Recovery of platinum(0) through the reduction of platinum ions by hydrogenase-displaying yeast

Biological technologies for recycling rare metals, which are essential for high-tech products, have attracted much attention because they could prove to be more environmentally friendly and energy-saving than other methods. We have developed biological recycling technologies by cell surface engineering for the selective recovery of toxic heavy metal ions and rare metal ions from aqueous wastes. In this study, we aimed to construct a unique biological technique to recover rare metals ‘in solid’ form by reducing rare metal ions, leading to a practical next-generation recovery system. Sulfate-reducing bacteria (SRB) can reduce Pt(II) to Pt(0), and hydrogenases of SRB contribute to the reduction. Therefore, we constructed yeasts displaying their hydrogenases on the ‘cell membrane’, and reduction experiments were performed under anaerobic conditions without any electron donors. As a result, hydrogenase-displaying yeasts produced black precipitates in PtCl4 2− solution. Based on X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) observations, the constructed yeasts were found to successfully produce the precipitates of Pt(0) through the reduction of Pt(II). Interestingly, the precipitates of Pt(0) were formed as nanoparticles, suitable for industrial usage.


Introduction
Rare metals are essential for high-tech products such as cell phones and automobiles, leading to ever-growing demands today. However, a stable supply of rare metals is difficult on a global scale because rare metals are expensive and localized, and their abundances are extremely low. Therefore, technologies for recovering rare metals from waste products, industrial wastewater, and sea are required.
Recently, various physical, chemical, and biological methods have been studied to prepare metallic forms for the recycling of rare metals (Du et al. 2008;Fredrickson et al. 2008;Cueto et al. 2011). However, physical and chemical methods have some disadvantages. For example, they involve toxic solvents and consume high energy. On the other hand, biological methods can produce metallic forms in a more environmentally friendly and energy-saving manner than other methods, because they only require microbes as the basic constituent. Therefore, biological methods have currently attracted much attention, and various microbes that produce metallic forms have been screened on the earth (Klaus-Joerger et al. 2001;Konishi et al. 2006).
Platinum, especially, is a rare precious metal used in catalytic converters or fuel cells. However, there are growing concerns about platinum exhaustion by 2050 because its crustal abundance is extremely low. Additionally, the Republic of South Africa accounts for about 70 % of platinum production. Therefore, developing efficient technologies to accumulate and recycle platinum is desired to ensure a stable and continuous supply of platinum.
It has been reported that sulfate-reducing bacteria (SRB) contribute to the bioremediation of toxic metal ions such as chromate or uranium by reducing them (Michel et al. 2001;Payne et al. 2002). SRB are anaerobic prokaryotes  (0)] and that hydrogenases on the cell membrane of SRB contribute to the reduction reaction (Riddin et al. 2009).
We have developed biological recycling technologies using cell surface engineering for the selective recovery of rare metals and toxic heavy metals causing environmental pollution (Ueda and Tanaka 2000;Ueda 2010, 2011;Nishitani et al. 2010;Kuroda et al. 2012). In cell surface engineering, any metal-binding proteins/peptides can be displayed on the yeast cell surface with maintenance of their activities. Furthermore, 'cell membrane' display of target proteins (Hara et al. 2012) has been achieved by fusing the target proteins to plasma membrane proteins such as Yps1p (Gagnon-Arsenault et al. 2006). The adsorbed metal ions can be easily recovered from yeast cells, because they are adsorbed on the yeast cell surface and a metal recovery process does not need breaking down the yeast cells. Therefore, we can repeatedly use the surface-engineered yeast cells as bioadsorbents in the next reaction.
In this study, we attempted to construct yeasts that display hydrogenase on their 'cell membrane' and use the constructed yeasts to recover platinum in 'solid form' by reducing Pt(II) to Pt(0). Yeast cells grow faster than SRB and are easily handled, because of the simple medium required to culture yeast cells and their safe biological features as compared to those of SRB. Therefore, it is likely that hydrogenase-displaying yeasts could recover platinum more effectively than SRB. In addition, platinum is mainly used as nanoparticles in catalytic converters of automobiles. Our method to recover platinum as nanoparticles, not simple solid forms, using hydrogenase-displaying yeasts could be useful as a next-generation technology for efficient metal recovery.

