Environmental Engineering Reference
In-Depth Information
wide variety of metal stressors is vital. Studies in this species with metals other than
mercury are needed.
The source of sulfides in organisms such as cyanobacteria and algae could be
directly or indirectly from an organic pool. For example, MT IIIs are known to be
associated with nanocrystalline sulfide particles in which metal ions are bound [55,
56, 124, 125]. Apparently, these nanoparticles stabilize the MT III complexes and,
as a consequence, improve resistance to metals. It remains to be seen if aerobic
metal sulfide production is mainly through this latter mechanism. The pathway has
not been determined for the source of sulfur used in metal sulfide biotransformation
even though thiol containing compounds have been implicated [24]. Kelly et al.
[75] showed that the synthesis of mercury sulfide followed two distinct phases. The
initial efficiency of the biotransformation process appears to be reliant on the size of
the initial phase and the effectiveness of longer term metal bioremediation should be
proportionate to the biotransformation rate associated with the slower second phase.
These are suspected to be reflected in the initial size and rate of replenishment of
a pool of readily available sulfur from compounds such as cysteine, glutathione or
metallothioneins.
3.9 Future Considerations
Aerobic microbiological processes offer attractive alternatives for the clean-up of
metal-containing waste water produced by mining, gas-scrubbing, soil-washing, and
processing of fossil fuels as well as from municipal waste. However, aerobic pho-
tosynthetic biotransformation has yet to be utilized as a method for bioremediation
of heavy metal contaminated sites. Further investigations are required to advance
the work spearheaded by Kelly and colleagues [24, 75]. In order to effectively
implement this biotechnology the efficiency of the process must be determined with
respect to a range of contaminating metal ions of varying concentration. Careful
consideration must also be given to choosing which organisms are best suited for
bioremediation of the different heavy metals and their combinations. These stud-
ies would lay the groundwork for further research and development of bioreactors
designed for large scale industrial applications.
Acknowledgements
The authors wish to thank the Natural Sciences and Engineering Research
Council (Canada) for supporting their research in metal bioremediation. Valuable advice on the
manuscript from two anonymous reviewers is gratefully acknowledged.
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