Evaluation of parental and transgenerational effects of clotrimazole in Daphnia magna - A multi-parametric approach

Azole antifungals inhibit the cytochrome P450 complex, decreasing the production of ergosterol in fungi, and compromising the biosynthesis of ecdysteroids in crustaceans, which are hormones regulating reproduction and ecdysis. The azole antifungal clotrimazole (CLO) raises environmental concerns due its toxicity. This work evaluated the effects on the number of moults, feeding rate, growth, reproduction, transgenerational reproductive effects on two different generations (F0, parental generation; and F1, organisms born from F0), and energetic balance in Daphnia magna. Neonates (<24 h) were exposed to sublethal concentrations (0, 2.7, and 3.4 mg/L) of CLO, to assess its effects on the moulting process. Neonates were also exposed to environmentally realistic concentrations of CLO (0, 30, 150, 750, and 3750 ng/L) for 24 and 96 h, to assess adverse effects on their feeding behaviour. Effects on energy reserves (fatty acids, glycogen, and protein levels) were also measured in animals exposed to CLO. A reproduction test was carried out to evaluate the amount and size of neonates from F0 and F1 generations. CLO exposure decreased the number of moults, and the size of organisms, but did not alter the feeding pattern of 5 days old individuals. However, neonates (<24 h) exposed to CLO had a significant decrease in their feeding pattern. CLO decreased the fatty acids content in exposed animals, but did not change glycogen and protein. CLO also decreased the size of adult daphnids from the third brood, born from animals exposed in F0; in F1 animals, the size of neonates from the third brood was decreased. This study evidenced the toxic effects caused by CLO on growth, feeding and reproduction of D. magna. Nevertheless, it is not possible to conclude whether the effects are due to the inhibition of cytochrome P450 enzymes, or to unspecific effects caused by general toxic stress and decreased nutrition.

The Effect of Brachionus calyciflorus (Rotifera) on Larviculture and Fatty Acid Composition of Pikeperch (Sander lucioperca (L.)) Cultured under Pseudo-Green Water Conditions

A new cultivation system with the chlorophyte Monoraphidium contortum combined with a self-sustaining culture of the freshwater rotifer Brachionus calyciflorus was applied for Sander lucioperca (L.) larviculture. Survival, morphometrics, as well as fatty acid composition of pikeperch larvae were analyzed after a ten-day feeding period. By using the pseudo-green water technique with improved aeration and water movement at the surface, survival rates reached up to 94%, with a total larval length of 8.1 ± 0.3 mm and a specific length growth rate of up to 4.1% day−1 for S. lucioperca. The biochemical composition of B. calyciflorus and especially its contents in C18 PUFAs and suitable n-3/n-6 ratios met the nutritional requirements of pikeperch larvae. The high abundance of highly unsaturated fatty acids (HUFAs) in the diet appeared to be less important in the first feeding due to a possible retention of essential fatty acids, which originate from the yolk sac reserves, at adequate levels. Exponential growth of microalgae and zooplankton under the applied conditions was most effective when stocking M. contortum five days and B. calyciflorus three days before adding the fish larvae. Appropriate timing and sufficient live feed density allowed a successful integration of B. calyciflorus into pikeperch larviculture. We hypothesize that feeding pikeperch larvae with a self-sustaining Brachionus-culture under pseudo-green water conditions with minor disruptions during larviculture will improve survival and growth. This system is a first step towards pikeperch larviculture inside recirculated aquaculture systems (RAS) under continuous feed supply with live feed within the same aquaculture unit.

