Temperature-induced changes in fatty acid dynamics of the intertidal grazer Platychelipus littoralis (Crustacea, Copepoda, Harpacticoida): Insights from a short-term feeding experiment

Functional characterization reveals a diverse array of metazoan fatty acid biosynthesis genes

Long-chain (≥C₂₀ ) polyunsaturated fatty acids (LC-PUFAs) are physiologically important fatty acids for most animals, including humans. Although most LC-PUFA production occurs in aquatic primary producers such as microalgae, recent research indicates the ability of certain groups of (mainly marine) invertebrates for endogenous LC-PUFA biosynthesis and/or bioconversion from dietary precursors. The genetic pathways for and mechanisms behind LC-PUFA biosynthesis remain unknown in many invertebrates to date, especially in non-model species. However, the numerous genomic and transcriptomic resources currently available can contribute to our knowledge of the LC-PUFA biosynthetic capabilities of metazoans. Within our previously generated transcriptome of the benthic harpacticoid copepod Platychelipus littoralis, we detected expression of one methyl-end desaturase, one front-end desaturase, and seven elongases, key enzymes responsible for LC-PUFA biosynthesis. To demonstrate their functionality, we characterized eight of them using heterologous expression in yeast. The P. littoralis methyl-end desaturase has Δ15/17/19 desaturation activity, enabling biosynthesis of α-linolenic acid, eicosapentaenoic acid and docosahexaenoic acid (DHA) from 18:2 n-6, 20:4 n-6 and 22:5 n-6, respectively. Its front-end desaturase has Δ4 desaturation activity from 22:5 n-3 to DHA, implying that P. littoralis has multiple pathways to produce this physiologically important fatty acid. All studied P. littoralis elongases possess varying degrees of elongation activity for saturated and unsaturated fatty acids, producing aliphatic hydrocarbon chains with lengths of up to 30 carbons. Our investigation revealed a functionally diverse range of fatty acid biosynthesis genes in copepods, which highlights the need to scrutinize the role that primary consumers could perform in providing essential nutrients to upper trophic levels.

Effects of temperature and combinational exposures on lipid metabolism in aquatic invertebrates

Studies of changes in fatty acids in response to environmental temperature changes have been conducted in many species, particularly mammals. However, few studies have considered aquatic invertebrates, even though they are particularly vulnerable to changes in environmental temperature. In this review, we summarize the process by which animals synthesize common fatty acids and point out differences between the fatty acid profiles of vertebrates and those of aquatic invertebrates. Unlike vertebrates, some aquatic invertebrates can directly synthesize polyunsaturated fatty acids (PUFAs), which can be used to respond to temperature changes. Various studies have shown that aquatic invertebrates increase the degree of saturation in their fatty acids through an increase in saturated fatty acid production or a decrease in PUFAs as the temperature increases. In addition, we summarize recent studies that have examined the complex effects of temperature and combinational stressors to determine whether the degree of saturation in aquatic invertebrates is influenced by other factors. The combined effects of carbon dioxide partial pressure, food quality, starvation, salinity, and chemical exposures have been confirmed, and fatty acid profile changes in response to high temperature were greater than those from combinational stressors.

Genome-wide identification of fatty acid synthesis genes, fatty acid profiles, and life parameters in two freshwater water flea Daphnia magna strains

The freshwater water flea Daphnia magna is a planktonic animal belonging to the Cladocera. To evaluate differences between two D. magna strains (KIT and NIES) in terms of life parameters and fatty acid profiles, we examined several endpoints. In the D. magna KIT strain, the numbers of total and cumulative offspring were lower at 23 °C and higher at 14 °C than in the D. magna NIES strain. However, at 14 °C, the D. magna KIT strain showed an increased lifespan. Although the n-3/n-6 polyunsaturated fatty acids (PUFA) ratio was always decreased at a low temperature, the PUFA ratio in the KIT strain had a higher value on day 3 than the NIES strain, which gave it higher adaptability to low temperature. In addition, we identified the elongation of very long chain fatty acids (elovl) and fatty acid desaturase (fad) genes, which are involved in fatty acid biosynthesis pathways, in the genomes of both D. magna KIT and NIES. The Elovl and Fad genes in both D. magna strains were highly conserved, including tandem duplicated Elovl 1/7 genes. This study provides new information about the molecular basis for the difference in temperature sensitivity between two strains of D. magna.

