Agriculture Reference
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small aggregates, so size increased and toxicity decreased with increasing rutile
proportion. The influence of the aggregate size was not unexpected, since the
reactivity of nanoparticles is driven by their specific surface area, which is reduced
by aggregation. To assess the nanoparticles dispersion state and their interactions
with bacteria, cryogenic transmission electron microscopy (TEM) and zeta poten-
tial measurements were performed. A higher nanoparticle aggregate sorption on
cells was observed in pH 5 water, compared to SRW, i.e., the observed toxicity was
not directly linked to the particles sorption onto the cell surfaces.
Another comprehensive study was conducted by Miller et al. ( 2012 ) toward the
effects of TiO 2 nanoparticles on phytoplankton (0.2-200- μ m single or clustered
cells), the dominant primary producers in marine ecosystems (Behrenfeld
et al. 2006 ), as they are the base of oceanic food webs and a dominant component
of the global carbon cycle, as well as other biogeochemical cycles. Also, the marine
and estuarine ecosystems are expected to be the destination of most industrially
discharged nanomaterials (Musee et al. 2011 ).
Significant suppression of population growth occurred for three of the species
used, under UV light (UVA averaged 4.5 W m 2 and UVB 4.1 W m 2 , levels
comparable to UV intensities near the ocean surface,
1 m depth in coastal waters).
In one species, Isochrysis galbana , toxicity was evident at the lowest concentration
tested, 1 mg L 1 , indicating a no-effect concentration (NOEC)
<
1mgL 1 . In the
other two species affected, Thalassiosira pseudonana and Dunaliella tertiolecta ,
significant toxicity was evident at 3 mg L 1 , although a slight depression of growth
rates was seen for D. tertiolecta at 1 mg L 1 . No significant effect on growth rates
of any species was seen in the blocked UV treatment except in the case of I. galbana
at the highest TiO 2 concentration tested, 7 mg L 1 . No significant effect of nano-
TiO 2 on growth rate was seen in any treatment for the diatom Skeletonema
costatum . Also in this study, scanning electron microscopy (SEM) revealed that
TiO 2 nanoparticles were adhering to the surfaces of phytoplankton cells as aggre-
gates with sizes between 10 and 100 nm.
Chen et al. ( 2012 ) investigated the toxicological effects of nanometer titanium
dioxide (21 nm, anatase-to-rutile mixture) on a unicellular green alga
Chlamydomonas reinhardtii , by investigating the changes of the physiology and
cyto-ultrastructure of this species under treatment. There was no significant differ-
ence between the control and the treatment group with nano-TiO 2 during an 8-h
exposure. After a 24-h exposure, there was significant reduction in the treated
groups and the control. 10, 20, and 100 mg L 1 nano-TiO 2 inhibited growth
significantly, and the cell density of those treated groups (10, 20, and 100 mg L 1
TiO 2 ) reduced gradually during the experiment, which showed that cell growth was
inhibited completely by high TiO 2 dosage. The photosynthetic activity (Fv/Fm, a
ratio of variable to maximum fluorescence, indicating photosynthesis activity) of
C. reinhardtii decreased sharply with high TiO 2 concentration (
<
1mgL 1 ). SEM
micrographs of C. reinhardtii treated with nano-TiO 2 indicated that TiO 2 NPs had
assembled on the cell wall surface. The content of soluble protein in algae cells
increased after 24-h treatment with TiO 2 at 0.1, 1, and 10 mg L 1 , while it
decreased in cells treated with TiO 2 at concentrations of 20 and 100 L 1 . After
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