A new insight into lead (II) tolerance of environmental fungi based on a study of Aspergillus niger and Penicillium oxalicum

Abstract

Summary Environmental microorganisms have been widely applied in heavy metal remediation. This study explored the mechanisms of lead tolerance of two typical filamentous fungi, Aspergillus niger and Penicillium oxalicum. It is shown that the mechanisms of reducing Pb toxicity by these two fungi have three major pathways. The secreted oxalic acid can react with Pb (II) to form insoluble Pb minerals, primarily lead oxalate. Then, the enhanced biosorption via forming new border of cell wall prevents the transportation of Pb (II) into hypha. In addition, the fungal activity could be maintained even at high Pb concentration due to the intracellular accumulation. It was confirmed that A. niger has the higher Pb tolerance (up to 1500 mg l−1 Pb level) compared with P. oxalicum (up to 1000 mg l−1). Meanwhile, Pb levels below 1000 mg l−1 partially stimulate the bioactivity of A. niger, which was confirmed by its elevated respiration (from 53 to 63 mg C l−1 medium h−1). This subsequently enhanced microbial functions of A. niger to resist Pb toxicity. A better understanding of Pb tolerance of these two fungi sheds a bright future of applying them to remediate lead‐contaminated environments.

Tracking the fungus‐assisted biocorrosion of lead metal by Raman imaging and scanning electron microscopy technique

“…This is analogous to the morphology of linear hyphae. It hence implied evident microbial regulation on biomineralization …”

“…niger , which is substantiated by its respiration. Some fungal functions of corroding Pb metal, for example, the production of oxalic acid, would hence be further enhanced …”

“…Furthermore, its high tolerance to Pb 2+ toxicity allows that A . niger maintains enough bioactivity under Pb stress . However, the growth of A .…”

Contrasting the Pb (II) and Cd (II) tolerance of Enterobacter sp. via its cellular stress responses

“…The specific biochemical mechanisms contain proton exchange, chelation and microprecipitation of metals (Comte et al ., ; Fang et al ., ). The extracellular zone is the first ‘defence line’ to resist metal toxicity (Tian et al ., ). Extracellular Pb sequestration via polysaccharides and proteins from extracellular polymeric substances (EPS) is tightly related to the resistance ability of microorganisms.…”

“…In contrast, bacteria have more Cd efflux pathways and would like to accumulate Cd in their cytoplasm, especially for the unaccommodated cells (Belliveau et al ., ; Nies, ). Furthermore, these extra‐ and intracellular responses to heavy metals usually accompany with morphological alternations of microbial membrane (Tian et al ., ). The cellular differences of tolerant capacity of bacteria towards Pb and Cd hence require great attention.…”