CO₂-induced pH reduction increases physiological toxicity of nano-TiO₂ in the mussel Mytilus coruscus

The increasing usage of nanoparticles has caused their considerable release into the aquatic environment. Meanwhile, anthropogenic CO₂ emissions have caused a reduction of seawater pH. However, their combined effects on marine species have not been experimentally evaluated. This study estimated the physiological toxicity of nano-TiO₂ in the mussel Mytilus coruscus under high pCO₂ (2500-2600 μatm). We found that respiration rate (RR), food absorption efficiency (AE), clearance rate (CR), scope for growth (SFG) and O:N ratio were significantly reduced by nano-TiO₂, whereas faecal organic weight rate and ammonia excretion rate (ER) were increased under nano-TiO₂ conditions. High pCO₂ exerted lower effects on CR, RR, ER and O:N ratio than nano-TiO₂. Despite this, significant interactions of CO₂-induced pH change and nano-TiO₂ were found in RR, ER and O:N ratio. PCA showed close relationships among most test parameters, i.e., RR, CR, AE, SFG and O:N ratio. The normal physiological responses were strongly correlated to a positive SFG with normal pH and no/low nano-TiO₂ conditions. Our results indicate that physiological functions of M. coruscus are more severely impaired by the combination of nano-TiO₂ and high pCO₂.