Alexandre Bec

Formation and Transfer of Fatty Acids in Aquatic Microbial Food Webs: Role of Heterotrophic Protists

The term protist was first coined by Haeckel in 1866 for diverse microorganisms including bacteria (Haeckel 1866). However, in 1925 in a paper on an amoeboid parasite of Daphnia, Chatton (1925) highlighted for the first time the fundamental difference between prokaryotic and eukaryotic organisms and the term protist to be now used to describe unicellular eukaryotes, which do not differentiate into tissues (see Adl et al. 2005).

From Aquatic to Terrestrial Food Webs: Decrease of the Docosahexaenoic Acid/Linoleic Acid Ratio

Fatty acid composition of the adipose tissue of six carnivorous mammalian species (European otter Lutra lutra, American mink Mustela vison, European Mink Mustela lutreola, European polecat Mustela putorius, stone marten Martes foina and European wild cat Felis silvestris) was studied. These species forage to differing degrees in aquatic and terrestrial food webs. Fatty acid analysis revealed significant differences in polyunsaturated fatty acid composition between species. More specifically, our results underline a gradual significant decrease in the docosahexaenoic acid (DHA)/linoleic acid (LNA) ratio of carnivore species as their dependence on aquatic food webs decreases. In conclusion, the use of the DHA/LNA ratio in long-term studies is proposed as a potential proxy of changes in foraging behaviour of semi-aquatic mammals.

Development of a Real-Time PCR assay for quantitative assessment of uncultured freshwater zoosporic fungi.

Recently, molecular environmental surveys of the eukaryotic microbial community in lakes have revealed a high diversity of sequences belonging to uncultured zoosporic fungi. Although they are known as saprobes and algal parasites in freshwater systems, zoosporic fungi have been neglected in microbial food web studies. Recently, it has been suggested that zoosporic fungi, via the consumption of their zoospores by zooplankters, could transfer energy from large inedible algae and particulate organic material to higher trophic levels. However, because of their small size and their lack of distinctive morphological features, traditional microscopy does not allow the detection of fungal zoospores in the field. Hence, quantitative data on fungal zoospores in natural environments is missing. We have developed a quantitative PCR (qPCR) assay for the quantification of fungal zoospores in lakes. Specific primers were designed and qPCR conditions were optimized using a range of target and non-target plasmids obtained from previous freshwater environmental 18S rDNA surveys. When optimal DNA extraction protocol and qPCR conditions were applied, the qPCR assay developed in this study demonstrated high specificity and sensitivity, with as low as 100 18S rDNA copies per reaction detected. Although the present work focuses on the design and optimization of a new qPCR assay, its application to natural samples indicated that qPCR offers a promising tool for quantitative assessment of fungal zoospores in natural environments. We conclude that this will contribute to a better understanding of the ecological significance of zoosporic fungi in microbial food webs of pelagic ecosystems.

Origins of carbon sustaining the growth of whitefish Coregonus lavaretus early larval stages in Lake Annecy: insights from fatty‐acid biomarkers

The hypothesis that diatom carbon (C) produced during the spring peak supported spring zooplankton production and, ultimately, the growth of Coregonus lavaretus early larval stages from March to May 2006 in Lake Annecy, France, was tested using gut content analyses and fatty acid biomarkers. Gut content results showed that C. lavaretus larvae from stages 1 to 4 preferentially fed on copepods with Daphnia sp. only a minor proportion of larval diet. The levels of diatom-marker fatty acids (C16:1n-7 and C20:5n-3) were high in Daphnia sp., but lower in both copepods and C. lavaretus larvae from stages 0 to 4. These results indicated that the spring diatom biomass was actually grazed by Daphnia sp., but, contrary to what was expected, the spring bloom was not the only C source supporting copepods secondary production and, consequently, the growth of C. lavaretus early larval stages. In contrast, levels of terrestrial fatty acid marker (C24:0) were low in Daphnia sp. but high in copepods and C. lavaretus larvae, indicating a significant contribution of terrestrial carbon to copepods and, ultimately, to the growth of C. lavaretus early larval stages.

Resource partitioning among cladocerans in a littoral macrophyte zone: implications for the transfer of essential compounds

Abstract The maintenance of species diversity in a given environment is strongly linked to resource partitioning. Littoral macrophyte zones are heterogeneous environments with high microcrustacean diversity, where zooplankton have dietary access to seston as well as organisms growing on macrophytic surfaces (epiphyton). We conducted a field study in a macrophyte-rich backwater of the river Allier to examine how seston and epiphyton were used as potential food sources by four dominant cladoceran species. Fatty acids were analyzed in these two food sources to assess how their differential uptake affects the trophic trajectory of essential compounds from these resources to cladocerans. Our results showed resource partitioning among the four cladocerans studied; while Eurycercus fed mostly on epiphyton, Daphnia mostly consumed phytoplankton, and Ceriodaphnia and Simocephalus were able to forage on sestonic and epiphytic resources. Based on their polyunsaturated fatty acid (PUFA) content, it was evident that epiphyton was of higher food quality than seston in this macrophyte-rich backwater system. Variability of PUFA compositions of seston and epiphyton, and diversity of foraging strategies of cladoceran species, which represent the major link between microorganisms and consumers at higher trophic levels, affect dietary energy pathways and point to a variable PUFA transfer efficiency in backwater food webs.

Temporal changes in essential fatty acid availability in different food sources in the littoral macrophyte zone

In littoral environments, different food resources are available for zooplankters. In addition to seston, species may feed on biofilms growing on sediments, plants (epiphyton), and at the air–water interface (neuston). However, despite a growing interest in these different biofilms, little is known about their food quality for microcrustaceans. In a field study, we measured changes in the food quality over time in terms of the essential fatty acid (EFA) content of different potential food sources for littoral consumers. The food quality of seston, neuston, and epiphyton growing on three different aquatic macrophytes were assessed. Our results showed that there is an important seasonal variability within each food source. However, in the system studied, epiphytic biofilms, especially those of Ludwigia and Callitriche offered the highest food quality, in terms of EFA content, throughout the year. As the highest EFA concentrations in each food source were found consecutively, high concentrations of these physiologically important compounds are maintained in the system throughout the year. Therefore, greater diversity of food resources could affect ecosystem productivity.

Phospholipid-bound eicosapentaenoic acid (EPA) supports higher fecundity than free EPA in Daphnia magna

Nutrition bioassays in which polyunsaturated fatty acids (PUFA)-deficient diets were supplemented with free longchain PUFA (≥C20) consistently revealed positive effects on somatic growth and fecundity of Daphnia. However, free PUFA are hardly available in natural diets. In general, PUFA are bound to other lipids, especially to phospholipids and triglycerides. Here, we evaluate the potential of free and phospholipid-bound dietary eicosapentaenoic acid (EPA) to support somatic growth and fecundity of Daphnia magna. In a growth experiment, supplementation of a C20 PUFA-deficient diet with free or phospholipid-bound EPA improved somatic growth rates of D. magna equally. However, the increase in fecundity was significantly more pronounced when phospholipid-bound EPA was provided. Free and phospholipid-bound EPA were provided in the same concentrations in our experiment, suggesting that the allocation to reproduction-related processes is affected differently by phospholipid-bound PUFA and free PUFA. Our finding stresses the need to consider the distribution of dietary PUFA in different lipid classes to gain a better understanding of how PUFA influence life history traits of Daphnids in the field.