Removal mechanism of Pb(II) by Penicillium polonicum: immobilization, adsorption, and bioaccumulation

Currently, lead (Pb) has become a severe environmental pollutant and fungi hold a promising potential for the remediation of Pb-containing wastewater. The present study showed that Penicillium polonicum was able to tolerate 4 mmol/L Pb(II), and remove 90.3% of them in 12 days through three mechanisms: extracellular immobilization, cell wall adsorption, and intracellular bioaccumulation. In this paper. the three mechanisms were studied by Raman, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The results indicated that Pb(II) was immobilized as lead oxalate outside the fungal cell, bound with phosphate, nitro, halide, hydroxyl, amino, and carboxyl groups on the cell wall, precipitated as pyromorphite [Pb5(PO4)3Cl] on the cell wall, and reduced to Pb(0) inside the cell. These combined results provide a basis for additionally understanding the mechanisms of Pb(II) removal by P. polonicum and developing remediation strategies using this fungus for lead-polluted water.

Analysis of organic acid secretion. It has been reported that fungi can secrete LMWOAs to alleviate heavy metal toxicity 16,25 . To evaluate extracellular organic acid content, oxalic and citric acids were measured (Fig. 2). Oxalic acid secretion by P. polonicum with Pb(II) treatment was significantly greater than without Pb(II) treatment from day 2 to day 4. Thereafter, the concentration of oxalic acid rapidly decreased to a plateau (Fig. 2a). In contrast, citric acid content varied somewhat under Pb(II) stress compared with the C1 group (Fig. 2b). Specifically, changes in citric acid secretion were moderate when fungi were exposed to Pb(II). The differences were likely due to inhibition of fungal correlative enzymatic activity by high Pb(II) concentrations 26 . Furthermore, this difference also confirmed that when P. polonicum is exposed to Pb(II) ions, a protective response was mediated through oxalic acid secretion rather than through nonspecific LMWOA secretion. Dry weight and Pb(II) uptake capacity of P. polonicum. Several heavy metals are toxic to cells and may slow down the growth of fungi 27 . The effect of Pb(II) ions on fungal growth was analyzed by measuring the dry weight of P. polonicum and calculating the uptake capacity. The results (Fig. 3a) show that the biomass of the 4 mM Pb group was higher than that of the C1 group during the early stages, indicating that Pb(II) ions may slightly stimulate the growth of P. polonicum. A stimulatory effect of high concentrations of heavy metals on fungal growth has been reported by Anahid 28 . After this initial stimulus, the biomass of the Ex group was constantly lower than that of the C1 group. The maximum dry biomass obtained in the presence of 4 mM Pb(II) was 2.43 g on day 10, which was lower than the dry biomass of the C1 group (2.94 g) on day 8. When exposed to 4 mM Pb(II), P. polonicum displays significant tolerance toward Pb(II) toxicity. To some degree, the high concentration of Pb(II) did cause some effects, as evidenced by the slower growth of the fungus in culture. The maximum uptake by P. polonicum was 78.95 mg/g on day 12, which was slightly higher than the uptake on days 6, 8, and 10, and much higher than on day 2 (Fig. 3b). This suggests that peak uptake capacity occurs on day 6 and subsequently varies slightly. Further, there was a positive liner relationship (R 2 = 0.91) between biomass and uptake capacity (Fig. 3c), indicating that uptake capacity was affected by growth, and also influenced by other factors such as metabolic activity and environmental pH/Eh. Figure 1. Removal of lead ions by P. polonicum 4 mM Pb represents liquid medium with 4 mmol/L Pb(II) and P. polonicum; C1 represents liquid medium with P. polonicum, but without Pb(II); C2 represents liquid medium with 4 mmol/L Pb(II), but without P. polonicum. Solid lines represent Pb(II) concentration remaining in liquid medium, and dashed lines represent Pb(II) removal rate. Fungi were incubated in liquid media under 30 °C and 180 rpm for 12 days. The samples for Pb(II) concentration analysis were collected at day 2, 4, 6, 8, 10, 12, respectively. Error bars represent standard deviation.
