exposure matrices that buffer against such changes, coupled with appropriate monitoring of redox potentials during the tests.
Nanoparticles that form colored suspensions may lead to a particular set of
confounding effects related to shading of light. This is relevant for algae and other
photosynthetic organisms that may experience lower light availability when used in
tests where nanoparticle suspensions are dense enough to reduce transmittance
(Handy et al. 2012b; Hjorth et al. 2016; Nguyen et al. 2018b). Algal growth rates
or measurements of photosynthesis, chlorophyll content and related endpoints
should thus account for confounding effects of shading.
28.3.2 Trophic Interactions
The trophic level of an organism can determine the way it is exposed to
nanomaterials. This is partly due to the feeding habits of organisms at different
trophic levels. Free-living microalgae may absorb nanoparticles directly from the
water suspending the particles, but particles may also affect the algae by affecting the
amount of available nutrient ions, or by shading the algae from light as to reduce
photosynthesis. A filter feeder grazing on these algae may experience a similar
concentration of nanomaterials through contact with water, but will in addition
ingest, e.g., the aforementioned microalgae that may contain nanomaterials.
Depending on whether bioaccumulation (increased concentrations in organisms
with increasing lifetime) or biomagnification (predators accumulating higher concentrations than found in their prey) occurs, the next level predator may experience
different exposure through the ingested food. So far, bioaccumulation has been
observed for some nanomaterials (Petersen et al. 2008; Wang et al. 2013), and in
some cases even biomagnification (Judy et al. 2011; Majumdar et al. 2016; Gupta
et al. 2017). No such studies have been made with nanomaterials used for remediation. In some cases, the bioaccumulation may be due to the fact that an element
contained in the nanoparticles in question is a micronutrient that the organism needs
and scavenges for, as observed for cobalt nanoparticles (Coutris et al. 2012). Iron,
found in many nanoparticles used for remediation, is likely to behave similarly if test
organisms are experiencing sub-optimal iron supply.
28.4 Standardized or Non-standardized Tests?
A major part of the research on nanoparticle toxicity has been made using
non-standardized tests, in the sense that they do not follow test protocols approved
by standardization organizations like the Organisation for Economic Co-operation
and Development (OECD) and International Organization for Standardization (ISO).
Non-standardized tests have the advantage of choosing freely among organisms,
endpoints, and exposure media. This allows for exposure optimization and
28 Ecotoxicity of Nanomaterials Used for Remediation
579
Nanoparticles that form colored suspensions may lead to a particular set of
confounding effects related to shading of light. This is relevant for algae and other
photosynthetic organisms that may experience lower light availability when used in
tests where nanoparticle suspensions are dense enough to reduce transmittance
(Handy et al. 2012b; Hjorth et al. 2016; Nguyen et al. 2018b). Algal growth rates
or measurements of photosynthesis, chlorophyll content and related endpoints
should thus account for confounding effects of shading.
28.3.2 Trophic Interactions
The trophic level of an organism can determine the way it is exposed to
nanomaterials. This is partly due to the feeding habits of organisms at different
trophic levels. Free-living microalgae may absorb nanoparticles directly from the
water suspending the particles, but particles may also affect the algae by affecting the
amount of available nutrient ions, or by shading the algae from light as to reduce
photosynthesis. A filter feeder grazing on these algae may experience a similar
concentration of nanomaterials through contact with water, but will in addition
ingest, e.g., the aforementioned microalgae that may contain nanomaterials.
Depending on whether bioaccumulation (increased concentrations in organisms
with increasing lifetime) or biomagnification (predators accumulating higher concentrations than found in their prey) occurs, the next level predator may experience
different exposure through the ingested food. So far, bioaccumulation has been
observed for some nanomaterials (Petersen et al. 2008; Wang et al. 2013), and in
some cases even biomagnification (Judy et al. 2011; Majumdar et al. 2016; Gupta
et al. 2017). No such studies have been made with nanomaterials used for remediation. In some cases, the bioaccumulation may be due to the fact that an element
contained in the nanoparticles in question is a micronutrient that the organism needs
and scavenges for, as observed for cobalt nanoparticles (Coutris et al. 2012). Iron,
found in many nanoparticles used for remediation, is likely to behave similarly if test
organisms are experiencing sub-optimal iron supply.
28.4 Standardized or Non-standardized Tests?
A major part of the research on nanoparticle toxicity has been made using
non-standardized tests, in the sense that they do not follow test protocols approved
by standardization organizations like the Organisation for Economic Co-operation
and Development (OECD) and International Organization for Standardization (ISO).
Non-standardized tests have the advantage of choosing freely among organisms,
endpoints, and exposure media. This allows for exposure optimization and
28 Ecotoxicity of Nanomaterials Used for Remediation
579