par Traboni, Claudia 
Président du jury Flot, Jean-François
Promoteur Gypens, Nathalie
Co-Promoteur Saiz, Enric
Publication Non publié, 2022-05-11
Président du jury Flot, Jean-François
Promoteur Gypens, Nathalie
Co-Promoteur Saiz, Enric
Publication Non publié, 2022-05-11
Thèse de doctorat
| Résumé : | The classical dichotomy poses strict autotrophic (phytoplankton) and heterotrophic (zooplankton) protists as the only “engines” at the base of marine food webs. This paradigm has been challenged by the existence of unicellular mixoplankton, able to photosynthesize and predate simultaneously. Due to only recently appraisal, our knowledge on the role of mixoplankton in food webs is still very limited. By laboratory and in silico approaches, this Thesis addressed the effect of protistan mixotrophy on copepods, ubiquitous crustacean zooplankton that act as consumers of protists and as prey for larger predators. The ecophysiological response of the calanoid copepod Paracartia grani to several autotrophic, mixotrophic and heterotrophic diets was examined. In most cases no substantial differences in the copepod vital rates emerged on the basis of the nutrition type, as differences were mainly due to species-specific traits of the prey. Nevertheless, when feeding on the dinoflagellate Karlodinium veneficum grown under contrasting trophic modes (auto- vs mixotrophic), copepod egestion rate and gross-growth efficiency varied significantly. This indicates that mixotrophy in certain protists has the potential to influence nutrient transfer, export and recruitment of next copepod generation. As interesting results had emerged from the species K. veneficum, this dinoflagellate was tested as prey for the calanoid copepods P. grani and Centropages typicus with the aim to investigate the effect of inorganic nutrient limitation (N and P) on the food quality of K. veneficum and to ascertain how these abiotic conditions might influence copepod ecophysiological response. Results show that nutrient-limited mixotrophic K. veneficum had more stable stoichiometric ratios in comparison to autotrophs. Both copepod species ingested and reproduced more when fed nutrient-depleted mixotrophic K. veneficum compared to autotrophic counterparts. In the light of this, it can be concluded that this mixoplankter can buffer inorganic nutrient shortage by feeding on a prey, increasing the nutrient transfer to copepods contrarily to autotrophs. Through in-silico experiments, the effect of future environmental changes on the dynamics of a food web was simulated. The model encompassed three plankton functional types (phyto-, mixo- and zooplankton) and three trophic strategies (auto-, mixo- and heterotrophy). Blooms of K. veneficum were simulated under autotrophic and mixotrophic strategies (feeding on cryptophytes) and copepods of the species P. grani were allowed to feed on both protists. The dinoflagellate growth was enhanced when supplied with phagotrophy, promoting more nutritious and abundant food landscape for copepods. Bottom-up restrictions such as low inorganic nutrients and low cryptophytes availability decreased K. veneficum biomass with consequent limitation of zooplankton production. Copepods improved protist food quality via nutrient regeneration, but young and high-biomass populations attenuated the dinoflagellate bloom. The trophic transfer efficiency from prey to predators increased in presence of mixotrophy, especially in oligotrophic condition. By virtue of expected changes in the abiotic and biotic landscape, these simulations pinpoint the need of addressing the mixoplankton-copepod link when describing food web dynamics. |
