The hydrogenotrophic methanogens were consistently less
susceptible to inhibition by the ZnO NPs than acetoclastic methanogens.
In order to confirm this finding, batch toxicity bioassays
were performed using non-exposed sludge obtained from R1.
Fig. 6A shows the maximum SMA observed at different ZnO NPs
concentrations. Since only one of the treatments was considerably
inhibited during the first feeding of substrate (w30% inhibition), a
second substrate spike was made after depletion of acetate or
hydrogen. After the second feeding of substrate, the toxicity of ZnO
NPs to methanogens in AGS clearly increased with the time of
exposure and considerable inhibition was observed in most assays.
Furthermore, the toxicity of ZnO NPs to acetoclastic and hydrogenotrophic
methanogenesis differed. The IC50 values for
acetoclastic methanogenesis was 12.2 mg Zn L1 and only 4% activity
compared to the control was observed at 80.3 mg Zn L1. The
IC50 value of 229 mg Zn L1 of ZnO NPs to the hydrogenotrophic
methanogenesis, on the other hand was significantly higher
(Table 1). The short-term exposure results correlated well with the
findings observed during the long-term exposure. As discussed
before, extended supplementation of ZnO NPs to the reactors
caused more inhibition of the acetoclastic than the hydrogenotrophic
methanogens (Fig. 5). The higher sensitivity of acetoclastic
methanogens to ZnO NPs is in agreement with previous
findings (Gonzalez-Estrella et al., 2013) and has important implications
since approximately 67% of electron flow in anaerobic
digestion proceeds through acetate (Gujer and Zehnder, 1983).
Thus, measures may need to be taken to remove ZnO NPs or
otherwise attenuate ZnO toxicity, such as promote conversion to
ZnS by biogenic sulfide (Lombi et al., 2012).
Several authors relate the toxicity of ZnO NPs to their dissolution
and the release of toxic Zn2þ (Franklin et al., 2007; Liu et al., 2011;
Wong et al., 2010). Thus, the soluble concentration of Zn was
measured at the end of the batch toxicity assays to investigate the
possible correlation to the ZnO NPs toxicity. Fig. 6B shows a plot of
the activity against the measured Zn2þ concentration. The dissolved
concentrations found in batch experiments were in the range of those measured in the reactors (between 0.06 and 0.35 mg L1).
These Zn2þ concentrations are much lower than the inhibitory
concentrations reported in the literature (Mu and Chen, 2011; Mu
et al., 2012), so it is unclear whether dissolved Zn alone could
have accounted for the toxicity observed in this study. Therefore,
the toxicity of ZnO NPs could be caused by the combination of
various mechanisms. On the one hand, ZnO NPs partially dissolve
and release of toxic Zn2þ ions. Zn2þ is accumulated inside the cells
by the unspecific CorA Mg uptake system. Moreover, Zn2þ may
interact with Mg2þ and inhibit the function of this physiological
cation (Nies, 1999). On the other hand, the toxic effect of ZnO NPs
themselves, which were shown to cause cell membrane damage to
bacteria (Brayner et al., 2006; Kumar et al., 2011). The membrane
disruption increases the permeability of the cells to ZnO NPs that,
once inside the cell, have shown to cause DNA damage (Kumar
et al., 2011).
The hydrogenotrophic methanogens were consistently lesssusceptible to inhibition by the ZnO NPs than acetoclastic methanogens.In order to confirm this finding, batch toxicity bioassayswere performed using non-exposed sludge obtained from R1.Fig. 6A shows the maximum SMA observed at different ZnO NPsconcentrations. Since only one of the treatments was considerablyinhibited during the first feeding of substrate (w30% inhibition), asecond substrate spike was made after depletion of acetate orhydrogen. After the second feeding of substrate, the toxicity of ZnONPs to methanogens in AGS clearly increased with the time ofexposure and considerable inhibition was observed in most assays.Furthermore, the toxicity of ZnO NPs to acetoclastic and hydrogenotrophicmethanogenesis differed. The IC50 values foracetoclastic methanogenesis was 12.2 mg Zn L1 and only 4% activitycompared to the control was observed at 80.3 mg Zn L1. TheIC50 value of 229 mg Zn L1 of ZnO NPs to the hydrogenotrophicmethanogenesis, on the other hand was significantly higher(Table 1). The short-term exposure results correlated well with thefindings observed during the long-term exposure. As discussedbefore, extended supplementation of ZnO NPs to the reactorscaused more inhibition of the acetoclastic than the hydrogenotrophicmethanogens (Fig. 5). The higher sensitivity of acetoclasticmethanogens to ZnO NPs is in agreement with previousfindings (Gonzalez-Estrella et al., 2013) and has important implicationssince approximately 67% of electron flow in anaerobicdigestion proceeds through acetate (Gujer and Zehnder, 1983).Thus, measures may need to be taken to remove ZnO NPs orotherwise attenuate ZnO toxicity, such as promote conversion toZnS by biogenic sulfide (Lombi et al., 2012).Several authors relate the toxicity of ZnO NPs to their dissolutionand the release of toxic Zn2þ (Franklin et al., 2007; Liu et al., 2011;Wong et al., 2010). Thus, the soluble concentration of Zn wasmeasured at the end of the batch toxicity assays to investigate thepossible correlation to the ZnO NPs toxicity. Fig. 6B shows a plot ofthe activity against the measured Zn2þ concentration. The dissolvedconcentrations found in batch experiments were in the range of those measured in the reactors (between 0.06 and 0.35 mg L1).These Zn2þ concentrations are much lower than the inhibitoryconcentrations reported in the literature (Mu and Chen, 2011; Muet al., 2012), so it is unclear whether dissolved Zn alone couldhave accounted for the toxicity observed in this study. Therefore,the toxicity of ZnO NPs could be caused by the combination ofvarious mechanisms. On the one hand, ZnO NPs partially dissolveand release of toxic Zn2þ ions. Zn2þ is accumulated inside the cellsby the unspecific CorA Mg uptake system. Moreover, Zn2þ mayinteract with Mg2þ and inhibit the function of this physiologicalcation (Nies, 1999). On the other hand, the toxic effect of ZnO NPsthemselves, which were shown to cause cell membrane damage tobacteria (Brayner et al., 2006; Kumar et al., 2011). The membranedisruption increases the permeability of the cells to ZnO NPs that,once inside the cell, have shown to cause DNA damage (Kumaret al., 2011).
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