Environmental context determines the impact of titanium oxide and silver nanoparticles on the functioning of intertidal microalgal biofilms

Coastal environments are receiving habitats for most nanoparticle (NP) waste. Coastal sediments, into which NPs accumulate, support microalgal biofilms that provide important ecosystem processes: primary production, enhanced sediment stabilisation, and nutrient recycling. We assessed the impact of realistic concentrations of titanium oxide (TiO 2 ) and silver (Ag) NPs on marine microalgal biofilms and associated ecosystem processes in simulated natural conditions, by exposing natural biofilms to TiO 2 and Ag-NPs for one-month periods in outdoor tidal mesocosms under three contrasted environmental contexts (seasons). Ag-NPs had no significant effects on microalgal biomass, sediment biostabilisation potential and sediment-water oxygen and nutrient fluxes, even at concentrations (25 μg l −1 ) higher than current estimated levels (25 ng l −1 ). TiO 2 -NPs had no significant effect at current expected concentrations (25 μg l −1 ), but higher concentrations (25 mg l −1 ) resulted in decreased microalgal biomass; decreased ability of biofilms to biostabilise sediment, therefore limiting their coastal protection potential; reduced primary production and modified nutrient recycling. TiO 2 -NPs impacts were dependent on the environmental context: most effect was seen in winter, while no toxicity on biofilms was demonstrated in early spring. Our findings demonstrate that while Ag-NPs, being liable to dissolution into Ag + ions under the conditions tested, are not expected to have an environmental impact if current predictions of environmental loading prevail, TiO 2 -NPs may have ecological consequences in coastal environments in addition to direct impacts on microbial biomass. TiO 2 -NPs reduce the growth of coastal microalgae in environmentally relevant conditions, and alter sediment biostabilisation, primary production and nutrient recycling. These effects vary between environmental contexts (seasons).