Effects of nanoparticles of TiO2 on food depletion and life-history responses of Daphnia magna.

The extent to which different forms of nanoparticles of titanium dioxide (nano-TiO2) aggregated with microalgae, decreased food levels and hence impaired growth, reproduction and fitness of Daphnia magna individuals were studied. Treatments included three different types of nano-TiO2 differing in their coating or crystalline structure but of similar primary size (20 nm) plus a micron-sized bulk material, two exposure levels (1, 10mg/l) and two food ration levels of the microalgae Chlorella vulgaris that included a non limiting (1.5 μgC/ml) and a limiting one (0.3 μgC/ml). Effects were assessed using standardized chronic tests and assays that maximized food depletion in the water column under semi-static and re-suspension conditions. Results indicated that the high ion levels in culture medium lead to the aggregation of nanoparticles followed by particle destabilization. Nanoparticle aggregates interacted with the algae cells, forming clusters. Large TiO2-algae agglomerates settled readily dramatically depleting the concentration of available food for D. magna. At limiting food rations food depletion by nanoparticle aggregation had dramatic effects on reproduction and fitness of exposed D. magna at 1mg/l irrespectively of the particle form. At high food rations effects were only observed for one of the nano-TiO2, P-25, at high exposure levels (10 mg/l) under both semi-static and particle re-suspension conditions, which suggest that P-25 effects were mediated by clogging the gut and hence diminishing food acquisition. These results indicate that nano-TiO2 may affect the transfer of energy throughout the planktonic aquatic food webs increasing the settlement of edible particles from the water column.