Abstract
Organic phosphates (OP) account for approximately 30–90% of total soil P. However, it is too stable to be utilized by plants as available P source. Aspergillus niger (A. niger) has considerable ability to secret phytase to decompose OP. Meanwhile, mineralization of lead (Pb) is efficient to achieve its remediation. This study hence investigated Pb immobilization by A. niger assisted decomposition of OP under variable acidic environments. A. niger can survive in the acidic environment as low as pH = 1.5. However, alternation of environmental pH within 3.5–6.5 significantly changed fungal phytase secretion. In particular, weakly acidic stimulation (pH of ~5.5) increased phytase activity secreted by A. niger to 0.075 µmol/min/mL, hence elevating P release to a maximal concentration of ~20 mg/L. After Pb addition, ATR-IR and TEM results demonstrated the formation of abundant chloropyromorphite [Pb5(PO4)3Cl] mineral on the surface of mycelium at pH = 5.5. Anglesite, with a higher solubility than pyromorphite, was precipitated massively in other treatments with pH lower or higher than 5.5. This study elucidated the great potential of applying OP for Pb immobilization in contaminated water.
Highlights
Lead (Pb) is the most widespread toxic metal in the world
Many metabolic processes of the organism are controlled by enzymes, whose catalytic responses could depend on pH39
In the EI treatments, A. niger had the biomass of 29 ± 2 mg at pH = 1.5, which was significantly lower than those of other treatments (Fig. 1B)
Summary
Lead (Pb) is the most widespread toxic metal in the world. It presents a more serious environmental and health toxicity than any other element[1,2,3,4,5]. Phytase is a generic term used to describe phosphohydrolase enzymes, which can promote the sequential release of inorganic orthophosphates from phytic acid and phytates[22]. It is widespread in nature, secreted from plants, animals or microorganisms[23]. Many phosphate-solubilizing microbes (PSM) can solubilize inorganic phosphate via secreting organic acids[25,26] as well as releasing P from organic phosphate sources via phosphatase catalysis[27,28]. PSM can be widely applied to increase P release from organic/inorganic phosphates[29]. Phosphate-solubilizing fungi (PSF) constitute 0.1–0.5% of total fungal populations in soil[30] and can produce more organic acids than bacteria[31]. The study regarding Pb immobilization assisted by fungal decomposition of OP under variable acidic environments is still poorly understood
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