Size dependent impacts of a model microplastic on nitrification induced by interaction with nitrifying bacteria

Two sizes of polystyrene (PS) were compared to investigate their impact on nitrification. The smaller PS (50 nm) had a higher impact than the larger PS (500 nm). Lower NO2- and NO3- accumulation was observed in the 50 nm PS treatment. There was no significant difference in DIN concentration between the control and 500 nm PS treatments. PS treatment did not have a significant influence on the specific ammonia oxidation rate, but the specific nitrite utilization rate was the lowest in the 50 nm PS treatment. The changes in transcript levels of amoA gene did not correspond well with the observed changes in DIN concentrations, suggesting that the effects of 50 nm PS treatment might be unrelated to biological phenomena, for which an actual uptake of PS is needed. The fluorescent images revealed that the smaller PS can easily access bacterial cells, which corroborated the results of inhibition of nitrification by the smaller PS. Notably, most of the PS particles did not penetrate bacterial cells, suggesting that the observed effects of 50 nm PS on nitrification might be due to disruption of the membrane potential of the cells.