Analysis of extracellular pb-containing minerals formed by P. polonicum. The morphology of extracellular Pb-containing minerals formed by P. polonicum was observed under a scanning electron microscope ( Fig. 5a-d), and energy spectra were determined (Fig. 5e,f). The size of Pb-containing minerals varied significantly. Some particles were more than 87.5 μm. Others were less than 5 μm. The morphology of minerals also varied. Some were short columns (Fig. 5c), other were prismatic needles (Fig. 5d). All particles contained a large amount of C, O, and Pb (Fig. 5a,e). Mycelium, after Pb(II) treatment, also contained a considerable amount of Pb (Fig. 5b,f), indicating that the removal of Pb(II) was not only through the formation of extracellular Pb-containing minerals but also through adsorption on cell walls or transfer into cells 18,21, 22 .

Analysis of a fourier transform infrared spectroscopy (ftiR) study.
To determine the impacts of Pb(II) on cell wall characteristics, the functional groups on P. polonicum cell walls with and without Pb(II) treatment were investigated using FTIR analysis. The changes in vibrational frequencies detected via FTIR confirmed the involvement of the cell wall in Pb(II) removal 19 .
FTIR spectra (Fig. 7) from P. polonicum show primary functional groups from untreated fungal cells: hydroxyl, amino, methyl/methylene, carboxyl, phosphoryl, and nitro 1,14,19,31 . The strong peak at 3269 cm −1 is characteristic of O-H and N-H stretching vibrations 14 . The peak at 2922 cm −1 is C-H asymmetric stretching, and that at 1643 cm −1 is C=O stretching and N-H deformation (amide II region) 19 . The carboxyl group appeared at 1544 and 1403 cm −1 32 , and the phosphate group appeared at 1236 cm −1 19 . The peaks at 1030 cm −1 belonged to the C-C, C=C, C-O-C, and C-O-P groups of polysaccharides 19 , and the peak at 530 cm −1 was assigned to the nitro group 19 .
Because lead atoms are unlikely to be directly attached to carbon atoms, the peaks at 2922, 1643, and 1030 cm −1 showed little difference with and without Pb(II) treatment. However, some peaks occurred with a slight shift (from 3269 to 3273 cm −1 , 1643 to 1644 cm −1 , 1544 to 1541 cm −1 , and 1403 to 1400 cm −1 ), indicating the intervention of hydroxyl, amino, and carboxyl groups during Pb(II) adsorption. Further, significant shifts in peaks (from 1236 to 1258 cm −1 and from 530 to 541 cm −1 ) after Pb(II)treatment might imply that the phosphate and nitro groups were involved in the binding of Pb. The emergence of new peaks at 1147 and 572 cm −1 suggests that the phosphate and halide groups may play an important role in Pb(II) removal at the cell wall surface 19,33 . In conclusion, negatively charged functional groups (e.g., hydroxyl, amino, carboxyl, phosphate, nitro, and halide groups) provide the electrostatic force for binding positively charged Pb(II) to the cell surface 34 .

Identification of Pb-containing minerals formed on cell walls and inside fungal cells. It is vital
to know lead speciation, not only to predict its mobility and bioavailability, but also to evaluate its risk to living organism 35 . Therefore, Transmission electron microscopy (TEM) investigations were used to identify mineral phases. The morphology and element composition of Pb-containing minerals were explored via TEM images and energy-dispersive spectroscopy (EDS) (Fig. 8). Moreover, crystal phases, interplanar spacing, and interplanar angles were obtained using high-resolution images and Fourier transformation analysis (Figs. 9 and 10).
According to the TEM images ( Fig. 8a,b), most minerals that formed on cell walls were fine pillared crystallized, about 90 × 20 nm, whereas a few particles were amorphous. The EDS pattern showed high concentrations of Pb, with Os, Cu, C, O, P, and Cl (Fig. 8c). Os and Cu result from osmic acid and the Cu TEM grid for fixing and supporting the sample, respectively. C and O may to some extent reflect TEM carbon film and organics. Thus, minerals are distinct Pb-containing grains associated with P and Cl. High-resolution TEM analysis was performed for additional speciation. The interplanar spacing of particles was measured and used for statistical analysis. As shown in Fig [35][36][37][38] .
