Decontaminating Heavy Metals from Water Using Photosynthetic Microbes

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 essential 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.