Influences of temperature and salinity on the physicochemical properties and toxicities of zinc oxide nanoparticles to microalgae

Abstract

As effective blockers of ultraviolet radiation, zinc oxide nanoparticles (ZnO-NPs) are widely used in sunscreens and can be easily released into aquatic environments. When ZnO-NPs enter the marine environment, their properties and toxicity may be altered by temperature and salinity. However, combined effects of temperature and salinity on their toxicity to aquatic organisms remain unknown. Moreover, ZnO-NPs are often coated with a synthetic layer in commercial sunscreens to enhance its adhesion to human skin and waterproof ability. Yet, what kind of coating could result in a low toxicity remains largely unknown. This study, therefore, aimed to (1) investigate the effect of environmental factors such as salinity, temperature and pH on physicochemical properties and toxicity of ZnO-NPs to microalgae, and (2) compare the physicochemical properties and toxicities of different coated and uncoated ZnO-NPs.

Increases in salinity, temperature and pH enhanced the aggregation and reduced dissolution of ZnO-NPs. An increase in water temperature from 10 to 30°C reduced the zeta potential of ZnO-NPs, but an increase in salinity from 12 to 32 psu and an increase of pH from 7 to 8 did not affect their zeta potential. The increased aggregate size and decreased ion release with increasing salinity, and consequently lower concentration of bioavailable zinc ions, resulted in decreased toxicity of ZnO-NPs to the marine diatom Thalassiosira pseudonana at higher salinity based on growth inhibition and photosynthetic responses. Their toxicity to the diatoms decreased from 10 to 25°C, and then increased at 30°C due to the incurred thermal stress. There were interacting effects of salinity and temperature on the toxicity of ZnO-NPs to T. pseudonana; their toxicity to the diatoms generally decreased with increasing salinity from 12 to 32 psu across temperatures between 10-25°C, but this trend did not hold at 30°C. Temperature is a more dominant factor than salinity, as it has strong influences on the diatom’s physiology and tolerance towards ZnO-NPs. Based on gene expression results, T. pseudonana exposed to ZnO-NPs at low salinity (12 psu) and extreme temperatures (10°C and 30°C), experienced diatom frustule damage, photosynthetic damage and oxidative stress earlier than the diatoms cultured at the high salinity (32 psu) and room temperature (25°C). Changes in gene expression patterns by temperature and salinity were related to zinc ions released from ZnO-NPs, particle-cell interactions and physiological changes in the diatoms.

ZnO-NPs covered with organosilane coating suppressed aggregation and dissolution of ZnO-NPs. Toxicities of uncoated and silane-coated ZnO-NPs to six different microalgae species were species-specific. ZnO-NPs coated with hydrophobic dodecyltrichlorosilane (D-ZnO-NPs) generally exhibited lower toxicity to the microalgae than the uncoated ZnO-NPs and ZnO-NPs coated with 3-aminopropyltrimethoxysilane (A-ZnO-NPs).D-ZnO-NPs and uncoated ZnO-NPs induced oxidative stress in T. pseudonana, while A-ZnO-NPs and uncoated ZnO-NPs impaired/disturbed their frustule formation, silicon transportation, and photosynthesis.

This study provided empirical toxicity data and useful information for ecological risk assessment of ZnO-NPs under different environmental conditions. The results also indicate that ZnO-NPs coated with hydrophobic dodecyltrichlorosilane can lead to a lower toxicity. Hence, functionalized coatings with similar chemical structures may offer better, eco-friendly sunscreen products. An abstract of 500 words