Effects of Microplastic Combined with Cr(Ⅲ) on Apoptosis and Energy Pathway of Coral Endosymbiont

Abstract The combined effect of polyethylene (PE) microplastic and chromium (Cr(III)) was investigated on scleractinian coral Acropora pruinosa ( A. pruinosa ). The endpoints analyzed in this study included the density of endosymbiont, chlorophyll a + c content, activity of enzymes involved in apoptosis (caspase-1; caspase-3), glycolysis (lactate dehydrogenase, LDH), pentose phosphate pathway (glucose-6-phosphate dehydrogenase, G6PDH) and electron transfer coenzyme (nicotinamide adenine dinucleotide, NAD + /NADH). During the 7-day exposure to PE and Cr(III) stress, the density of endosymbiont and the chlorophyll content were decreased gradually. Caspase-1 and-3 activities were increased in the high concentration Cr(III) exposure group. Furthermore, the LDH and G6PDH activities were decreased significantly, and the ratio of NAD + /NADH was decreased significantly. In summary, the results showed that PE and Cr(III) stress had inhibited the energy metabolism enzymes of A. pruinosa , and further led to the apoptosis of endosymbiont in coral. In addition, the combination of stressors exposure revealed as the concentration of Cr(III) keep at 10 µg/L, the toxic effects of heavy metals on endosymbiont was temporarily relieved with elevated PE In contrary, coral polyps were kept under the concentration of PE at 5 mg/L and increasing Cr(III), the metabolic activities of corals were seriously disturbed, which was increased the burden of energy consumption. In the short term, the deleterious effects of Cr(III) is more obvious due to it led to the apoptosis of endosymbiont and its irreversible damage. It is the first time to give insights into the combined effect of microplastic and Cr(III) stress on the apoptosis and energy pathway of coral endosymbiont. The results suggest that microplastic combined with Cr(III)contamination is an important factor affecting apoptosis and energy metabolism of coral A. pruinosa .