The unicellular photosynthetic organisms known as microalgae are becoming one of the most important models for aquaticsystem studies. Among them, Chlamydomonas reinhardtii is widely used as a bioindicator of pollution or of different changes inthe environment. Numerous pollutants are present in aquatic environments, particularly plastics and nanoplastics. Physiologicalvariations after an environmental change highlight variation in the macromolecular composition of microalgae (proteins, nucleicacids, lipids and carbohydrates). Recently, Fourier transform infrared vibrational spectroscopy has been described as a reliabletool, sensitive and allowing rapid measurement of macromolecular composition of microalgae. Coupled with preprocessing andprincipal component analysis, it is well adapted to monitoring the effect of environmental stress on biochemical composition. Inthis study, infrared spectroscopy, combinedwithmultivariate analysis, has been tested first on known environmental stresses suchas light intensity variation and nitrogen limitation. Then, this technique has been applied to monitor the interaction and potentialimpacts of polystyrene nanoparticles on microalgae. The results showed slight variations on protein and carbohydrates bands inthe presence of nanoplastics, suggesting that their presence led to modifications in the biochemical composition of themicroalgae. To confirm the interaction between microalgae and nanoplastics, visualization by confocal microscopy and cytotoxicitymeasurement has been carried out. Results showed that polystyrene nanoparticles seemed to adsorb on microalgaesurface, leading to a loss of plasma membrane integrity. The resulting chemical modifications, even if moderate, could bedetected by infrared spectroscopy‚ showing that this tool could be very helpful in the understanding of nanoparticlemicroalgaeinteraction mechanisms.