Ecotoxicity study of titania (TiO₂) NPs on two microalgae species: Scenedesmus sp. and Chlorella sp.

Abstract

In view of their increasing commercial applications metal oxide NPs like titania have elevated chances of entry to the environment. The ecotoxicity analyses are required to assess their environmental risks. The present work aims to demonstrate the effect of titania NPs on microalgae isolated from freshwater environment (Scenedesmus sp. and Chlorella sp.). The growth inhibitory effect of titania NPs was observed for both the species (72 h EC₅₀ value, 16.12 mg/L for Chlorella sp.; 21.2 mg/L for Scenedesmus sp.). Bulk micron-sized titania also showed toxicity though to a lesser extent (72 h EC₅₀ value, 35.50 mg/L for Chlorella sp.; 44.40 mg/L for Scenedesmus sp.). A concentration dependent decrease in chlorophyll content was observed in the treated cells compared to the untreated ones, more effect being notable in case of NPs. Preliminary results based on FT-IR studies and microscopic images suggest interaction of the NPs with the cell surface.

Introduction

Metal oxide NPs have been recently manufactured at the industrial level and have tremendous applications in water treatment, medicine, cosmetics, and engineering (et al., 2004, U.S. Environmental Protection Agency, 2005), for example aluminum NPs are already used in explosive combinations (Kaste and Rice, 2004). Titanium dioxide NPs are mostly used as photo-catalyst and UV absorbent in consumer products like sunscreen lotion and they also act as catalyst in sterilization (Fujishima et al., 2000, Meng et al., 2005, Pena et al., 2005). Since the applications of engineered metal oxide NPs increased in recent years, it is expected that metal oxide NPs such as titanium dioxide will find their way into the aquatic environment, where their fate and behavior are largely unknown (Maynard et al., 2006, Nowack and Bucheli, 2007). The photocatalytic activity of titanium dioxide NPs has been found to depend on the concentration, crystal structure and light intensity (Kim and Lee, 2005). The titania NPs were shown to cause toxicity to organisms by producing reactive oxygen species (ROS) upon interaction with UV light; leading to cell membrane damage (Yeung et al., 2009). Recently we have studied the effect of aluminum oxide NPs on Escherichia coli (Sadiq et al., 2009). Toxicity effect of alumina (Al₂O₃) nano-particle on bacteria was suggested to be due to the attachments of NPs to bacterial cells in 24–48 h growth period (Jiang et al., 2009).

Algal toxicity tests are extensively applied to assess the effects of hazardous substances in water since algae play an important role in the equilibrium of aquatic ecosystems, being the first level of the trophic chain to produce organics and oxygen. Ecotoxicity of titanium dioxide, zinc oxide and copper oxide NPs to algae such as Pseudokirchneriella subcapitata and Desmodesmus subspicatus was reported recently (Aruoja et al., 2009). A number of toxicity studies were reported with oxide NPs including titanium with pure cultures freshwater algae (Peller et al., 2007, Hartmann et al., 2010, Ji et al., 2010) and marine algae (Miller et al., 2010). As a part of our ongoing ecotoxicological assessment of manufactured NPs we have demonstrated the growth inhibitory effect of alumina NPs (Sadiq et al., 2011) for freshwater algae (72 h EC₅₀ value, 45.4 mg/L for Chlorella sp.; 39.35 mg/L for Scenedesmus sp.).

With this background, the aim of the current investigation was to study the difference in (if any) toxic response of micron-sized and nano-sized titania particles towards microalgae isolated from aquatic environment.