After the removal of Pb(II) ions by P. polonicum, Pb-containing minerals were also present intracellularly (Fig. 8a,b). Through EDS analysis, intracellular minerals contained large amounts of C, Pb, O, S, Cu, and Os ( (Fig. 8d), indicating  www.nature.com/scientificreports www.nature.com/scientificreports/ that Pb(II) ions were transferred into the cells and formed Pb-containing minerals. After excluding the elements present due to sample preparation, Pb and S showed a good correlation. Further, Fourier transformation and high-resolution image analysis (Fig. 10a,b) indicated that the mineral particles formed inside cells develop a monocrystalline structure. The interplanar spacing values were 0.298, 0.301, and 0.301 nm, and the interplanar angles were 60.08° and 60.43° (Fig. 10c). Through comparisons with standard XRD patterns (Fig. 10d, JCPDS No. 044-0872), intracellular Pb-containing mineral was identified as elemental lead with a hexagonal structure. The ability of P. polonicum to transfer Pb(II) ions into cells and reduce it to a zero valence state was previously unknown. The discovery of this ability will supplement our understanding of the underlying mechanisms of fungal tolerance to and accumulation of Pb(II).

Discussion
Lead(Pb) is of particular concern and can be locally present in enormous quantities 13 . Filamentous fungi are regarded as versatile bioremediation agents for Pb (II) and have been widely studied. Recently, investigations of the underlying mechanisms of the fungal bioremediation and biorecovery of Pb(II) have been widely reported 19,22,39 . In this study, extracellular immobilization, cell wall adsorption, and intracellular bioaccumulation of Pb(II) were investigated.
Lately, it has been widely reported that fungi can secrete oxalic acid and citric acid to precipitate Pb(II) ions and reduce Pb(II) toxicity 14, 25,40 . The reactions of Pb(II) ions with oxalic acid to produce lead oxalate precipitates are as shown in "eqs. (1-4)" 41 :  (K is the equilibrium constant of the chemical reaction. The larger the K value, the easier the reaction; K sp is the solubility product constant. The K sp value of insoluble matter is less than 10 −5 .) When a fungus is exposed to a high Pb(II) environment, for the sake of decreasing the toxicity of heavy metal, Pb(II) stress responses are triggered. Specifically, oxalic acid secretion by P. polonicum is enhanced and rapidly peaks. The resulting precipitation of lead oxalate reduces the toxicity and allows continued fungal growth. However, high Pb(II) concentrations over a long period still cause cell damage via thiol binding andprotein denaturation, displacement of essential metals involved in biological reactions, or a secondary effect of oxidative 42,43 . Therefore, after day 6, fungi cultured in the absence of Pb(II) grew better than Pb-exposed organisms. Toxicity was assessed via measurement of dry weight and examination of surface morphology.
The drop in oxalic acid content may be due to decreased secretion, chelation of oxalic acid with Pb(II), with subsequent precipitation of lead oxalate, or degradation of oxalate by oxalate decarboxylase 44 . In fact, lead oxalate has been identified in fungal mycelia under Pb(II) stress by XRD and Raman analysis. Similar phenomenon was also observed by Jarosz-Wilkolazka and Gadd 45 and Machuca 46 , further confirming the importance of extracellular detoxification with oxalic acid 15 . Therefore, the formation of lead oxalate can be a detoxification pathway  www.nature.com/scientificreports www.nature.com/scientificreports/ used by P. polonicum for the removal of Pb. Unfortunately, the proportion of lead removal by extracellular oxalate immobilization is difficult to determine.
