Plastic pollution, in particular micro and nano plastics, has a range of impacts on aquatic and terrestrial environments. One area where new insights are needed is the effect on key microbial processes that control the carbon cycle (marine organic matter production and degradation). The present study explores plastic-related changes in organic matter in a controlled laboratory experiment with a non-axenic culture of the diatom species Chaetoceros socialis. The phytoplankton culture was grown in three different 2-L flasks: one served as control and contained the algae, heterotrophic bacteria, and the growing medium prepared with artificial seawater. The other two flasks were additioned with particles of similar concentration and size ( 30-µm): polystyrene microspheres in one, and SiO2 particles in the other, to compare the effects of microplastics and inorganic particles. Following the algal growth phase, the water from the three flasks was filtered to remove the algal cells and the particles. A 5-day dark experiment was performed on these new samples containing only bacteria and dissolved organic matter. Optical changes in CDOM (Chromophoric Dissolved Organic Matter) were monitored daily to examine CDOM quality and degradation in the three systems. CDOM absorbance (as a proxy for concentration) was higher in the control samples with respect to samples from both particles-enriched systems. This suggests different rates of CDOM production and degradation in the presence or absence of particles, with possible lower CDOM degradation in the control. The further comparison among CDOM indicators of microbial activity, such as spectral slope and slope ratio, showed that CDOM from the particle-enriched systems was more subject to microbial alteration, pointing to a higher microbial turnover of this material, and especially in the presence of inorganic particles (SiO2). These results indicate the potential role of particles, and among these, microplastics, to modify marine microbial organic matter dynamics.