# Bioremediation Methods for the Recovery of Lead-Contaminated Soils: A Review
## 4 Bioremediation via Fungi and Bacteria
### Bioremediation by Fungi  
As stated in Section 3.1, lead bioavailability is strictly connected to its dangerousness towards living organisms. Microorganisms existing in the soil are deeply involved in lead speciation through processes as varied as biosorption, intracellular accumulation, biomineralization, and chelation with self-produced organic acids (Figure 2). All these pathways can be exploited to increase (or decrease) the bioavailability of lead through its mobilization (or immobilization) [68]. [PERSON] et al. [69] identified two fungi, _Paecilomyces javanicus_ and _Metathizium anisopliae_, isolated from a former lead mining area in Scotland and capable of transforming metallic lead into chloropyromorphite. They incubated lead shots in the presence and absence of fungi and verified that the metabolites produced by the fungi, in particular organic acids, contributed to the release of mobile lead species, which subsequently precipitated as pyromorphite. Energy-dispersive X-ray analysis (EDXA) and X-ray powder diffraction analysis (XRPD) confirmed that, in soils treated with fungi, phosphorus and chlorine were also present in addition to the main elements found in abiotic conditions (namely lead, carbon, and oxygen); furthermore, X-ray mapping allowed for the identification of the position of phosphorus and chlorine and confirmation of the formation of chloropyromorphite as the main product. [PERSON] et al. [68] studied biomineralization (the process by which living forms influence the precipitation of mineral materials [70]) of lead phosphate by fungal strains isolated from decaying woods. In their work, they assessed the tolerance of three different fungi, _Aserigillum niger_, _Penicillium chrysogenum_, and _Trichoderma viride_, to Pb(II) by testing the minimum inhibitory concentration (MIC, lowest concentration of metal in which no visible fungal growth occurs). _A. niger_ and _P. chrysogenum_ showed resistance to high concentrations of lead with MICs \(>8\) mM. The authors also evaluated the lead phosphate formation after incubation of _P. chrysogenum_ with lead shot. Environmental scanning electron microscopy (ESEM) showed the precipitation of lead phosphate mineral, while the analysis by high resolution transmission electron microscopy (HRTEM) confirmed the deposition of lead mineral phases on fungi cell walls. This may be related to the enhancement of the biosorption process by the fungus in response to the stress conditions caused by the presence of lead. Thirty-six microorganisms (fungi and yeasts) were isolated from contaminated soil in Morocco [71]. Among them, _Aserigillus_ and _Penicillium_ strains proved to be the most tolerant of heavy metals, capable of growing with high concentrations of metals (MIC between 20 and 25 mM for Pb), and as such they are interesting candidates for the bioremediation of lead-contaminated soils. The behaviors of _Aserigillus niger_, _Penicillium biliaie_, and a _Penicillium_ sp. in lead-polluted soils characterized by scarce or rich nutrient content were studied by [PERSON] et al. [72]. In particular, fungal response was studied in terms of production of exudates such as low molecular weight organic acids (LMWOAs). It is well known that in the presence of heavy metals fungi produce organic acids  
Figure 2: Schematic representation of the principal mechanisms involved in bioremediation of lead by fungi and bacteria. with complex capacities, so inducing the immobilization of metals through their bonds to fungal cell walls and the formation of soluble metal complexes. During the study [72], the latter phenomenon was investigated by evaluating the possible use of chelating agents produced by the fungal strains to mobilize the lead and subsequently extract it to operate a bioremediation on the soil. The organic acids exuded from fungi were mainly oxal acid and citric acid; the maximum concentration of LMWOAs was in the range of 60 to 80 mM with a corresponding decrease of pH to values between 2 and 4 mM for _A. niger_ and 15 and 20 mM for _P.