costly. Indeed, large amounts of bacterial, fungal, or algal
biomass originating from waste treatment plants, agriculture, or food processing can be made available for such
treatments. Most often they complete physicochemical treatment (flocculation) to remove the remaining contamination.
The economic value of these techniques has also been shown
as part of the decontamination of radionuclides polluted sites
(Kalin et al. 2005). Today they tend to be developed and
improved through genetic modification of bacterial strains to
increase their ability to complex metals of interest. The
improvement also concerned processes, particularly with
the development of continuous bioreactors.
In addition to bioremediation, the biorecovery uses the
capabilities of microorganisms to extract toxic metals or
metals of economic interest. One example is gold, which is
accumulated and reduced on biological membranes allowing
its recovery in the form of usable metal (Au0).
16.9.5 Microbial Bioprecipitation of Metals
16.9.5.1 Involved Mechanisms
The precipitation of metals leads to ore production from
soluble ionic forms. The precipitation produces an inert
form of the metal (Fig. 16.47) obtained by the precipitation
of metal by products of bacterial metabolism (sulfides,
hydroxides, carbonates, phosphates, silica, etc.). Bioprecipitation has been studied for many elements:
(i) Many studies concern iron (formation of iron
hydroxides) because of its quantitative importance
(fourth most abundant element in the earth’s crust) and
qualitative interest (many metabolisms are based on iron)
as well as, in a lesser extent, because of its involvement
in the formation of manganese oxides.
(ii) Other studies focus on the role of microorganisms in the
formation of clays and in precipitation processes of
carbonates, phosphates, and sulfate (for more details,
see Konhauser 2007). In the case of carbonates, calcite
formation can be achieved via cyanobacteria (see
Microbialites) through the adsorption of calcium at the
surface of the cells (bioadsorption) and then precipitation of the carbonates with calcium. Microorganisms are
also involved in the formation of dolomite (CaMg
(CO 3 ) 2 ), siderite (FeCO 3 ), and rhodochrosite (MnCO 3 ).
(iii) Finally, the most successful processes in terms of bioremediation techniques involve the formation of sulfidic
ores that can be of abiotic origin (including hydrothermal) or biogenic (Ehrlich 1995). In the latter case, the
process involves sulfate-reducing microorganisms.
These chemoorganotrophic or chemolithotrophic
microorganisms share the ability to use oxidized sulfur
compounds (sulfate, sulfur, sulfite, thiosulfate) as terminal electron acceptor in a conservative energy
metabolism. The result of their metabolism is the production of sulfide (S
2À ), which precipitates many metals,
including divalent metals (cadmium, cobalt, copper,
iron, lead, manganese, mercury, nickel, zinc).
16.9.5.2 Environmental Applications
The efficiency of bioprecipitation has been demonstrated by
White and Gadd (1996) on the removal of metals from acidic
leachate. In this context, the removal of metals could reach
95 %. There is a commercial application of this method of
treatment
(THIOPAQ
technology,
Paques,
The
Netherlands). Such processes have demonstrated their efficiency in terms of treatment on several sites (Budelco BV,
Netherlands and Kennecott Utah, USA) for the removal of
different metals such as zinc, aluminum, copper, iron, and
manganese. The treatment process is carried out in two
phases. The first allows for the production of sulfide by
sulfate-reducing microorganisms in the presence of hydrogen as an electron donor. Hydrogen sulfide is used in the
precipitation of metals that can be recovered and the sulfides
produced in excess can be subsequently reoxidized by
chemolithotrophic aerobic microorganisms that will produce
elemental sulfur. The use of such methods has also been
proposed for the treatment of groundwater rich in arsenic
(Bangladesh).
16.9.6 Other Microbial Activities Related
to Metal Contaminants
The metals can be highly toxic to living organisms, especially those who have no known biological function, such as
mercury, lead, cadmium, or tin. Microorganisms have developed different strategies to withstand the presence of these
metals in the environment:
(i) Mechanisms preventing the penetration in the cell
(ii) Mechanisms for their excretion (transport) out of the cell
The active mechanisms of excretion involve sequestration phenomena, active transport, and speciation. Some
genes encoding systems for active transport of toxic metals
are known (Silver and Phung 2005) as it is the case of
cadmium (cad), copper (cop), lead (pbr), or arsenic (ars).
Other resistance systems, specific or not, involve methylation processes that produce volatile forms of tin, arsenic,
selenium, lead, etc. (Thayer and Brinkman 1982). These
volatile forms must be considered as losses from the ecosystem under consideration and therefore induce a reduction in
the toxicity of these metals.
These resistance systems have been described in natural
environment and are rarely used as bioremediation
techniques. Similarly, the microbial reduction of mercury
through the Mer detoxification system (Box 14.15) implies a
16 Applied Microbial Ecology and Bioremediation
745
biomass originating from waste treatment plants, agriculture, or food processing can be made available for such
treatments. Most often they complete physicochemical treatment (flocculation) to remove the remaining contamination.
