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. This dataset reports cell numbers measured by optical density in each of the three culturing flasks, three replicate measurements per flask..