Fatty acid composition of the heterotrophic nanoflagellate Paraphysomonas sp. : influence of diet and de novo synthesis

International audienceWe compared the relative importance of dietary factors versus de novo synthesis in determining the polyunsaturated fatty acid (PUFA) composition of the heterotrophic nanoflagellate Paraphysomonas sp. The flagellate was fed with different mutants of the picocyanobacterial strain Synechocystis PCC6803, which differ in their capability to synthesize specific PUFAs. The desA, desB, and desD genes of Synechocystis PCC6803 encode lipid desaturases at the Δ12, Δ15, and Δ6 positions of 18C fatty acids (FAs), respectively. Thus, the use of desA–, desB–, desD–, and desA–/des D– mutants of Synechocystis PCC6803 as food sources permitted us to provide the heterotrophic flagellate with decreasing levels of unsaturated FAs. In each treatment, Paraphysomonas sp. exhibited the same FA composition pattern, i.e. high levels of 16:0 and 18:1, and significant amounts of 18C PUFAs and long-chain PUFAs such as 20:4(n-6), 20:5(n-3), and 22:6(n-3), which indicated that Paraphysomonas sp. is capable of synthesizing these PUFAs de novo. Results also showed that dietary 18C PUFAs seem to be preferentially accumulated in Paraphysomonas sp. lipids. This demonstrates that heterotrophic protists could play a key role in transferring essential compounds from primary producers to metazoan consumers

Sex-Specific Differences in Essential Lipid Requirements of Daphnia magna

Sex-specific differences in nutritional requirements may crucially influence the performances of the sexes, which may have implications for sexual reproduction and thus is of great ecological and evolutionary interest. In the freshwater model species Daphnia magna, essential lipid requirements have been extensively studied. Dietary deficiencies in sterols and polyunsaturated fatty acids (PUFA) have been shown to constrain somatic growth and parthenogenetic reproduction of female Daphnia. In contrast, nutrient requirements of male Daphnia have not been studied yet. Supplementation experiments were conducted to investigate differences in sterol (cholesterol) and PUFA (eicosapentaenoic acid, EPA) requirements between female and male D. magna. Thresholds for sterol-limited juvenile growth were higher in females than in males, suggesting that females are more susceptible to dietary sterol deficiencies than males. Sex-specific differences in maximum somatic growth rates were evident primarily in the presence of dietary EPA; females could not exploit their generally higher growth potential in the absence of dietary PUFA. However, the thresholds for EPA-limited growth did not differ between sexes, suggesting that both sexes have similar dietary EPA requirements during juvenile growth. During a life history experiment, the gain in body dry mass was higher in females than in males, irrespective of food treatment. In both sexes, the gain in body dry mass increased significantly upon EPA supplementation, indicating that both sexes benefited from dietary EPA supply also later in life. However, the positive effects of EPA supplementation were most pronounced for female reproduction-related traits (i.e., clutch sizes, egg dry masses, and total dry mass investment in reproduction). The high maternal investment in reproduction resulted in a depletion of nutrients in female somata. In contrast, the comparatively low paternal investment in reproduction allowed for the accumulation of nutrients in male somata. We conclude that males are generally less susceptible to dietary nutrient deficiencies than females, because they can rely more on internal body stores. Our data suggest that the performances of the sexes are differentially influenced by lipid-mediated food quality, which may have consequences for sexual reproduction and thus the production of resting eggs and the maintenance of Daphnia populations

Flux of the biogenic volatiles isoprene and dimethyl sulfide from an oligotrophic lake

Biogenic volatile organic compounds (BVOCs) affect atmospheric chemistry, climate and regional air quality in terrestrial and marine atmospheres. Although isoprene is a major BVOC produced in vascular plants, and marine phototrophs release dimethyl sulfide (DMS), lakes have been widely ignored for their production. Here we demonstrate that oligotrophic Lake Constance, a model for north temperate deep lakes, emits both volatiles to the atmosphere. Depth profiles indicated that highest concentrations of isoprene and DMS were associated with the chlorophyll maximum, suggesting that their production is closely linked to phototrophic processes. Significant correlations of the concentration patterns with taxon-specific fluorescence data, and measurements from algal cultures confirmed the phototrophic production of isoprene and DMS. Diurnal fluctuations in lake isoprene suggested an unrecognised physiological role in environmental acclimation similar to the antioxidant function of isoprene that has been suggested for marine biota. Flux estimations demonstrated that lakes are a currently undocumented source of DMS and isoprene to the atmosphere. Lakes may be of increasing importance for their contribution of isoprene and DMS to the atmosphere in the arctic zone where lake area coverage is high but terrestrial sources of BVOCs are small

