Decontaminating Heavy Metals fromWater Using Photosynthetic Microbes - Emerging Environmental Technologies - page 55

Chapter 3
Decontaminating Heavy Metals from Water
Using Photosynthetic Microbes

Daniel D. Lefebvre and Chad Edwards

Abstract Elevated levels of heavy metals in our environment can pose serious
problems to a wide variety of living organisms, including humans. This is because
transporters in cell membranes can absorb toxic non-essential metal ions and essen-
tial metal ions to excess, both of which can deleteriously affect important metabolic
processes. Organisms have responded to this threat by evolving coping mechanisms
that biotransform the metals into forms possessing low toxicity. By their very nature
these mechanisms also act to make metals less bioavailable in the environment,
and it is this property that can be exploited for bioremediation purposes. Sulfur
and its metabolism is often central to these coping mechanisms. It is absorbed by
cells in the form of sulfate that in turn is converted to sulfite and subsequently into
thiols via energy input and reduction. Metal ions can bind to these thiol groups
in cysteine, glutathione and metallothioneins rendering them essentially detoxified.
Furthermore, some organisms such as the sulfate reducing bacteria biotransform
metal ions into metal sulfides that have very low solubilities and hence, very low
bioavailabilities. However from the perspective of applying metal bioremediation,
the sulfate reducing bacteria require anoxic environments that would be unlike the
conditions associated with most anthropogenic sources of heavy metals. Recently,
photosynthetic microbes have also been shown to form metal sulfides. Here, we
present the potential of these microbes for the effective aerobic bioremediation of
heavy metals.

Keywords Heavy metals · Metal biotransformation · Bioremediation ·

Cyanobacteria · Algae · Sulfur metabolism · Thiol · Metal sulfide