Broadband Linear-to-Circular Polarization Conversion Realized by the Solid State Plasma Metasurface

In this paper, a solid-state plasma metasurface (SSPM) for linear-to-circular polarization conversion is designed and investigated. The unit cell of such a SSPM consists of three layers from bottom to top, which are the copper coating, the dielectric layer, and the solid-state plasma resonators, respectively. By exciting solid-state plasma resonance units in different regions, we successfully achieved two operating modes (modes I and II). In mode I, the axial ratio band which is less than 3 dB (3 dB AR band) is mainly obtained in 14.34–19.61 GHz (the relative bandwidth is 31.25%). When the proposed SSPM is regulated to mode II, the 3 dB AR band is changed to 8.81–14.34 GHz (the relative bandwidth is 40.90%). The polarization conversion rate, phase difference, AR curves, and surface current diagrams are analyzed to explore the performances of the proposed SSPM. Our design promotes the potential applications of tunable devices.

     

Wideband LTE Power Amplifier with Integrated Novel Analog Pre-Distorter Linearizer for Mobile Wireless Communications

For the first time, a new circuit to extend the linear operation bandwidth of a LTE (Long Term Evolution) power amplifier, while delivering a high efficiency is implemented in less than 1 mm² chip area. The 950 µm × 900 µm monolithic microwave integrated circuit (MMIC) power amplifier (PA) is fabricated in a 2 µm InGaP/GaAs process. An on-chip analog pre-distorter (APD) is designed to improve the linearity of the PA, up to 20 MHz channel bandwidth. Intended for 1.95 GHz Band 1 LTE application, the PA satisfies adjacent channel leakage ratio (ACLR) and error vector magnitude (EVM) specifications for a wide LTE channel bandwidth of 20 MHz at a linear output power of 28 dBm with corresponding power added efficiency (PAE) of 52.3%. With a respective input and output return loss of 30 dB and 14 dB, the PA’s power gain is measured to be 32.5 dB while exhibiting an unconditional stability characteristic from DC up to 5 GHz. The proposed APD technique serves to be a good solution to improve linearity of a PA without sacrificing other critical performance metrics.     

Dielectric Resonator Antenna Loaded with Reconfigurable Plasma Metamaterial Polarization Converter

In this paper, a plasma based metamaterial (MM) polarizer is optimized for linear to circular polarization conversion in Ku-band wireless applications. The plasma MM polarizer is design based on the transmission mode. The proposed plasma MM polarizer unit-cell element consists of back-to-back plasma arcs placed on the top and the bottom faces of a dielectric substrate. Polarization Conversion from linear polarized (LP) wave to circular polarized (CP) wave requires that the incident wave is splitted into two orthogonal components with equal magnitudes and π/2 phase difference. The polarization characteristics of the plasma MM polarizer have bandwidth of 17.5% centered around 13.7 GHz. A linearly polarized dielectric resonator antenna (DRA) is designed with impedance bandwidth of 27.05%. The LP-DRA loaded with plasma MM polarizer radiates CP-waves with improved gain of 8.89 dBi. Reconfigurable CP radiation right-hand, left-hand, or linear polarization is achieved when the argon gas in the lower, upper, or both arc-shaped containers is ionized to plasma state. The proposed plasma MM polarizer and the DRA structures are examined utilizing a full-wave simulator.

     

Wide Bandpass and Narrow Bandstop Microstrip Filters Based on Hilbert Fractal Geometry: Design and Simulation Results

This paper presents new Wide Bandpass Filter (WBPF) and Narrow Bandstop Filter (NBSF) incorporating two microstrip resonators, each resonator is based on 2^nd iteration of Hilbert fractal geometry. The type of filter as pass or reject band has been adjusted by coupling gap parameter (d) between Hilbert resonators using a substrate with a dielectric constant of 10.8 and a thickness of 1.27 mm. Numerical simulation results as well as a parametric study of d parameter on filter type and frequency responses are presented and studied. WBPF has designed at resonant frequencies of 2 and 2.2 GHz with a bandwidth of 0.52 GHz, −28 dB return loss and −0.125 dB insertion loss while NBSF has designed for electrical specifications of 2.37 GHz center frequency, 20 MHz rejection bandwidth, −0.1873 dB return loss and 13.746 dB insertion loss. The proposed technique offers a new alternative to construct low-cost high-performance filter devices, suitable for a wide range of wireless communication systems.     

