Interaction of water salinity and titanium dioxide nanoparticle (TiO2) exposure in the Pacific oyster, Crassostrea gigas: immune and antioxidant system responses

Abstract

Titanium dioxide nanoparticles (TiO2) are released into the natural waters in different ways and cause toxicity to aquatic life. Although various studies have reported the toxicity of nanoparticles for aquatic animals, there is limited information regarding the effects of environmental factors, especially salinity, on their toxicity. Simultaneous effects of TiNPs and water salinity were studied in the Pacific oyster, Crassostrea gigas. Oysters were exposed to sub-lethal concentrations of TiNPs (low concentration: 50 µg/l, middle concentration: 500 µg/l, high concentration: 5000 µg/l) at different water salinities (Extremely low salinity: 5 g/l, low salinity: 15 g/l, middle salinity: 25 g/l, high salinity (normal salinity): 35 g/l) for 96 h. After nano-TiO2 exposure, the immune components in hemolymph (total hemocyte concentration [THC], phagocytosis), antioxidant enzyme activities in digestive gland (superoxide dismutase, catalase, glutathione peroxidase, glutathione S-transferase), oxidation stress index (malondialdehyde) in gills and the expression of immune-related genes (Vav-3 protein, cofilin, Galectin 4, pro-Cathepsin L, Rho protein, Ficolin, LPS/-1,3-glucan protein, Lysozyme, Defensin) in hemocytes were measured. The THC, phagocytosis, antioxidant enzyme activities, and immune-related gene expressions mostly decreased at salinities of 5 and 15 g/l in response to 500–5000 µg/l nano-TiO2 compared to non-exposed oysters and 50 µg/l nano-TiO2 group. An increase in water salinity up to 35 g/l mitigated the depressing effects of the nano-TiO2 on the immune and antioxidant components of the oysters. Exposure of oysters to 500–5000 µg/l nano-TiO2-induced oxidative stress at salinities of 5 and 15, since malondialdehyde levels were higher in these treatments compared to control and those in 50 µg/l nano-TiO2 group. The increase of salinity up to 35 g/l ameliorated the TiO2-induced oxidative stress. The results of the present study showed that the toxic impacts of nano-TiO2 can be ameliorated in the Pacific oyster with increase in water salinity, as the immune components, antioxidant enzyme activities recovered and oxidative stress mitigated when the salinity increased up to 35 g/l.