Construction of plasmids for cell membrane display
All primers used in the plasmid construction are listed in Table 1. The genomic DNAs were extracted from D. vulgaris Hildenborough and D. desulfuricans cell cultures using Blood & Cell Culture DNA Kit (Qiagen, Hilden, Germany). The DNA fragment encoding Hyn B hydrogenase (Gene-ID: 2793337) was amplified from D. vulgaris Hildenborough genomic DNA by PCR using primers Hyn B FW and Hyn B RV. In addition, the DNA fragment encoding Ni-Hyd hydrogenase (Gene-ID: 7284719) was amplified from D. desulfurican genomic DNA by PCR using primers Ni-Hyd FW and Ni-Hyd RV. The amplified DNA fragments Hyn B and Ni-Hyd were inserted into BamHI section of the pYS0 plasmid (Hara et al. 2012) for the Yps1p-based display on yeast cell membrane by using the In-Fusion HD Cloning Kit (Clontech, CA, USA). The resulting plasmids were named pYS-Hyn B and pYS-Ni-Hyd, respectively. The plasmid pYS0 for the cell membrane display of the FLAG-tag was used as a negative control for the platinum reduction experiments.

Transformation of S. cerevisiae
Saccharomyces cerevisiae W303-1A was transformed with the constructed plasmids using the Frozen-EZ Yeast Transformation II kit (Zymo Research, Irvine, CA, USA). Transformed cells were isolated by incubation  Table 2.

Immunofluorescence microscopy
To confirm the display of hydrogenase on yeast cell membrane, immunofluorescent labeling of cells with or without cell wall digestion was performed using the antibody against the FLAG epitope tag as described previously (Nishitani et al. 2010

Pt(II) reduction experiment
Prior to Pt(II) reduction, hydrogenase-displaying yeasts and control yeast were precultivated anaerobically in SDC + AHLU medium bubbled with N 2 gas at 30 °C for 24 h. The cells were collected after centrifugation at 3000×g for 5 min at room temperature and incubated in PBS (pH7.4) containing 100 μM PtCl 4 2− with an optical density of 20 at 600 nm. The reduction reaction was anaerobically performed at 37 °C for 72 h with shaking. After the incubation, cells and black precipitates were separated from the reaction solution by centrifugation at 3000×g for 10 min at room temperature. Then, the residual platinum ions in the supernatant were quantified by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 7500cx, Agilent Technologies, Santa Clara, CA, USA).

X-ray photoelectron spectroscopy and transmission electron microscopy (TEM)
After Pt(II) reduction, black precipitates and cells were collected from cell suspensions by centrifugation at 3000×g for 10 min at room temperature, and then lyophilized. Quantera SXM (Physical Electronics, Chanhassen, USA) was used for X-ray photoelectron spectroscopy (XPS) measurements using Kα lines of Al (100 μm, 24.8 W, 15 keV) as an X-ray source. The pass energy was 112.0 eV, and the step size was 0.100 eV. Transmission electron microscopy (TEM) using a 200 kV TEM instrument (JEM-2010F, JEOL, Tokyo, Japan) was employed to investigate the microstructure of the precipitates.

Construction of hydrogenase-displaying yeasts
Display of hydrogenases on yeast cell membrane was attempted to mimic SRB that produce hydrogenases on their cell membrane for reduction activity. The constructed multicopy plasmids for cell membrane display of Hyn B from D. vulgaris and Ni-Hyd from D. desulfuricans (pYS-Hyn B and pYS-Ni-Hyd) contain a glucoamylase secretion signal (GA s.s.), hydrogenase genes (Hyn B or Ni-Hyd), a FLAG-tag, and a cell-membraneanchoring domain of Yps1p (Fig. 1). To confirm the cell membrane display of the hydrogenases, immunofluorescent labeling of the transformed yeasts and wild-type yeast was performed with or without cell wall digestion, which improves the accessibility of the antibody to the membrane-displayed proteins (Fig. 2). The transformed yeasts after cell wall digestion showed a green fluorescence derived from fluorescent antibodies binding to the cell membrane, while other cells showed no fluorescence. The result indicates that transformed yeasts successfully displayed hydrogenases on their cell membrane.

