Nanoparticles (NPs), or particles with dimensions that go below 100 nanometers, can be released into the environment in various ways, such as through cosmetic products like sunscreen. Titanium dioxide (TiO2) is a commonly used nanoparticle in sunscreen, and it has the potential to be harmful to various aquatic organisms including algae. The purpose of this study was to examine how the crystalline phase of TiO2 NPs (brookite or rutile) influenced their toxicity to the microalgae species Chlorella. It was hypothesized that brookite phase TiO2 NPs would inhibit the growth of Chlorella more than rutile phase TiO2 NPs due to the similarity of brookite to anatase phase TiO2, which is known to be more toxic than rutile. To test this hypothesis, Chlorella were exposed to various concentrations of the two types of nanoparticles, and their fluorescence was measured over four days. An independent samples t-test was conducted to analyze the significance of the data. At an alpha level of 0.05, results showed that brookite Chlorella (M=0.02, SD=0.00926) had significantly lower growth than rutile Chlorella (M=0.05, SD=0.0469) over the course of 70 hours, t(40)=-3.11301, p=.003414. These findings suggest that the hypothesis is supported. Therefore, rutile TiO2 is less toxic to Chlorella than brookite TiO2 and should continue to be used as the preferred form of TiO2 in sunscreens.

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