Comparison of lipids and fatty acids among tissues of two semiterrestrial crabs reveals ecophysiological adaptations in changing coastal environments

Decapods have successfully colonized changing coastal habitats throughout the world by adapting their behavior, physiology, and biochemistry. Biochemical reserves, such as lipids and fatty acids (FAs), play fundamental roles in this adaptation process. These energy reserves are key for the development of decapods and their composition mainly depends on the type and quality of food available in their habitats. This study evaluated the lipid content and FA composition of three tissues (hepatopancreas, gills, and muscle) in two widely distributed, semi-terrestrial coastal crab species in Chile, Cyclograpsus cinereus from the upper intertidal and Hemigrapsus crenulatus from estuaries. This evaluation aimed to assess the physiological role of the bioenergetic reserves of these crabs, which tolerate fluctuating environmental conditions. Our results showed that both species had a higher lipid content in the hepatopancreas and a lower lipid content in its gills and muscle. All three of the evaluated tissues in C. cinereus showed high contents of saturated fatty acids (SFAs), and its hepatopancreas displayed the highest contents of monounsaturated (MUFAs) and polyunsaturated fatty acids (PUFAs). In turn, H. crenulatus had the highest contents of MUFAs and PUFAs in its gills and muscle tissues, including an important amount of eicosapentaenoic acid (EPA). The FA content of C. cinereus may indicate an adaptive physiological response aimed at maintaining its cellular fluid balance during periods of desiccation in the upper intertidal zone. In contrast, the FAs found in H. crenulatus may be linked to the high activity of the sodium‑potassium pump in its gills, in order to maintain osmoregulation in estuaries.

Seasonal and spatial fatty acid profiling of the calanoid copepods Temora longicornis and Acartia clausi linked to environmental stressors in the North Sea

The Belgian part of the North Sea (BPNS) is subjected to multiple environmental stressors. The impact of these stressors includes the modulation of fatty acid (FA) composition of the zooplankton. This study recorded temporal and spatial patterns of the FA profiles of two dominant calanoid copepods within the BPNS: Temora longicornis (Müller, 1785) and Acartia clausi (Giesbrecht, 1889). By means of distance-based linear modelling and by applying multi model inference to generalized additive models, environmental stressors were linked to patterns of the FA profiles of these species. The FA profiles of A. clausi and T. longicornis showed distinct intraspecific, spatial and temporal differences within the BPNS. Temperature and algal food quality (marked by the ratio of silicate concentration to dissolved inorganic nitrogen concentration, SiO₄/DIN) were the most important drivers of seasonal fluctuations in the DHA/EPA ratio of both species. DHA/EPA ratio can be used as marker for stress in copepods in the BPNS in order to have a quick indication of food quality changes at the basis of the food web.

Characterization of Humic Substances in the Soils of Ophiocordyceps sinensis Habitats in the Sejila Mountain, Tibet: Implication for the Food Source of Thitarodes Larvae

