Absence of sterols constrains carbon transfer between cyanobacteria and a freshwater herbivore (Daphnia galeata) A key process in freshwater plankton food webs is the regulation of the efficiency of energy and material transfer. Cyanobacterial carbon (C) in particular is transferred very inefficiently to herbivorous zooplankton, which leads to a decoupling of primary and secondary production and the accumulation of cyanobacterial biomass, which is associated with reduced recreational quality of water bodies and hazards to human health. A recent correlative field study suggested that the low transfer efficiency of cyanobacterial C is the result of the absence of long-chain polyunsaturated fatty acids (PUFA) in the diet of the zooplankton. By supplementation of single-lipid compounds in controlled growth experiments, we show here that the low C transfer efficiency of coccal and filamentous cyanobacteria to the keystone herbivore Daphnia is caused by the low sterol content in cyanobacteria, which constrains cholesterol synthesis and thereby growth and reproduction of the herbivore. Estimations of sterol requirement in Daphnia suggest that, when cyanobacteria comprise more than 80% of the grazed phytoplankton, growth of the herbivore may be limited by sterols and Daphnia may subsequently fail to control phytoplankton biomass. Dietary sterols therefore may play a key role in freshwater food webs and in the control of water quality in lakes dominated by cyanobacteria.
Microalgae commonly used as feed for bivalves, Pavlova lutheri (P), Isochrysis affinis galbana (T) and Chaetoceros calcitrans forma pumilum (Cp), were fed to Pacific oyster Crassostrea gigas to assess their nutritional value for larval development and metamorphosis during two experiments. Monospecific, bispecific and trispecific diets were firstly evaluated during 3 weeks from D larvae to young postlarvae. Then bispecific diets, based on different T and Cp proportions, were assessed during a similar period. Concurrently, ingestion was studied through the whole larval and postlarval development for each diet and/or diet mixture. Because lipids are assumed to be a key nutrient for bivalves, biochemical analysis was undertaken on the second set of trials focused on fatty acids and sterols. Compared to the other diet mixtures (mono and plurispecific diet) TCp induced the best larval growth performance (13.2 μm day− 1), a high larval survival (98%) but did not result in higher metamorphosis (72%). In contrast, monospecific diet P was the poorest for larvae with low growth and low survival. When varying T and Cp proportions, best larval developments were induced with 25T/75Cp and 50T/50Cp diets, though quite similar to that obtained with 75T/25Cp. In contrast, unbalanced diets (95T/5Cp and 95Cp/5T) led to low larval performances. In addition, grazing experiences showed preferential uptake of microalgae with P < PT much less-than T much less-than Cp much less-than TCp = PCp = PTCp. For mixed diets a low daily consumption (< 10 000 microalgae per larvae) was noted during the first week followed by a second phase (next 8-10 days) with a sharp increase and regular intake, reaching 90 000 microalgae per larvae per day. Finally, a marked drop (40 000 microalgae per larvae) was observed at the beginning of metamorphosis from days 20 to 21. Principal component analysis between main fatty acids (19) and sterols (7) detected in larvae and postlarvae was used to discriminate profiles according to diets and/or metamorphosis competence. The correlation circle representation showed that the 26 variables are well explained by these combined variables (78%) with a repartition along the first principal component according to diets with a gradient from 5T/95Cp to 95T/5Cp. In contrast, postlarvae and larvae were discriminated on the second principal component while no relationships were found between competent and incompetent larvae.
Microalgae offer a high potential for energetic lipid storage as well as high growth rates. They are therefore considered promising candidates for biofuel production, with the selection of high lipid-producing strains a major objective in projects on the development of this technology. We developed a mutation-selection method aimed at increasing microalgae neutral lipid productivity. A two step method, based on UVc irradiation followed by flow cytometry selection, was applied to a set of strains that had an initial high lipid content and improvement was assessed by means of Nile-red fluorescence measurements. The method was first tested on Isochrysis affinis galbana (T-Iso). Following a first round of mutation-selection, the total fatty acid content had not increased significantly, being 262 ± 21 mgTFA (gC)-1 for the wild type (WT) and 269 ± 49 mgTFA (gC)-1 for the selected population (S1M1). Conversely, fatty acid distribution among the lipid classes was affected by the process, resulting in a 20% increase for the fatty acids in the neutral lipids and a 40% decrease in the phospholipids. After a second mutation-selection step (S2M2), the total fatty acid content reached 409 ± 64 mgTFA (gC)-1 with a fatty acid distribution similar to the S1M1 population. Growth rate remained unaffected by the process, resulting in a 80% increase for neutral lipid productivity.
The purpose of this study was to examine the responses of the oyster Crassostrea gigas to oxygen levels at subcellular and whole organism levels. Two experiments were carried out. The first experiment was designed to measure the clearance and oxygen consumption rates of oysters exposed at different concentrations of oxygen at 15, 20 and 258C for 20 h. The goal of this first part was to estimate the hypoxic threshold for oysters below which their metabolism shifts towards anaerobiosis, by estimating the oxygen critical point (PcO 2 ) at 15, 20 and 258C. The second experiment was carried out to evaluate the metabolic adaptations to hypoxia for 20 days at three temperatures: 12, 15 and 208C. The metabolic pathways were characterized by the measurement of the enzymes pyruvate kinase (PK) and phosphoenolpyruvate carboxykinase (PEPCK), the alanine and succinate content and the adenylate energy charge. Respiratory chain functioning was estimated by the measurement of the activity of the electron transport system (ETS). The values of PcO 2 were 3.0290.15, 3.4390.20 and 3.2890.24 mg O 2 l (1 at 15, 20 and 258C, respectively. In whole oysters, hypoxia involved the inhibition of PK whatever the temperature, but PEPCK was not stimulated. Succinate accumulated significantly only at 128C and alanine at 12 and 158C. A negative relationship between the PK activity and the alanine content was only found in hypoxic oysters. Finally, hypoxia increased significantly the activity of ETS. With high PcO 2 values, the metabolic depression occurred quickly, showing that oysters had a low capacity to regulate their respiration when oxygen availability is reduced, particularly in the summer.
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