amino acids, proteins, and exopolysaccharides which facilitate metal solubilization.
Heterotrophs contribute to metal leaching of non-sulfur containing solids and sustain
higher pH. The use of fungi has a drawback since the metals would be entrapped in
the mycelial network or adsorbed in the cells [7]. Fungi such as Aspergillus
sp. [24, 34], Penicillium sp. [29, 35], etc. have been reported for the bioleaching
of metals. The bacterial genera majorly exploited for the bioleaching of metals from
e-waste are cyanogenic and include Pseudomonas sp. [36–38], Chromobacterium
violaceum [39–41], and Bacillus sp. [12, 42].
3.1.2 Inoculum Size and Growth
The inoculum size has a vital role in the bioleaching of metals from e-waste by
influencing the substrate utilization rate and consequently the metabolites released.
The increase in size of inoculum was found to increase the production of organic
acids to leach metals [24], increase bioleaching of heavy metal [43], and enhance
copper recovery [25] from e-waste. Inoculum size beyond a certain limit results in
turbidity and clogging in the bioreactor, which limits the contact between the solids
and cells and thus the time provided for bioleaching would not be sufficient and
incomplete bioleaching of metals would result [25]. The advantage of optimizing
inoculum size is to shorten the lag phase and obtain higher growth rate while
preventing clogging. The growth rate indirectly triggers the process, analogous to
natural biogeochemical pathways at a faster rate and hence bioleaching [44]. Pham
and Ting [45] have found that, in the presence of e-waste, Pseudomonas fluorescens
has higher cyanide production as a result of higher growth rate to achieve better
bioleaching efficiency [45].
3.1.3 Omics of Bioleaching Microorganisms
A detailed study on the genome, proteome, and the metabolome of the organism
signifies its use in the bioleaching of metals from e-waste. At present the omics
approach has revealed the population dynamics in the natural leaching environments
and laboratory scale operations using a consortium. Next-generation sequencing
technologies will help improve the design and operational control of bioleaching
with respect to the organism involved in the process [46]. Additional information on
the genotype of the cell including the genomic islands (GI) of metal resistance will
give insights into the ability of the organism to adapt to the metal-containing
environment, understand the cryptic passive or active regulatory networks for
enzyme production, etc. One such example is the discovery of ATPases of
Sulfolobus solfataricus for copper resistance [47]. Few attempts have been made
to identify specific genes that express for bioleaching of metals, by knockout studies.
Further, the genes are subjected to heterologous expression for bioleaching, but the
limitation with releasing the engineered strains into the natural environment is
inevitable, and hence a natural selection is endorsed [13]. The gene expression is
32
M. Minimol et al.
Heterotrophs contribute to metal leaching of non-sulfur containing solids and sustain
higher pH. The use of fungi has a drawback since the metals would be entrapped in
the mycelial network or adsorbed in the cells [7]. Fungi such as Aspergillus
sp. [24, 34], Penicillium sp. [29, 35], etc. have been reported for the bioleaching
of metals. The bacterial genera majorly exploited for the bioleaching of metals from
e-waste are cyanogenic and include Pseudomonas sp. [36–38], Chromobacterium
violaceum [39–41], and Bacillus sp. [12, 42].
3.1.2 Inoculum Size and Growth
The inoculum size has a vital role in the bioleaching of metals from e-waste by
influencing the substrate utilization rate and consequently the metabolites released.
The increase in size of inoculum was found to increase the production of organic
acids to leach metals [24], increase bioleaching of heavy metal [43], and enhance
copper recovery [25] from e-waste. Inoculum size beyond a certain limit results in
turbidity and clogging in the bioreactor, which limits the contact between the solids
and cells and thus the time provided for bioleaching would not be sufficient and
incomplete bioleaching of metals would result [25]. The advantage of optimizing
inoculum size is to shorten the lag phase and obtain higher growth rate while
preventing clogging. The growth rate indirectly triggers the process, analogous to
natural biogeochemical pathways at a faster rate and hence bioleaching [44]. Pham
and Ting [45] have found that, in the presence of e-waste, Pseudomonas fluorescens
has higher cyanide production as a result of higher growth rate to achieve better
bioleaching efficiency [45].
3.1.3 Omics of Bioleaching Microorganisms
A detailed study on the genome, proteome, and the metabolome of the organism
signifies its use in the bioleaching of metals from e-waste. At present the omics
approach has revealed the population dynamics in the natural leaching environments
and laboratory scale operations using a consortium. Next-generation sequencing
technologies will help improve the design and operational control of bioleaching
with respect to the organism involved in the process [46]. Additional information on
the genotype of the cell including the genomic islands (GI) of metal resistance will
give insights into the ability of the organism to adapt to the metal-containing
environment, understand the cryptic passive or active regulatory networks for
enzyme production, etc. One such example is the discovery of ATPases of
Sulfolobus solfataricus for copper resistance [47]. Few attempts have been made
to identify specific genes that express for bioleaching of metals, by knockout studies.
Further, the genes are subjected to heterologous expression for bioleaching, but the
limitation with releasing the engineered strains into the natural environment is
inevitable, and hence a natural selection is endorsed [13]. The gene expression is
32
M. Minimol et al.