Simultaneous Effects of Light Intensity and Phosphorus Supply on the Sterol Content of Phytoplankton

Sterol profiles of microalgae and their change with environmental conditions are of great interest in ecological food web research and taxonomic studies alike. Here, we investigated effects of light intensity and phosphorus supply on the sterol content of phytoplankton and assessed potential interactive effects of these important environmental factors on the sterol composition of algae. We identified sterol contents of four common phytoplankton genera, Scenedesmus, Chlamydomonas, Cryptomonas and Cyclotella, and analysed the change in sterol content with varying light intensities in both a high-phosphorus and a low-phosphorus approach. Sterol contents increased significantly with increasing light in three out of four species. Phosphorus-limitation reversed the change of sterol content with light intensity, i.e., sterol content decreased with increasing light at low phosphorus supply. Generally sterol contents were lower in low-phosphorus cultures. In conclusion, both light and phosphorus conditions strongly affect the sterol composition of algae and hence should be considered in ecological and taxonomic studies investigating the biochemical composition of algae. Data suggest a possible sterol limitation of growth and reproduction of herbivorous crustacean zooplankton during summer when high light intensities and low phosphorus supply decrease sterol contents of algae

Fatty acid composition of Turbatrix aceti and its use in feeding regimes of Coregonus maraena (Bloch, 1779): is it really a suitable alternative to Artemia nauplii?

By incorporating the free-swimming nematode Turbatrix aceti into early feeding regimes of the European whitefish Coregonus maraena, the suitability of this nematode species was investigated as an alternative to Artemia nauplii. During a 14-day feeding trial in a total of 25 aquaria each 1.7 L (each treatment n = 5, 255 larvae/tank) T. aceti was used either as the sole live food or in combination with Artemia nauplii or microdiet to determine the effect of T. aceti on growth performance and survival rate of C. maraena. By analysing the fatty acid composition of T. aceti prior to and after enrichment with INVE spresso® it was investigated whether the amount of n3-polyunsaturated fatty acids (n3-PUFA) in T. aceti could be further enhanced. Supplementation of Artemia nauplii with T. aceti increased growth significantly within the first 5 days of rearing in comparison to the non-supplemented food treatments (14.39 ± 0.15 mm compared to 13.44 ± 0.18 mm; mean ± SE). However, growth and survival of juvenile C. maraena on nematode-supplemented Artemia nauplii did not differ significantly from non-supplemented Artemia nauplii at the end of the 14-day rearing period (15.22 ± 0.15 mm compared to 14.86 ± 0.24 mm). All feeding treatments containing Artemia nauplii showed significantly higher growth and lower mortality at the end of the experiment in comparison to diets containing only the microdiet or T. aceti or a combination thereof. The overall low performance of T. aceti alone can most likely be explained by an insufficient capacity of C. maraena to digest this nematode species efficiently. Enrichment with INVE spresso® successfully increased the proportion of DHA in the T. aceti tissue. The results reveal that T. aceti cannot be considered a full alternative to Artemia nauplii, at least not in the rearing of C. maraena, but might be a useful vector of essential fatty acids within the early rearing period of this and potentially other fish species when provided as live food along with Artemia nauplii

Dietary supply with polyunsaturated fatty acids and resulting maternal effects influence host -- parasite interactions

