The present dataset relates to a study that explored dissolved organic matter dynamics under three scenarios: a particle-free environment, a particle-enriched system with polystyrene microplastics, and a particle-enriched system with inorganic particles (water insoluble SiO₂). In part 1 of the experiment, natural marine organic matter was obtained by culturing a non-axenic strain of Chaetoceros socialis in 2 L flasks under each of three scenarios (1C = control, 2PS = polystyrene, 3S = silica). After the growth phase, filtered samples from the three flasks containing dissolved organic matter and bacteria were incubated separately in the dark in 4 replicates closed quartz cuvettes per treatment (total = 12 cuvettes, 28 mL capacity quartz cuvettes 10 cm path length, Hellma 120-QS, Quartz SUPRASIL, Hellma Analytics) at a temperature of of 20 °C ± 2 °C for 5 days to monitor changes in CDOM. In this phase, Chromophoric dissolved organic matter (CDOM), a bulk optical property, was monitored daily to examine changes in its quality and quantity and to compare degradation dynamics in the three systems (1C, 2PS and 3S). The dataset for part 2 reports CDOM absorption coefficient at 355 nm (a355, m⁻¹) as well as spectral slope parameters in different wavelengths as indicators of CDOM degradation processes and microbial activity: spectral slope S between 302 and 322 nm (S302-322, nm⁻¹), between 275 and 295 nm (S275-295, nm⁻¹), and between 350 and 400 nm (S350-400, nm⁻¹), as well as slope ratio SR, as SR = S275-295:S350-400.OC was estimated estimated in each cuvette from CDOM absorbance by applying the approach developed by Fichot and Benner (Fichot, C.G.; Benner, R. Geophys. Res. Lett. 2011, 38, doi:10.1029/2010GL046152) using culture specific parameters previously calibrated (Galgani, L. et al., Sci. Rep. 2018, 8, doi:10.1038/s41598-018-32805-4).