Phenotypic convergence in a natural Daphnia population acclimated to low temperature

Fluidity of a given membrane decreases at lower ambient temperatures, whereas it rises at increasing temperatures, which is achieved through changes in membrane lipid composition. In consistence with homeoviscous adaptation theory, lower temperatures result in increased tissue concentrations of polyunsaturated fatty acids (PUFAs) in Daphnia magna, suggesting a higher PUFA requirement at lower temperatures. However, so far homeoviscous adaptation has been suggested for single or geographically separated Daphnia genotypes only. Here, we investigated changes in relative fatty acid (FA) tissue concentrations in response to a lower temperature (15°C) within a D. magna population. We determined juvenile growth rates (JGR) and FA patterns of 14 genotypes that were grown on Chlamydomonas klinobasis at 15°C and 20°C. We report significant differences of JGR and the relative body content of various FAs between genotypes at either temperature and between temperatures. Based on slopes of reaction norms, we found genotype-specific changes in FA profiles between temperatures suggesting that genotypes have different strategies to cope with changing temperatures. In a hierarchical clustering analysis, we grouped genotypes according to differences in direction and magnitude of changes in relative FA content, which resulted in three clusters of genotypes following different patterns of changes in FA composition. These patterns suggest a lower importance of the PUFA eicosapentaenoic acid (EPA, C20:5ω3) than previously assumed. We calculated an unsaturation index (UI) as a proxy for membrane fluidity at 15°C, and we neither found significant differences for this UI nor for fitness, measured as JGR, between the three genotype clusters. We conclude that these three genotype clusters represent different physiological solutions to temperature changes by altering the relative share of different FAs, but that their phenotypes converge with respect to membrane fluidity and JGR. These clusters will be subjected to different degrees of PUFA limitation when sharing the same diet.

Taxonomic Shift Over a Phosphorus Gradient Affects the Stoichiometry and Fatty Acid Composition of Stream Periphyton

Phosphorus enrichment of stream ecosystems generally increases primary production in the benthos, but the consequences of eutrophication for the nutritional quality of periphyton for grazers are less clear. On short timescales, high phosphorus inputs may lead to reduced C:P ratios and high essential fatty acid contents of periphyton, which are both considered important determinants of food quality for grazers. However, nutrient enrichment may alter the taxonomic composition of periphyton and favor the growth of less palatable algal taxa. In this study, periphyton was grown under a gradient of dissolved phosphorus availability from 5 to 100 µg P · L^-1 , to investigate eutrophication effects on periphyton taxonomy, C:N:P stoichiometry, and fatty acid composition. After 1 month, periphyton grown under oligotrophic conditions was mainly composed of diatoms (~86%). With increasing phosphorus availability, diatoms were gradually outcompeted by chlorophytes and cyanobacteria, which were the predominant taxon under eutrophic conditions. Unexpectedly, periphyton C:P ratios increased with greater phosphorus supply, from ~280 under oligotrophic conditions up to ~790 at 100 µg · L^-1 , reflecting a tendency of chlorophytes and cyanobacteria to produce more biomass per unit of assimilated phosphorus compared to diatoms. Periphyton content of essential polyunsaturated fatty acids relative to biomass followed a unimodal relationship with phosphorus availability and peaked at intermediate phosphorus levels, likely as a result of both taxonomic and nutrient effects. Our results demonstrate that phosphorus-driven eutrophication of freshwater ecosystems may worsen periphyton nutritional quality due to taxonomic sorting, which may further lead to lower growth and reproduction of herbivores.

Fatty acid bioconversion in harpacticoid copepods in a changing environment: a transcriptomic approach

By 2100, global warming is predicted to significantly reduce the capacity of marine primary producers for long-chain polyunsaturated fatty acid (LC-PUFA) synthesis. Primary consumers such as harpacticoid copepods (Crustacea) might mitigate the resulting adverse effects on the food web by increased LC-PUFA bioconversion. Here, we present a high-quality de novo transcriptome assembly of the copepod Platychelipus littoralis, exposed to changes in both temperature (+3°C) and dietary LC-PUFA availability. Using this transcriptome, we detected multiple transcripts putatively coding for LC-PUFA-bioconverting front-end fatty acid (FA) desaturases and elongases, and performed phylogenetic analyses to identify their relationship with sequences of other (crustacean) taxa. While temperature affected the absolute FA concentrations in copepods, LC-PUFA levels remained unaltered even when copepods were fed an LC-PUFA-deficient diet. While this suggests plasticity of LC-PUFA bioconversion within P. littoralis, none of the putative front-end desaturase or elongase transcripts was differentially expressed under the applied treatments. Nevertheless, the transcriptome presented here provides a sound basis for future ecophysiological research on harpacticoid copepods. 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'.

