Structure de la communauté phytoplanctonique des fleuves côtiers en réponse aux contraintes hydrodynamique (une approche basée sur les traits morpho-fonctionnels)

Les traits fonctionnels sont de plus en plus utilisés en écologie pour expliquer les changements de composition et de structure de la communauté phytoplanctonique en fonction des contraintes environnementales. Ce succès tient au fait que les traits fonctionnels sont des caractéristiques reliées aux processus de croissance et de perte des organismes, et qu'ils déterminent ainsi l'occurrence, la dominance ou l'absence d'une espèce dans des conditions environnementales données. Parmi les traits fonctionnels, on distingue les traits morpho-fonctionnels qui présentent l'avantage d'être facilement observables et mesurables. Les approches basées sur les traits morpho-fonctionnels ont majoritairement été utilisées en lac mais pourraient toutefois s'avérer pertinentes en rivière où l'importance relative des contraintes environnementales agissant sur la communauté phytoplanctonique est très variable d'un système à l'autre. L'objectif de cette thèse a donc été de tester l'hypothèse selon laquelle la communauté phytoplanctonique en rivière pouvait être structurée selon les traits morpho-fonctionnels. Dans les rivières étudiées, nous avons montré que la communauté est structurée selon les traits morpho-fonctionnels reliés à la vitesse de sédimentation, en mettant en évidence le rôle clé de la forme des organismes, mais aussi de leur motilité et de leur densité via la présence de mucilage. A travers la sélection de différentes valeurs pour ces traits morpho-fonctionnels, nous avons pu mettre en évidence l'impact majeur de la turbulence sur la communauté phytoplanctonique de ces rivières. L'effet de ce paramètre a été testé expérimentalement en conditions contrôlées sur une communauté simplifiée afin de tester l'hypothèse selon laquelle sous l'effet de la turbulence, la composition de la structure de la communauté pouvait être prédite par les traits morpho-fonctionnels. Les résultats de cette approche expérimentale ont permis de réitérer le rôle clé de la forme dans la structure de la communauté corroborant ainsi les résultats de rivières et ont également souligné le rôle de la plasticité phénotypique. L'approche basée sur les traits a également été testée en estuaires, mais s'est cependant montré infructueuse. Plusieurs hypothèses ont été proposées pour expliquer ce résultat, notamment l'absence de trait identifié comme étant relié à la salinité.Functional traits are increasingly used in ecology to explain changes in the composition and structure of phytoplankton community in relation to environmental constraints. Functional traits are related to growth and loss processes, and thus they determine the occurrence, dominance or absence of species in given environmental conditions. Among the functional traits, morpho-functional traits display the advantage of their ease to be measured. Approaches based on morpho-functional traits were mainly used in lakes, but could however be relevant in rivers where the relative significance of environmental constraints acting on the phytoplankton community varies from one system to another. The aim of this thesis was therefore to test the hypothesis that riverine phytoplankton community could be structured according to morpho-functional features. In the rivers studied, we showed that the community is structured according to morpho-functional traits related to sedimentation rate, highlighting the key role of the shape of organisms, but also of their motility and density via the presence of mucilage. Through the selection of different values for these morpho-functional traits, we highlighted the main impact of turbulence on the phytoplankton community within these rivers. The effect of this parameter has been tested experimentally on a simplified community under controlled conditions in order to test the hypothesis that as a result of turbulence, the composition of community structure could be predicted through morpho-functional traits. The results of this experimental approach helped to reiterate the key role of the shape in community structure thus corroborating the results found in rivers. It also stressed the role of phenotypic plasticity. This approach based on features was also tested in estuaries, but was, however, unsuccessful. Several hypotheses have been proposed to explain this result, including the lack of traits identified as related to salinity.RENNES1-Bibl. électronique (352382106) / SudocSudocFranceF

Evidence of trophic transfer of microcystins from the gastropod Lymnaea stagnalis to the fish Gasterosteus aculeatus.