Tunable Spoof Surface Plasmons Bulleye Antenna

A tunable spoof surface plasmons antenna using sinusoidally modulated corrugated reactance surface based on a bulleye structure is proposed in this paper. The designed antenna is made of concentric metallic grooves etched on a metal plate, the depth of which is of sinusoidal periodic variation in the radial direction. This makes it possible that highly confined spoof surface plasmons along corrugated surface can be converted to radiation modes. The proposed bulleye antenna can work from 25.8 to 33 GHz and a bandwidth of 7.2 GHz and its main lobe can be directed at 30^∘ from the vertical direction at 30 GHz. This antenna has a maximum gain of 15 dB and its main lobe can scan from 14^∘ to 58^∘ by tuning the frequency from 28 to 32 GHz.

     

Electronic Beam Switching of Circularly Polarized Plasma Magneto-Electric Dipole Array with Multiple Beams

In this paper, different array arrangements based on magneto-electric (ME) dipole antenna with wideband circular polarization (CP) characteristics are designed and investigated. Planar, triangular prism, square prism, and hexagonal prism array arrangements are considered. Each prism face has a sub-array comprises 2 × 2 ME-dipole elements. Each sub-array has wide impedance matching of 73.7%, a maximum gain of 16.6 dBi, and CP bandwidth of 78.2%. It employs the plasma frequency of the ME-dipole antenna to control its radiation characteristics. Frequency-independent lumped element equivalent circuit is constructed for a single antenna element. It is used to represent the antenna input impedance at different plasma electron densities with fixed physical structure. The proposed equivalent circuit comprises a single series section used for matching enhancement with feeder circuit, and three parallel tuned circuits corresponding to the three resonance frequencies in the input impedance. The best values of the equivalent circuit elements are computed using the particle swarm optimization (PSO) technique. Different array arrangements, planar, triangular, square, and hexagonal prism are designed to create single or multiple beams in different directions. An electronic beam switching is achieved by tuning in the plasma inside the ME-dipole in the desired direction. The radiation characteristics are analyzed and investigated using the finite integration technique (FIT).

     

Design and Dosimetric Analysis of an Exposure Facility for Investigating Possible Effects of 2.45 GHz Wi-Fi Signals on Human Sleep

A new head exposure system for double-blind provocation studies investigating possible effects of 2.45 GHz Wi-Fi exposure on human sleep was developed and dosimetrically analyzed. The exposure system includes six simultaneously radiating directional antennas arranged along a circle (radius 0.6 m) around the test subject's head, and enables a virtually uniform head exposure, i.e. without any preferred direction of incidence, during sleep. The system is fully computer-controlled and applies a real wireless local area network (WLAN) signal representing different transmission patterns as expected in real WLAN scenarios, i.e. phases of “beacon only” as well as phases of different data transmission rates. Sham and verum are applied in a double-blind crossover study design and all relevant exposure data, i.e. forward and reverse power at all six antenna inputs, are continuously recorded for quality control. For a total antenna input power (sum of all antennas) of 220 mW, typical specific absorption rate (SAR) in cortical brain regions is approximately 1–2 mW/kg (mass average SAR over respective brain region), which can be seen as a realistic worst-case exposure level in real WLAN scenarios. Taking into account variations of head positions during the experiments, the resulting exposure of different brain regions may deviate from the given average SAR levels up to 10 dB. Peak spatial 10 g average SAR in all brain and all head tissues is between 1.5–3.5 and 10.4–25 mW/kg, respectively. Bioelectromagnetics. © 2020 Bioelectromagnetics Society.     

