The toxicological effects of low-density polyethylene microplastic particles (LDPE-MPs) on the growth and metabolic activities of the marine diatom Chaetoceros muellerii

The aim of the current study is to examine the response of the marine diatom Chaetoceros muellerii upon exposure to LDPE-MPs. The toxic effects of LDPE-MP treatment on C. muellerii cultures were dependent on its concentration, particle size, and exposure time. The highest percentage of growth inhibition (60.87%) was observed in cultures treated with a dose of 100 mg L⁻¹ and a particle size of 100 µm of LDPE-MP after 6 days of exposure. A notable reduction was also recorded for the chlorophyll, carotenoids, carbohydrate, and protein contents of the exposed C. muellerii cultures compared to the control. In contrast, exposure to LDPE-MPs promoted the lipid content by 47.78 and 51.78% over control at 100 and 250 µm particle sizes, respectively. Also, enhanced the antioxidant activities of CAT (by 41.76 and 33.33%) and SOD (by 57.26 and 44.87%) of C. muellerii cultures at both tested particle size, respectively. As a defense mechanism, C. muellerii cells secreted exopolysaccharides (EPS) which reached 12.75 and 19.98 folds over control in cultures of 10 mg L⁻¹ LDPE-MPs at both tested particle size, respectively. The EPS triggered the adsorption of LDPE-MPs on C. muellerii surfaces forming hetero-aggregate clusters, obviously shown in the Scanning Electron Microscopy (SEM) images. The Diffraction Scanning Calorimetric (DSC) technique and combustion techniques were applied for quantifying the adsorbed LDPE-MPs on the surfaces of C. muellerii cells. The accumulated LDPE-MPs on C. muellerii cells at 100 mg L⁻¹ recorded 0.334 and 0.167 g g^-1 DW at 100 and 250µm treatment, respectively. To our knowledge, this is the first work to applying both techniques for quantifying MPs accumulated on the microalgal cells, which could be adopted in future studies.