International audienceAccording to our previous results the gastropod Lymnaea stagnalis exposed to MC-producing cyanobacteria accumulates microcystins (MCs) both as free and covalently bound forms in its tissues, therefore representing a potential risk of MC transfer through the food web. This study demonstrates in a laboratory experiment the transfer of free and bound MCs from L. stagnalis intoxicated by MC-producing Planktothrix agardhii ingestion to the fish Gasterosteus aculeatus. Fish were fed during five days with digestive glands of L. stagnalis containing various concentrations of free and bound MCs, then with toxin-free digestive glands during a 5-day depuration period. MC accumulation was measured in gastropod digestive gland and in various fish organs (liver, muscle, kidney, and gills). The impact on fish was evaluated through detoxification enzyme (glutathion-S-transferase, glutathion peroxydase and superoxyde dismutase) activities, hepatic histopathology, and modifications in gill ventilation, feeding and locomotion. G. aculeatus ingestion rate was similar with intoxicated and toxin-free diet. Fish accumulated MCs (up to 3.96 ± 0.14 μg g−1 DW) in all organs and in decreasing order in liver, muscle, kidney and gills. Hepatic histopathology was moderate. Glutathion peroxydase was activated in gills during intoxication suggesting a slight reactive oxygen species production, but without any impact on gill ventilation. Intoxication via ingestion of MC-intoxicated snails impacted fish locomotion. Intoxicated fish remained significantly less mobile than controls during the intoxication period possibly due to a lower health condition, whereas they showed a greater mobility during the depuration period that might be related to an acute foraging for food. During depuration, MC elimination was total in gills and kidney, but partial in liver and muscle. Our results assess the MC transfer from gastropods to fish and the potential risk induced by bound MCs in the food web

Evidence of the Cost of the Production of Microcystins by Microcystis aeruginosa under Differing Light and Nitrate Environmental Conditions

The cyanobacterium Microcystis aeruginosa is known to proliferate in freshwater ecosystems and to produce microcystins. It is now well established that much of the variability of bloom toxicity is due to differences in the relative proportions of microcystin-producing and non-microcystin-producing cells in cyanobacterial populations. In an attempt to elucidate changes in their relative proportions during cyanobacterial blooms, we compared the fitness of the microcystin-producing M. aeruginosa PCC 7806 strain (WT) to that of its non-microcystin-producing mutant (MT). We investigated the effects of two light intensities and of limiting and non-limiting nitrate concentrations on the growth of these strains in monoculture and co-culture experiments. We also monitored various physiological parameters, and microcystin production by the WT strain. In monoculture experiments, no significant difference was found between the growth rates or physiological characteristics of the two strains during the exponential growth phase. In contrast, the MT strain was found to dominate the WT strain in co-culture experiments under favorable growth conditions. Moreover, we also found an increase in the growth rate of the MT strain and in the cellular MC content of the WT strain. Our findings suggest that differences in the fitness of these two strains under optimum growth conditions were attributable to the cost to microcystin-producing cells of producing microcystins, and to the putative existence of cooperation processes involving direct interactions between these strains

Cross talk: two way allelopathic interactions between toxic Microcystis and Daphnia.

Due to eutrophication, freshwater ecosystems frequently experience cyanobacterial blooms, many of which produce bioactive metabolites that can affect vertebrates and invertebrates life traits. Zooplankton are able to develop tolerance as a physiological response to cyanobacteria and their bioactive compounds, however, this comes with energetic cost that in turn influence Daphnia life traits and may impair populations. Vice versa, it has been suggested that Daphnia are able to reduce cyanobacterial dominance until a certain cyanobacterial density; it remains unclear whether Daphnia metabolites alone influence the physiological state and bioactive metabolites production of cyanobacteria. Hence, this study investigates mutual physiological reactions of toxic Microcystis aeruginosa PCC7806 and Daphnia magna. We hypothesize that a) the presence of D. magna will negatively affect growth, increase stress response and metabolites production in M. aeruginosa PCC7806 and b) the presence of M. aeruginosa PCC7806 will negatively affect physiological responses and life traits in D. magna. In order to test these hypotheses experiments were conducted in a specially designed co-culture chamber that allows exchange of the metabolites without direct contact. A clear mutual impact was evidenced. Cyanobacterial metabolites reduced survival of D.magna and decreased oxidative stress enzyme activity. Simultaneously, presence of D.magna did not affect photosynthetic activity. However, ROS increase and tendencies in cell density decrease were observed on the same day, suggesting possible energy allocation towards anti-oxidative stress enzymes, or other protection mechanisms against Daphnia infochemicals, as the strain managed to recover. Elevated concentration of intracellular and overall extracellular microcystin MC-LR, as well as intracellular concentrations of aerucyclamide A and D in the presence of Daphnia, indicating a potential protective or anti-grazing function. However, more research is needed to confirm these findings

Cell free Microcystis aeruginosa spent medium affects Daphnia magna survival and stress response.