When extracellular lead oxalate immobilization is insufficient to remove abundant Pb(II), cell wall adsorption via ion exchange, hydrolytic adsorption, functional group binding, and surface precipitation are also Pb(II) removal mechanisms 18,21 . As shown in the FTIR spectra, phosphate, nitro, and halide groups play an important role in the binding of Pb(II). Hydroxyl, amino, and carboxyl groups could also be involved. Similar phenomena were observed by Gola et al. 47 and Wang et al. 14 . Moreover, the Ca(II) and Mg(II) concentrations in culture medium were measured before and after Pb(II) treatment. The amount of Ca(II) consumed by P. polonicum was 3 and 10.9 mg/g with and without Pb(II) treatment, respectively, and the amount of Mg(II) was 5.4 a nd 9.2 mg/g. Specifically, ion exchange between Pb (II) and Ca (II), Mg (II) occurred, because phosphate, hydroxyl, and carboxyl groups were more likely to bind Pb(II) instead of Ca(II) and Mg(II). This finding is consistent with the results reported by Wu and Li 48 .
In addition, TEM images, EDS patterns, and high-resolution TEM images indicated that Pb(II) was observed as Pb 5 (PO 4 ) 3 Cl (PDF: 01-089-4339) on the cell wall. According to the FTIR pattern, phosphate groups were involved in Pb(II) binding. Therefore, the formation of Pb 5 (PO 4 ) 3 Cl may start with the binding of Pb(II) by a phosphate group that may serve as a stable nucleation site for the aggregation of Cl − and Pb 2+ ions. In this nucleation process, stable Pb-containing minerals are formed on the cell wall. Another mechanism for the formation of Pb 5 (PO 4 ) 3 Cl could exist. Under aerobic conditions, fungi continuously produce polyphosphate, used as an energy source for growth and metabolism. Under anaerobic conditions, polyphosphate is degraded to produce ATP and large amounts of phosphate. In the process of phosphate efflux, excess phosphate combines with heavy metal and chloride ion to produce chlorophosphate minerals that precipitate on cell walls. The combined results suggest that adsorption and mineral precipitation processes on cell walls may be another important pathway for the removal of Pb(II) by P. polonicum.
A number of studies have confirmed that many fungi can accumulate considerable heavy metals in cells, without destroying the integrity of cells [20][21][22]49 . IntracellularPb(II) transport occurs by intracellular metal binding or sequestration sites with higher affinity than binding sites at the cell surface, providing a driving force for the intracellular uptake 21, 22 . Under these conditions, fungi have developed a detoxification mechanism within the cell. TEM images, EDS patterns, and high-resolution TEM images confirmed that lead(pb) exists in elemental www.nature.com/scientificreports www.nature.com/scientificreports/  www.nature.com/scientificreports www.nature.com/scientificreports/ form inside cells. Several reports propose that microbes can reduce U(VI) to U(IV) 50 , Au(III) to Au(0) 51 , Hg(II) to H(0) 52 and Ag(I) to Ag(0) 53 . To the best of our knowledge, the fungal reduction of Pb(II) to Pb(0) has not been reported previously. When metal ions enter cells, microbes may synthesize a variety of metal binding peptides, enzymes, and proteins, such as GSH, metallothionein (MT), and oxidoreductase, to regulate metal ion homeostasis and decrease toxicity [54][55][56] . As we all know, glutathione(GSH) and metallothionein(MT) have a large amount of sulfhydryl groups, which correspond to abundant S shown in the EDS patterns of intracellular minerals. Therefore, it can be deduced that, when Pb(II) enters the cell, GSH and MT may bind Pb(II) via sulfhydryl groups, and then deliver the Pb(II) into reductase vicinity. Some Pb(II) may be reduced to Pb (0), and the remainder is sequestrated as Pb-bearing organic chelates. These combined results suggest that fungi may use both biosynthesis and metabolic processes to detoxify via Pb(II) bioaccumulation.