Humic substances in soil are considered to be an alternative food to the tender plant roots for Thitarodes larvae in the habitats of Ophiocordyceps sinensis in the Qinghai-Tibetan Plateau. However, there is no report involving the evaluation of their potential as a food source from the composition and structure of habitat soils. In this work, the composition and structure of humic substances in habitat soils from the Sejila Mountain, Tibet were characterized by diverse techniques for evaluating the nutritional value and possibility of humus as the food source for Thitarodes larvae. Fourier transform infrared spectroscopy revealed that humic acid may possess superior ability to provide the molecular segments for biosynthesizing lipids more than other humic fractions. Combining with the analysis of solid-state 13C nuclear magnetic resonance spectrum, the fractions of hydrophobic fulvic acid and hydrophilic fulvic acid are further considered as a potential food source for Thitarodes larvae. Overall, humic substances in habitat soils are rich in the molecular segments for biosynthesizing lipids and other important nutrients, which may provide the energy and material sources for maintaining the survival of Thitarodes larvae in the absence of tender plant roots, particularly in the annual cold winter. Combining with the evidence of physico-chemical parameters of habitat soils and stable carbon isotopic composition of major tender plant roots in the Sejila Mountain, the composition and structure of humic substances in habitat soils may provide a novel idea for the eco-friendly and semi-wild cultivation of Thitarodes larvae with low cost.

Feeding response of the tropical copepod Acartia erythraea to short-term thermal stress: more animal-derived food was consumed

The objective of this study was to explore the feeding response of tropical copepods to short-term thermal shock and provide insight into the potential impact of coastal power plants on the trophic dynamics of tropical coastal ecosystems. Feeding experiments were conducted at three different temperatures (29 °C, 33 °C, and 35 °C) using the copepod Acartia erythraea, collected from Sanya Bay, China. The grazing rate of A. erythraea decreased dramatically in the high temperature treatment. Analysis of 18S rDNA clone libraries revealed that the diet of copepods from different treatments was mainly comprised of diatoms, metazoans, and protozoans; A. erythraea exhibited an obvious feeding preference shift with temperature, with a change from a diatom-dominated diet at 29 °C to a metazoan-dominated diet at 35 °C, and the omnivory index shifted from 0.1 to 2.84 correspondingly. Furthermore, A. erythraea showed a positive feeding response to plant food (i.e., phytoplankton and land plants) in the control treatment (29 °C), but a positive response to animal prey (i.e., metazoans and protozoans) at temperatures exceeding 33 °C, as evaluated by the Ivlev’s selectivity index. Our results suggest that copepods could regulate their food intake by considering their energy demands when exposed to short-term thermal stress, which might influence the pathway of materials moving up the trophic system. However, further studies are required to elucidate the effects of elevated temperature on feeding of different organisms in order to predict the influence of thermal pollution on the food web of tropical coastal ecosystems.

Climate warming is predicted to reduce omega-3, long-chain, polyunsaturated fatty acid production in phytoplankton

Phytoplankton are the main source of energy and omega-3 (n-3) long-chain essential fatty acids (EFA) in aquatic ecosystems. Their growth and biochemical composition are affected by surrounding environmental conditions, including temperature, which continues to increase as a result of climate warming. Increasing water temperatures may negatively impact the production of EFA by phytoplankton through the process of homeoviscous adaptation. To investigate this, we conducted an exploratory data synthesis with 952 fatty acid (FA) profiles from six major groups of marine and freshwater phytoplankton. Temperature was strongly correlated with a decrease in the proportion of n-3 long-chain polyunsaturated FA (LC-PUFA) and an increase in omega-6 FA and saturated FA. Based on linear regression models, we predict that global n-3 LC-PUFA production will be reduced by 8.2% for eicosapentaenoic acid (EPA) and 27.8% for docosahexaenoic acid (DHA) with an increase in water temperature of 2.5 °C. Using a previously published estimate of the global production of EPA by diatoms, which contribute to most of the world's supply of EPA, we predict a loss of 14.2 Mt of EPA annually as a result of ocean warming. The n-3 LC-PUFA are vitally important for an array of key physiological functions in aquatic and terrestrial organisms, and these FA are mainly produced by phytoplankton. Therefore, reduced production of these EFA, as a consequence of climate warming, is predicted to negatively affect species that depend on these compounds for optimum physiological function. Such profound changes in the biochemical composition of phytoplankton cell membranes can lead to cascading effects throughout the world's ecosystems.