Platinum reduction by hydrogenase-displaying yeasts
To evaluate the platinum-reduction abilities of the constructed hydrogenase-displaying yeasts, platinum reduction experiment was performed with the hydrogenase-displaying yeasts and control yeast under anaerobic conditions. The platinum concentration in the supernatant solution before and after the reduction was measured by using ICP-MS as described in "Materials and methods" section. Then, the decrease ratio of platinum ions in the reaction solutions after the reduction experiment was calculated (Fig. 3a). Two types of hydrogenase-displaying yeasts decreased more PtCl 4 2− in the reaction solutions than the control yeast. In addition, after the reduction experiment, black precipitates were produced by hydrogenase-displaying yeasts (Fig. 3b). These results suggest that black precipitates produced by hydrogenase-displaying yeasts contained platinum(0).

Analysis of precipitates produced by hydrogenase-displaying yeasts
It was unclear whether the produced black precipitates contained other platinum compounds as well as elemental platinum [Pt(0)]. Therefore, to reveal the chemical form of the platinum included in the black precipitates, the precipitates were analyzed by XPS that is used for examination of the chemical-bonding state and the valence of the element (Fig. 4). In the black precipitates produced by hydrogenase-displaying yeasts, Pt(0) peak (binding energy: 70.7-71.3 eV) was detected. In contrast, Pt(0) peak was not detected in the precipitates produced by control yeast. Therefore, the result indicates that black precipitates produced by hydrogenase-displaying yeasts contained Pt(0).
The black precipitates were observed by TEM to confirm the configuration of Pt(0) in the black precipitates (Fig. 5). The TEM image indicates that Pt(0) in the black precipitates existed as an aggregated nanoparticle. Furthermore, it was found that these nanoparticles had a crystalline structure because lattice fringes were present in the platinum nanoparticles (Fig. 5b, red dot circle).

Discussion
We constructed yeasts displaying hydrogenase on their cell membrane and achieved the recovery of Pt(II) as nanoparticles using the constructed yeasts. So far, the concentration recovery of metal ions from aqueous solutions has been performed by the surface-engineered yeasts displaying the proteins/peptides that bind metal ions Ueda 2010, 2011). In addition to this strategy, recovery of metals in a solid state from an aqueous solution is a remarkable strategy for practical use, in terms of the usability of the recovered metals. Especially, platinum is mainly used as nanoparticles in industries such as those producing catalytic converters of automobiles. In the case of platinum recovery as ions, an additional process for reducing the ions to nanoparticles is necessary. Therefore, our biological technique used to recover Pt(II) as nanoparticles can save cost and time, and is ecological because any electron donors are unnecessary in this system.
According to the result of XPS analysis (Fig. 4), the black precipitates produced by hydrogenase-displaying yeasts included Pt(0) with binding energies of 70.7-71.3 eV, suggesting that the hydrogenases displayed on yeast cell membrane contributed to the Pt(II) reduction to Pt(0). In addition, Pt(0) produced by hydrogenase-displaying yeasts formed nanoparticles with lattice fringes and the size of the platinum nanoparticles was about a few nanometers in diameter (Fig. 5b). Although XPS analysis revealed that hydrogenase-displaying yeasts produced Pt(0), there is a possibility that black precipitates produced by hydrogenase-displaying yeasts contained other platinum compounds such as PtO or Pt(OH) 2 . Although further analysis is required to demonstrate the composition of the black precipitates, the produced black precipitates could be used as material in fuel cells or catalytic converters. Platinum was used as a reduction target metal in this study. However, there are some reports that SRB could recover Au(III) as Au (0), in addition to Pt(II) (Creamer et al. 2006). If hydrogenases of SRB contribute to the reduction of Au(III), hydrogenase-displaying yeasts also could reduce Au(III). Furthermore, there are five kinds of metals bearing chemical and physical properties similar to those of platinum [ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), and iridium (Ir)]. These five metals and platinum are together called platinum group metals (PGMs), and are important because of their catalytic properties (Kettler 2003). Owing to the similarity in their properties, hydrogenase-displaying yeasts could reduce these metal ions. Therefore, the constructed hydrogenase-displaying yeasts could be available as an environmentally friendly technology for the recovery of platinum group metal ions.