope analysis (CSIA) of fatty acids was established to evaluate its applicability in establishing the origin traceability of Apostichopus japonicus in the coastal areas of China. Moreover, principal component analysis (PCA) and discriminant analysis (DA) were applied to distinguish between the origins of A. japonicus.

RESULTS

The results show that the stable carbon isotope compositions of fatty acids of A. japonicus significantly differ in terms of both season and origin. They also indicate that the stable carbon isotope composition of fatty acids could effectively discriminate between the origins of A. japonicus, except for between Changhai Island and Zhangzi Island in the spring of 2016 because of geographical proximity or the similarity of food sources. The fatty acids that have the highest contribution to identifying the geographical origins of A. japonicus are C22:6n-3, C16:1n-7, C20:5n-3, C18:0 and C23:1n-9, when considering the fatty acid contents, the stable carbon isotope composition of fatty acids and the results of the PCA and DA.

CONCLUSIONS

We conclude that CSIA of fatty acids, combined with multivariate statistical analysis such as PCA and DA, may be an effective tool for establishing the traceability of A. japonicus in the coastal areas of China. The relevant conclusions of the present study provide a new method for determining the traceability of seafood or other food products. © 2017 Society of Chemical Industry.