conclusion
Mechanisms for the removal of Pb(II) by P. polonicum were systematically studied under high Pb(II) concentration stress. The strain could tolerate 4 mM Pb(II) with slight abnormity, and remove 90.3% of them in 12 days through three ways, namely, extracellular immobilization, cell wall adsorption, and intracellular bioaccumulation. P. polonicum secretes oxalic acid extracellularly to immobilize Pb(II) as form of lead oxalate crystals and thus reduce free Pb(II) and associated toxicity. The cell wall adsorption via ion exchange, functional group binding, and surface precipitation are also Pb(II) removal mechanisms. In order to prevent strain cell from injury, the cell wall binds Pb(II) to phosphate, nitro, halide, hydroxyl, amino, and carboxyl groups, conducts ion exchange between Pb (II) and Ca (II), Mg (II), and precipitates Pb(II) as form of pyromorphite on it. In addition, once Pb(II) enters the cell, intracellular organic chelates, such as glutathione(GSH) and metallothionein(MT), and reductase can detoxify Pb(II) via chelation or reduction Pb(II) to Pb(0), which plays an important role in the process of accumulating and tolerating Pb(II). These results help clarify mechanisms for Pb(II) removal by fungi and broaden the spectrum of fungal biogenic minerals.

Methods and materials
Strain, medium and chemical reagents. The fungus P. polonicum was obtained from the Microbial Geochemical Laboratory of Peking University and was verified to withstand Pb(II) up to 12 mmol/L (2486.4 mg/L) and to have a high removal efficiency for Pb(II). In this study, all incubations were carried out at 30 °C ± 0.5 °C. The medium used contained (per liter) 2 g NaCl, 0.5 g NaNO 3 , 5 × 10 −3 g MgSO 4 ·7H 2 O, 0.1 g NH 4 Cl, 1 g beef extract, 3 g tryptone, 0.5 g yeast extract, 3 g glucose, and 1 L deionized water 13 . The medium was adjusted to pH 5.0 using 0.1 mol/L HNO 3 13, 23,57 . To test the pb removal rate and efficiency of P. polonicum, after sterilization with a High Pressure Steam Autoclave (Panasonic MLS-3751L, Shanghai, China), 500 ml Conical flasks were placed on a clean bench containing an ultraviolet lamp for 3 hours, and then, 250 ml medium (with or without Pb(II)) solution were added into flasks. All reagents used in the experiment were of analytical grade and were purchased from the Beijing Chemical Plant (Sigma-Aldrich-Fluka, Yuanye biotechnology corporation, Shanghai, China). All experiments were carried out in an LRH-150 incubator (Shanghai Qixin Scientific Instruments, Shanghai, China).

Lead ion concentration and fungal biomass determination.
In order to determine lead ion concentration, 3 mL of culture solution was taken from Conical flasks by a 5 ml pipette every 2 days. And then the culture solution was filtered by 0.22 μm filter membrane. The filter solution was acidified by 10% nitric acid, and inductively coupled plasma atomic emission spectroscopy (XSEKIES2, Thermo Fisher Scientific, USA) was used to assay the Pb(II) concentration 58 . Measurements were performed in triplicate. All the relative standard deviations for replicates were <5%. The amount of Pb(II) uptake (q, mg/g) was calculated using the "eq. (5)" 10,59 : f (where q (mg/g) is milligrams of Pb(II) uptake per gram biomass, C i (mg/L) is the initial Pb(II) concentration, C f (mg/L) is the final Pb(II) concentration, m (g) is the amount of dry biomass, and V (L) is the volume of the medium.) All mycelia in flasks were collected at days 2, 4, 6, 8, 10, and 12 through 0.22 μm filter membrane, respectively. Subsequently, mycelia were washed with deionized water three times, and were dried in an oven (Shanghai Shinbae industrial corporation, Shanghai, China) to determine dry biomass (45 °C, 24 h).
Surface morphology observation. The surface morphologies of P. polonicum with and without Pb(II) treatment were examined with field emission environment scanning electron microscopy (ESEM) (FEI Quanta 200, FEI, USA). After Pb(II) treatment, the morphologies and compositions of extracellular Pb-containing minerals formed by P. polonicum were observed and determined by ESEM and EDS (Oxford, UK) at 15 kV and 120 Pa.