Heat stress mediates toxicity of rutile titanium dioxide nanoparticles on fertilisation capacity in the broadcast spawning mussel Mytilus galloprovincialis

Titanium dioxide nanoparticle (nTiO₂) pollution of marine environments is rapidly increasing with potentially deleterious effects on wildlife. Yet, the impacts of nTiO₂ on reproduction remain poorly understood. This is especially the case for broadcast spawners, who are likely to be more severely impacted by environmental disturbances because their gametes are directly exposed to the environment during fertilisation. In addition, it is unclear whether rising water temperatures will further exacerbate the impact of nTiO₂ toxicity. Here, in a series of fertilisation trials, we systematically examine the main and interactive effects of nTiO₂ exposure and seawater temperature on fertilisation success in the Mediterranean mussel Mytilus galloprovincialis. Specifically, our fertilisation trials explored whether nTiO₂ exposure influences fertilisation rates when (i) eggs alone are exposed, (ii) both sperm and eggs are exposed simultaneously, and (iii) whether increases in seawater temperature interact with nTiO₂ exposure to influence fertilisation rates. We also ask whether changes in nTiO₂ concentrations influence key sperm motility traits using computer-assisted sperm analysis (CASA). In fertilisation trials for treatment groups (i) and (ii), we found no main effects of nTiO₂ at environmentally relevant concentrations of 5, 10 and 50 μg L^-1 on fertilisation capacity relative to the control. Consistent with these findings, we found no effect of nTiO₂ exposure on sperm motility. However, in treatment group (iii), when fertilisation trials were conducted at higher temperatures (+6 °C), exposure of gametes from both sexes to 10 μg L^-1 nTiO₂ led to a reduction in fertilisation rates that was significantly greater than when gametes were exposed to elevated temperature alone. These interacting effects of nTiO₂ exposure and seawater temperature demonstrate the toxic potential of nTiO₂ for fertilisation processes in a system that is likely to be impacted heavily by predicted future increases in sea surface temperatures.