The economic value of these techniques has also been shown
as part of the decontamination of radionuclides polluted sites
(Kalin et al. 2005). Today they tend to be developed and
improved through genetic modification of bacterial strains to
increase their ability to complex metals of interest. The
improvement also concerned processes, particularly with
the development of continuous bioreactors.
In addition to bioremediation, the biorecovery uses the
capabilities of microorganisms to extract toxic metals or
metals of economic interest. One example is gold, which is
accumulated and reduced on biological membranes allowing
its recovery in the form of usable metal (Au0).
16.9.5 Microbial Bioprecipitation of Metals
16.9.5.1 Involved Mechanisms
The precipitation of metals leads to ore production from
soluble ionic forms. The precipitation produces an inert
form of the metal (Fig. 16.47) obtained by the precipitation
of metal by products of bacterial metabolism (sulfides,
hydroxides, carbonates, phosphates, silica, etc.). Bioprecipitation has been studied for many elements:
(i) Many studies concern iron (formation of iron
hydroxides) because of its quantitative importance
(fourth most abundant element in the earth’s crust) and
qualitative interest (many metabolisms are based on iron)
as well as, in a lesser extent, because of its involvement
in the formation of manganese oxides.
(ii) Other studies focus on the role of microorganisms in the
formation of clays and in precipitation processes of
carbonates, phosphates, and sulfate (for more details,
see Konhauser 2007). In the case of carbonates, calcite
formation can be achieved via cyanobacteria (see
Microbialites) through the adsorption of calcium at the
surface of the cells (bioadsorption) and then precipitation of the carbonates with calcium. Microorganisms are
also involved in the formation of dolomite (CaMg
(CO 3 ) 2 ), siderite (FeCO 3 ), and rhodochrosite (MnCO 3 ).
(iii) Finally, the most successful processes in terms of bioremediation techniques involve the formation of sulfidic
ores that can be of abiotic origin (including hydrothermal) or biogenic (Ehrlich 1995). In the latter case, the
process involves sulfate-reducing microorganisms.
These chemoorganotrophic or chemolithotrophic
microorganisms share the ability to use oxidized sulfur
compounds (sulfate, sulfur, sulfite, thiosulfate) as terminal electron acceptor in a conservative energy
metabolism. The result of their metabolism is the production of sulfide (S
2À ), which precipitates many metals,
including divalent metals (cadmium, cobalt, copper,
iron, lead, manganese, mercury, nickel, zinc).
16.9.5.2 Environmental Applications
The efficiency of bioprecipitation has been demonstrated by
White and Gadd (1996) on the removal of metals from acidic
leachate. In this context, the removal of metals could reach
95 %. There is a commercial application of this method of
treatment
(THIOPAQ
technology,
Paques,
The
Netherlands). Such processes have demonstrated their efficiency in terms of treatment on several sites (Budelco BV,
Netherlands and Kennecott Utah, USA) for the removal of
different metals such as zinc, aluminum, copper, iron, and
manganese. The treatment process is carried out in two
phases. The first allows for the production of sulfide by
sulfate-reducing microorganisms in the presence of hydrogen as an electron donor. Hydrogen sulfide is used in the
precipitation of metals that can be recovered and the sulfides
produced in excess can be subsequently reoxidized by
chemolithotrophic aerobic microorganisms that will produce
elemental sulfur. The use of such methods has also been
proposed for the treatment of groundwater rich in arsenic
(Bangladesh).
16.9.6 Other Microbial Activities Related
to Metal Contaminants
The metals can be highly toxic to living organisms, especially those who have no known biological function, such as
mercury, lead, cadmium, or tin. Microorganisms have developed different strategies to withstand the presence of these
metals in the environment:
(i) Mechanisms preventing the penetration in the cell
(ii) Mechanisms for their excretion (transport) out of the cell
The active mechanisms of excretion involve sequestration phenomena, active transport, and speciation. Some
genes encoding systems for active transport of toxic metals
are known (Silver and Phung 2005) as it is the case of
cadmium (cad), copper (cop), lead (pbr), or arsenic (ars).
Other resistance systems, specific or not, involve methylation processes that produce volatile forms of tin, arsenic,
selenium, lead, etc. (Thayer and Brinkman 1982). These
volatile forms must be considered as losses from the ecosystem under consideration and therefore induce a reduction in
the toxicity of these metals.
These resistance systems have been described in natural
environment and are rarely used as bioremediation
techniques. Similarly, the microbial reduction of mercury
through the Mer detoxification system (Box 14.15) implies a
16 Applied Microbial Ecology and Bioremediation
745