Transcriptome sequencing of a keystone aquatic herbivore yields insights on the temperature-dependent metabolism of essential lipids

Background

Nutritional quality of phytoplankton is a major determinant of the trophic transfer efficiency at the plant-herbivore interface in freshwater food webs. In particular, the phytoplankton’s content of the essential polyunsaturated omega-3 fatty acid eicosapentaenoic acid (EPA) has been repeatedly shown to limit secondary production in the major zooplankton herbivore genus Daphnia. Despite extensive research efforts on the biological model organism Daphnia, and the availability of several Daphnia genomes, little is known regarding the molecular mechanisms underlying the limitations in Daphnia related to dietary EPA availability.

Results

We used RNA-seq to analyse the transcriptomic response of Daphnia magna which were fed with two different diets — each with or without supplementation of EPA — at two different temperature levels (15 and 20 °C). The transcripts were mapped to the D. magna genome assembly version 2.4, containing 26,646 translations. When D. magna fed on green alga, changing the temperature provoked a differential expression of 2001 transcripts, and in cyanobacteria-fed daphnia, 3385 transcripts were affected. The supplementation of EPA affected 1635 (on the green algal diet), or 175 transcripts (on the cyanobacterial diet), respectively. Combined effects for diet and temperature were also observed (669 for the green algal and 128 transcripts for the cyanobacterial diet). Searching for orthologous genes (COG-analysis) yielded a functional overview of the altered transcriptomes. Cross-matched transcript sets from both feed types were compiled to illuminate core responses to the factors temperature and EPA-supplementation.

Conclusions

Our highly controlled eco-physiological experiments revealed an orchestrated response of genes involved in the transformation and signalling of essential fatty acids, including eicosanoid-signalling pathways with potential immune functions. We provide an overview of downstream-regulated genes, which contribute to enhance growth and reproductive output. We also identified numerous EPA-responsive candidate genes of yet unknown function, which constitute new targets for future studies on the molecular basis of EPA-dependent effects at the freshwater plant-herbivore interface.

Nitrogen enrichment leads to changing fatty acid composition of phytoplankton and negatively affects zooplankton in a natural lake community

Secondary production in freshwater zooplankton is frequently limited by the food quality of phytoplankton. One important parameter of phytoplankton food quality are essential polyunsaturated fatty acids (PUFAs). Since the fatty acid composition of phytoplankton is variable and depends on the algae’s nutrient supply status, inorganic nutrient supply may affect the algal PUFA composition. Therefore, an indirect transfer of the effects of nutrient availability on zooplankton by changes in algal PUFA composition is conceivable. While the phosphorus (P) supply in lakes is largely decreasing, nitrogen (N) inputs continue to increase. This paper presents data from a mesocosm field experiment in which we exposed phytoplankton communities to increasing N enrichment. As a consequence, the PUFA composition of the phytoplankton community changed. With increasing nitrogen fertilisation, we observed lower quantities of essential PUFAs, together with a decrease in the abundances of the dominant herbivorous zooplankton Daphnia sp. Their biomass was significantly correlated with phytoplankton PUFA content (C18:3 ω3, C20:5 ω3, C18:2 ω6). Our data therefore indicate that changes in nitrogen supply, together with the resultant changes in phytoplankton food quality, can negatively affect the secondary production of herbivorous zooplankton by reducing the availability of essential polyunsaturated fatty acids.