Environmental Engineering Reference
In-Depth Information
3.1 Introduction
Many heavy metals occur naturally at elevated levels in the earth's crust and environ-
mental pollution from heavy metals has become widespread as industrial activities
have increased over the last two centuries. During this period these pollutants have
arisen from a variety of anthropomorphic sources such as urban and agricultural
runoff, industrial effluents, sewage treatment plants, mining operations and refining
of fossil fuels. As a consequence, detrimental effects are now witnessed in a wide
assortment of ecosystems [1, 2].
When heavy metals are introduced in their elemental forms or in organic-
metalloid compounds, they can have dramatic health implications for human
populations. Exposure has been linked to neurological impairment and cellular
senescence [3] renal and hepatic failure [4] and carcinogenesis [5-7]. Consequently,
it is not surprising that the United States Environmental Protection Agency (EPA)
has ranked mercury, cadmium, copper, lead, nickel and zinc on the priority list for
hazardous pollutants [8]. Bridges and Zalups [9] provide a worthwhile review of the
medical implications of heavy metal toxicity.
Heavy metal decontamination has been studied in a wide variety of species
across all phylogenic kingdoms [10-13]. Many species of various taxa contain genes
encoding metallothionein proteins and peptides that actively bind to heavy metals
[14-19]). Prokaryotic, algal and fungal capacities to cope with metal stress have
been extensively investigated because of their relative biological simplicity, ease of
culture, and the molecular similarity of their decontamination mechanisms to mam-
malian counterparts [20]. These mechanisms are known to deal with metals such as
Zn(II), Cu(II), and Cd(II) [21, 22]. Furthermore, prokaryotic species possess
mer
operons capable of regulating the stress response associated with ameliorating high
concentrations of mercury [15, 23]. Prokaryotes, algae and fungi also have the abil-
ity to biotransform heavy metals into metal sulfides that are relatively unavailable
biologically because of their insolubility [24].
Vascular plants, including aquatic macrophytes, are known to possess the ability
to bind and detoxify heavy metals, and much of this knowledge has been applied
to understanding heavy metal detoxification in algae [25, 26]. For example, various
algal species have been investigated for their abilities to accumulate heavy metals
and for their increased biomass by comparison with metal tolerant aquatic macro-
phytes [25]. Nevertheless, both macrophytes and algae have been given attention
because they can remove and retain these harmful contaminants from the environ-
ment. For example, cadmium resistant strains of
Chlamydamonas reinhardtii
have
been identified and studied for their potential to bind large quantities of heavy metals
[27-30].
This chapter focuses on the process of heavy metal tolerance and bioconversions
in micro-organisms with particular emphasis being placed on the mechanisms of
decontamination in the photosynthetic micro-organisms, cyanobacteria and algae.
These organisms do not require fixed carbon as a source of energy, possess aerobic
metabolisms and, under the appropriate conditions, can be very effective at the bio-
transformation of heavy metal ions. Because there is a strong possibility for similar
