Are Carbon Nanotube Effects on Green Algae Caused by Shading and Agglomeration?

Abstract

Due to growing production, carbon nanotubes (CNT) may soon be found in a broad range of products and thus in the environment. In this work, an algal growth test was developed to determine effects of pristine and oxidized CNT on the green algae Chlorella vulgaris and Pseudokirchneriella subcapitata. CNT suspensions were prepared in algal test medium and characterized taking into account the suspension age, the reduced light transmittance of nanoparticle suspensions defined as shading of CNT and quantified by UV/vis spectroscopy, and the agglomeration of the CNT and of the algal cells. Growth inhibition and photosynthetic activity were investigated as end points. Growth of C. vulgaris was inhibited with effect concentrations of 50% (EC(50)) values of 1.8 mg CNT/L and of 24 mg CNT/L in well dispersed and in agglomerated suspensions, respectively, and 20 mg CNT/L and 36 mg CNT/L for P. subcapitata, respectively. However, the photosynthetic activity was not affected. Growth inhibition was highly correlated with the shading of CNT and the agglomeration of algal cells. This suggests that the reduced algal growth might be caused mainly by indirect effects, i.e. by reduced availability of light and different growth conditions caused by the locally elevated algal concentration inside of CNT agglomerates.

The Effect of DNA and Sodium Cholate Dispersed Single-Walled Carbon Nanotubes on the Green Algae Chlamydomonas reinhardtii

“…This is supported by previous work reporting that SWCNT toxic effects on the green algae Pseudokirchneriella subcapitata were lost with higher concentrations of the surfactant Gum Arabic [36]. In another study, Schwab et al found that carbon nanotubes only partially dispersed by natural organic matter had no direct toxic effect but that agglomerated nanotubes have indirect effects [37]. Therefore, it is likely that very welldispersed and nonagglomerated SWCNTs do not have any toxic effect, as is described herein.…”

Stability, metal leaching, photoactivity and toxicity in freshwater systems of commercial single wall carbon nanotubes

“…Interestingly, much work has focused on the research and development of applications of CNTs, but only a few studies have focused on the behavior and risks associated with CNTs in environmentally relevant conditions (Petersen et al, 2011;Hutchison, 2008;Wang et al, 2008;Robichaud et al, 2005;Lecoanet et al, 2004). Preliminary ecotoxicity studies have shown that CNTs are toxic to algae, fish, daphnids and other organisms (Schwab et al, 2011;Petersen et al, 2009;Smith et al, 2007). Transport studies showed that humic acid, a common environmental constituent, dispersed CNTs and increased their mobility in the environment (Wang et al, 2008).…”

“…While purified CNTs may contain only a small percentage of the metal catalysts, raw CNTs can contain as much as 40% metal by weight. To date little work has been done to quantify the release of metal residues from CNT matrices in natural waters, or the toxicological implications of the bioavailable metals (Schwab et al, 2011;Hull et al, 2009;Blaise et al, 2008). It was previously thought that CNT purification removed almost all metals that could leach in solution, but there may be some residual metals since metal dissolution is kinetically limited and most purification processes are too short in duration (Liu et al, 2008).…”

“…However, Schwab et al (2011) reported no effect of residual metals and that shading by the CNTs accounted for the observed reduced growth rates (Schwab et al, 2011). In fact, Schwab et al (2011) showed that algae and CNTs form irreversible aggregates, ostensibly due to hydrogen binding between the CNTs and the algal surface, which reduces the photosynthetic capability of the algae (Schwab et al, 2011).…”

Control experiments to avoid artifacts and misinterpretations in nanoecotoxicology testing

“…4.12 Shading control: The purpose of this control is to assess the extent to which light intensity reduction caused by ENMs could impact the ecotoxicity endpoint measured (17,18). The first step is to analyze the decrease in the transmittance through the test container containing the ENM from the specified light source; this could be conducted at the highest ENM concentration or at a range of ENM concentrations if a dose-response relationship is being tested.…”