Primary consumers in freshwater ecosystems, such as the zooplankton organism Daphnia magna, are highly affected by cyanobacteria, both as they may use it as a food source but also by cyanobacterial metabolites present in the water. Here, we investigate the impacts of cyanobacterial metabolites focussing on the environmental realistic scenario of the naturally released mixture without crushing cyanobacterial cells or their uptake as food. Therefore, D. magna were exposed to two concentrations of cell free cyanobacterial spent medium from Microcystis aeruginosa PCC 7806 to represent higher and lower ecologically-relevant concentrations of cyanobacterial metabolites. Including microcystin-LR, 11 metabolites have been detected of which 5 were quantified. Hypothesising concentration and time dependent negative impact, survival, gene expression marking digestion and metabolism, oxidative stress response, cell cycle and molting as well as activities of detoxification and antioxidant enzymes were followed for 7 days. D. magna suffered from oxidative stress as both catalase and glutathione S-transferase enzyme activities significantly decreased, suggesting enzyme exhaustibility after 3 and 7 days. Moreover, gene-expressions of the 4 stress markers (glutathione S-transferase, glutathione peroxidase, catalase and thioredoxin) were merely downregulated after 7 days of exposure. Energy allocation (expression of glyceraldehyde-3-phosphate dehydrogenase) was increased after 3 days but decreased as well after 7 days exposure. Cell cycle was impacted time dependently but differently by the two concentrations, along with an increasing downregulation of myosin heavy chain responsible for cell arrangement and muscular movements. Deregulation of nuclear hormone receptor genes indicate that D. magna hormonal steering including molting seemed impaired despite no detection of microviridin J in the extracts. As a consequence of all those responses and presumably of more than investigated molecular and physiological changes, D. magna survival was impaired over time, in a concentration dependent manner. Our results confirm that besides microcystin-LR, other secondary metabolites contribute to negative impact on D. magna survival and stress response

Accumulation and detoxication responses of the gastropod Lymnaea stagnalis to single and combined exposures to natural (cyanobacteria) and anthropogenic (the herbicide RoundUp® Flash) stressors.

Freshwater gastropods are increasingly exposed to multiple stressors in the field such as the herbicide glyphosate in Roundup formulations and cyanobacterial blooms either producing or not producing microcystins (MCs), potentially leading to interacting effects. Here, the responses of Lymnaea stagnalis to a 21-day exposure to non-MC or MC-producing (33μgL−1) Planktothrix agardhii alone or in combination with the commercial formulation RoundUp®Flash at a concentration of 1μgL−1glyphosate, followed by 14days of depuration, were studied via i) accumulation of free and bound MCs in tissues, and ii) activities of anti-oxidant (catalase CAT) and biotransformation (glutathione-S-transferase GST) enzymes. During the intoxication, the cyanobacterial exposure induced an early increase of CAT activity, independently of the MC content, probably related to the production of secondary cyanobacterial metabolites. The GST activity was induced by RoundUp®Flash alone or in combination with non MC-producing cyanobacteria, but was inhibited by MC-producing cyanobacteria with or without RoundUp®Flash. Moreover, MC accumulation in L. stagnalis was 3.2 times increased when snails were concomitantly exposed to MC-producing cyanobacteria with RoundUp®, suggesting interacting effects of MCs on biotransformation processes. The potent inhibition of detoxication systems by MCs and RoundUp®Flash was reversible during the depuration, during which CAT and GST activities were significantly higher in snails previously exposed to MC-producing cyanobacteria with or without RoundUp®Flash than in other conditions, probably related to the oxidative stress caused by accumulated MCs remaining in tissues

Daphnia magna exudates impact physiological and metabolic changes in microcystis aeruginosa.

While the intracellular function of many toxic and bioactive cyanobacterial metabolites is not yet known, microcystins have been suggested to have a protective role in the cyanobacterial metabolism, giving advantage to toxic over nontoxic strains under stress conditions. The zooplankton grazer Daphnia reduce cyanobacterial dominance until a certain density, which may be supported by Daphnia exudates, affecting the cyanobacterial physiological state and metabolites’ production. Therefore, we hypothesized that D. magna spent medium will impact the production of cyanobacterial bioactive metabolites and affect cyanobacterial photosynthetic activity in the nontoxic, but not the toxic strain. Microcystin (MC-LR and des-MC-LR) producing M. aeruginosa PCC7806 and its non-microcystin producing mutant were exposed to spent media of different D. magna densities and culture durations. D. magna spent medium of the highest density (200/L) cultivated for the shortest time (24 h) provoked the strongest effect. D.magna spent medium negatively impacted the photosynthetic activity of M. aeruginosa PCC7806, as well as the dynamics of intracellular and extracellular cyanobacterial metabolites, while its mutant was unaffected. In the presence of Daphnia medium, microcystin does not appear to have a protective role for the strain. On the contrary, extracellular cyanopeptolin A increased in M. aeruginosa PCC7806 although the potential anti-grazing role of this compound would require further studies

Influence du forçage climatique global et local sur la croissance des cyanobactéries.

communication oral

Les causes des proliférations de cyanobactéries

conférence invité