Microorganisms influence the chemical state – speciation and hence the mobility of the metals by numerous and complex mechanisms ranging from direct processes like metal transformation and intracellular uptake, to more indirect processes via production of substances that render the metals more or less mobile.
Principal ways in which microorganisms can influence metal mobility. M=Metal species, (Ledin and Pedersen, 1996)
The contact of the soluble metal species with the microbial cells results in in-situ transformations of the targeted species. As a consequence, the metals are immobilised by the microbial biomass. Soluble metal and metalloid species may be sequestered from water streams via their interaction with microbial cells by active i.e. metabolic – energy dependent and passive i.e. non metabolic – energy independent processes.
Metabolism mediated immobilisation of metal species by active microbial cells includes different mechanisms such as bioprecipitation and biological reduction / oxidation.
Passive metal uptake by microbial cells is described by the general term biosorption and includes different mechanisms of physico-chemical interaction between the microbial cell biopolymers and the metal species such as complexation, chelation, ion exchange and precipitation.
Both living-metabolizing and non-metabolizing microbial cells, may be used in technology development for:
- The removal of metals from aqueous industrial effluents,
- Metal recovery from industrial process streams,
- Bioremediationof contaminated surface waters and groundwater.
Biosorption can be defined as the selective sequestering of metal soluble species that result in the immobilization of the metals by microbial cells. It refers to physicochemical mechanisms of inactive (i.e. non-metabolic) metal uptake by microbial biomass. Metal sequestering by different parts of the cell can occur via various processes .
Immobilization may be the result of more than one mechanism, for example, metal complexation may be followed by metal reduction or metal precipitation.
Metabolically active and inactive cells behave in different ways. Thus inactive microbial cells can only immobilize metals by biosorption, whereas active microbial cells may immobilize soluble metal species both by biosorption and by other mechanisms that are part of and/or are due to the microbial metabolism. The cell wall is usually the first cellular structure in contact with the soluble species of the metals if we exclude the possibility of metal species interactions and retention by extracellular excretions produced by some microbial cells.
The functional groups available in the biopolymers, constituents of the cell wall and the other parts of the cell, have a significant potential for metal binding. These biopolymers, constituents of the cell wall and the other parts of the cell possess functional groups that have a significant potential for metal binding.Furthermore, intracellular biopolymers such as proteins and DNA may also contribute to metal immobilisation. In many cases, extracellular polymeric substances such as Exo-PolySaccharides (EPS) that are closely related to the cell membrane can also participate in metal immobilisation.
How bacteria influence speciation of mercury in the environment
Mercury is a contaminant of global concern, as bioaccumulation of methylmercury poses significant risk to aquatic ecosystems and human health. Controlling the transport of mercury in the environment is challenging due to deposition of airborne mercury at locations far from point sources. Mobility of mercury is strongly dependent on its chemical form, with the elemental mercury being volatile and hence mobile in the environment, while oxidized forms are much less mobile (though more toxic). This study, led by Dr. Mishra along with follow researchers of molecular environmental science group at Argonne National Laboratory, has provided improved understanding of the role of bacteria in controlling the chemical form of mercury in subsurface environments. Using X-ray absorption spectroscopy experiments at the Advanced Photon Source to study the sorption of oxidized HgII to Bacillus subtilis, a gram positive soil bacterium, they determined that HgII sorbs to bacterial cells via high and low affinity sulfhydryl and carboxyl binding groups on the cell surfaces. Additionally, they found that HgII that is sorbed to cells via high affinity sulfhydryl groups remains unavailable for reduction by magnetite, a reactive iron-containing mineral often found in sediments, even after two months of reaction time. This is in sharp contrast to their observation of complete reduction of HgII to Hg0 within two hours when HgII is sorbed to cells via the lower affinity carboxyl groups. Since binding of HgII to high-affinity sulfhydryl groups on bacteria could have important implications for the overall mobility of Hg in subsurface environments, these results identify a mechanism by which mercury might be immobilized in the environment.