Spectroscopies infrarouge et Raman de microalgues : étude des interactions avec des micro et nanoparticules

In a context of contamination of natural waters by anthropogenic micro and nanoparticles (plastics, metals, oxides...), the study of their interactions with freshwater microalgae, the first link in the trophic chain, is essential. Techniques such as vibrational spectroscopies (Raman and infrared), which allow sensitive and rapid analysis without sample preparation, may be of interest. They have been tested here on the model microalga Chlamydomonas reinhardtii to quickly detect the effects of environmental stresses (light, drop of nutrients and presence of metals). Infrared spectroscopy proved to be a good tool, allowing rapid variation in biochemical composition to be monitored, helping to easily discriminate between the different stresses on the microalgae. This technique was then used to assess the impact of the presence of particulate pollutants (metallic and plastic nanoparticles in particular). The short- and medium-term impact of nanoparticles on freshwater microalgae was characterized in parallel with another known stress (nitrogen nutrient deficiency) in order to compare the different responses. The main results of exposure to nanoparticles show overall the immediate non-toxicity of nanoparticles at the tested concentrations. However, a loss of membrane integrity in contact with polystyrene nanoparticles suggests a stronger interaction of the microalgae studied with nanoplastics than with gold nanoparticles of similar size. Infrared spectra of C. reinhardtii showed a modification of protein and carbohydrate bands following environmental changes. In order to understand the variations observed at the level of carbohydrates, components of the microalgae structure and representing an important and complex part of the infrared spectrum, an in-depth study of this area was carried out by deconvolution of the infrared spectra. In order to better understand the response mechanisms involved, a study of the expression of genes (RT-QPCR) related to stress and carbohydrates production was carried out in parallel with infrared spectroscopy. The results thus obtained showed that polystyrene nanoparticle and C. reinhardtii interactions resulted in a genetic and macromolecular response at the carbohydrate level. Thus, the nanoparticle type effect was confirmed by the expression of the gene linked to the biosynthesis of galactose. Understanding the role of the cell-wall in the interaction with nanoparticles, particularly in case of polymers, was also studied by comparing two strains of C. reinhardtii (with and without cell-wall). Finally, microalgae response as a function of the interaction time with nanoparticles was investigated in order to try to understand whether, in the medium term, the responses presented a temporal variation. In addition, in order to better understand the effect of plastic particles at a higher trophic level, complementary work on the impact of microplastics on daphnia was carried out.