Interactions between hosts and parasites can be substantially modulated by host nutrition. Polyunsaturated fatty acids (PUFAs) are essential dietary nutrients; they are indispensable as structural components of cell membranes and as precursors for eicosanoids, signalling molecules which act on reproduction and immunity. Here, we explored the potential of dietary PUFAs to affect the course of parasitic infections using a well-established invertebrate host -- parasite system, the freshwater herbivore Daphnia magna and its bacterial parasite Pasteuria ramosa.; Using natural food sources differing in their PUFA composition and by experimentally modifying the availability of dietary arachidonic acid (ARA) and eicosapentaenoic acid (EPA) we examined PUFA-mediated effects resulting from direct consumption as well as maternal effects on offspring of treated mothers. We found that both host and parasite were affected by food quality. Feeding on C20 PUFA-containing food sources resulted in higher offspring production of hosts and these effects were conveyed to a great extent to the next generation. While feeding on a diet containing high PUFA concentrations significantly reduced the likelihood of becoming infected, the infection success in the next generation increased whenever the maternal diet contained PUFAs. We suggest that this opposing effect was caused by a trade-off between reproduction and immunity in the second generation.; Considering the direct and maternal effects of dietary PUFAs on host and parasite we propose that host -- parasite interactions and thus disease dynamics under natural conditions are subject to the availability of dietary PUFAs

Allocation of essential lipids in Daphnia magna during exposure to poor food quality

1. In nature, food conditions change temporally and force consumers into trade-offs during resource allocation. In particular, under poor food conditions, for example during cyanobacterial blooms, herbivores have to optimize their resource allocation to maximize fitness, and face two decisions: (i) an individual might attempt to allocate acquired essential resources to reproductive tissues or use them for its own maintenance; and (ii) an individual might decide to optimize the chemical quality of its eggs.2. As cyanobacteria feature a deficiency in some essential lipids that leads to a decline in the growth and fecundity of Daphnia, an important freshwater herbivore, we investigated Daphnia magna s Strauss allocation of lipids such as polyunsaturated fatty acids (PUFAs) and cholesterol during an experimental non-toxic cyanobacterial bloom.3. Generally, we found a substantial maternal investment of the particularly important omega-3 (n-3) PUFAs, in particular eicosapentaenoic acid (EPA), into the eggs. The concentration of EPA was 2·4-fold higher in eggs than in somatic tissue, a cumulative effect, which was not significantly changed under cyanobacterial food conditions.4. Under poor conditions, D. magna not only decreased the number of eggs produced but, in principle, reduced the previously high concentrations of EPA in both eggs and somatic tissues to a similar degree. In contrast to EPA, the concentrations of α-linolenic acid and cholesterol, although lower than EPA, were more homeostatic in eggs than in somatic tissues, in which concentrations decreased.5. When food quality was improved, D. magna were able to recover completely the fatty acid concentrations in their somatic tissues and eggs.6. This study shows that the content of particular lipids in its food clearly affects resource allocation in D. magna, and suggests that cholesterol is important for somatic growth, while PUFAs are primarily needed for reproduction. As a decreasing investment of essential lipids into eggs implies a reduced fitness of the animals progeny under poor food conditions, this could have a strong impact on population dynamics, which might also be valid for other species

Daphnia's Adaptive Molecular Responses to the Cyanobacterial Neurotoxin Anatoxin-α Are Maternally Transferred

Cyanobacterial blooms are an omnipresent and well-known result of eutrophication and climate change in aquatic systems. Cyanobacteria produce a plethora of toxic secondary metabolites that affect humans, animals and ecosystems. Many cyanotoxins primarily affect the grazers of phytoplankton, e.g., Daphnia. The neurotoxin anatoxin-α has been reported world-wide; despite its potency, anatoxin-α and its effects on Daphnia have not been thoroughly investigated. Here, we investigated the effects of the anatoxin-α-producing Tychonema on life-history parameters and gene expression of nicotine-acetylcholine receptors (NAR), the direct targets of anatoxin-α, using several D. magna clones. We used juvenile somatic growth rates as a measure of fitness and analyzed gene expression by qPCR. Exposure to 100% Tychonema reduced the clones’ growth rates and caused an up-regulation of NAR gene expression. When 50% of the food consisted of Tychonema, none of the clones were reduced in growth and only one of them showed an increase in NAR gene expression. We demonstrate that this increased NAR gene expression can be maternally transferred and that offspring from experienced mothers show a higher growth rate when treated with 50% Tychonema compared with control offspring. However, the addition of further (anthropogenic) stressors might impair Daphnia’s adaptive responses to anatoxin-α. Especially the presence of certain pollutants (i.e., neonicotinoids), which also target NARs, might reduce Daphnia’s capability to cope with anatoxin-α.publishe