Design of Site-Specific Microwave Phased Array Hyperthermia Applicators Using 434 MHz Reduced Cavity-Backed Patch Antenna

Cancers of the neck, breast, and lower extremities are common malignancies diagnosed in India with a higher incidence of advanced-stage disease. Phased array (PA) applicators reported for hyperthermia treatment (HT) of the breast have small focal region and high cross-coupling, and those reported for lower extremities provide regional heating and limited steering. In this study, we present the numerical design of site-specific PA applicators for HT of large solid tumors in the neck, breast, and lower extremities using a miniaturized 434 MHz cavity-backed water-loaded patch antenna. The fabricated antenna has 38 × 36 mm² aperture, more than 90% power coupling, 25 MHz bandwidth, and good agreement between simulated and measured specific absorption rate (SAR) in phantom. The site-specific applicators demonstrated less power reflection (<−17.9 dB) and cross-coupling (<−26.8 dB) for 5 mm inter-ring spacing. SAR indicators for 64 cc tumor at varying locations in simplified layered three-dimensional (3D) tissue models of the neck, breast, and leg showed average power absorption ratio (aPA_ratio) ≥ 3.16, target to hotspot quotient (THQ) ≥ 0.57, 25% iso-SAR coverage (TC₂₅) ≥ 81%, and 50% iso-SAR coverage (TC₅₀) ≥51.8%. Simulation results of site-specific applicators for 3D inhomogeneous patient models showed aPA_ratio ≥ 5.98, THQ ≥ 0.9, TC₅₀ ≥ 86%, and 100% TC₂₅ for all sites. It is concluded that the 434 MHz miniaturized cavity-backed patch antenna can be used to develop high-density PA applicators with 12–24 antennas for HT of large solid tumors (≥4 cm) in the neck, breast, and lower extremities with 3D steering ability and less cross-coupling (≤−26.8 dB). © 2020 Bioelectromagnetics Society     

Design and Dosimetric Characterization of a Broadband Exposure Facility for In Vitro Experiments in the Frequency Range 18–40.5 GHz

A novel exposure facility for exposing cell monolayers to centimeter and millimeter waves (18–40.5 GHz) used by future 5G mobile communication technology and similar applications has been developed. A detailed dosimetric characterization of the apparatus for frequencies of 27 and 40.5 GHz and 60 mm petri dishes, used in a presently ongoing study on human dermal fibroblasts and keratinocytes, was carried out. The exposure facility enables a well-defined, randomized, and blinded application of sham exposure and exposure with selectable values of incident power flux density, and additionally provides the possibility of continuous monitoring of the sample temperature during exposure while it does not require significant deviations from routine in vitro handling procedures, i.e. petri dishes are not required to be placed inside waveguides or TEM cells. Mean specific absorption rate (SAR) values inside the cell monolayer of 115 W/kg (27 GHz) and 160 W/kg (40.5 GHz) per watt antenna input power and corresponding transmitted power density (S_t) values at the bottom of the cell monolayer of 65 W/m² (27 GHz) and 70 W/m² (40.5 GHz) per watt antenna input power can be achieved, respectively. For reasonable amounts of harvested cells (80% of petri dish bottom area), the variation (max/min) of SAR and S_t over the cell monolayer remains below 3.7 dB (27 GHz) and 3.0 dB (40.5 GHz), respectively. © 2021 Bioelectromagnetics Society.     

Core temperature measurement by microwave radiometry

A ‘double-container model’ was used for core temperature (T_c) measurement by microwave emission radiometry (MR) of warm fluid inside a tube, placed in a container with a cooler fluid. The intensity of microwaves emitted from the warmer fluid inside the tube were measured using a MR metering device, consisting of an antenna linked to a low-noise radio frequency amplifier (bandwidth 500 MHz, centered at 4.0 GHz). Based on the MR measurements, a T_c prediction model was developed for measuring the temperature of fluid inside a tube, achieving a sensitivity of ±0.5 °C at environmental temperature of 33–37 °C.     

Operant behavior and colonic temperature of Macaca mulatta exposed to radio frequency fields at and above resonant frequencies

Five food-deprived rhesus monkeys were exposed to 225-MHz continuous-wave, and 1.3-GHz, and 5.8-GHz pulsed radiation to determine the minimal power densities affecting performance. The monkeys were trained to press a lever (observing-response) thereby producing signals that indicated availability of food. In the presence of the aperiodically appearing food signals, a detection response on a different lever was reinforced by a food pellet. Continuous, stable responding during 60-min sessions developed and was followed by repeated exposures to radiofrequency radiation. The subjects, restrained in a Styrofoam chair, were exposed to free-field radiation while performing the task. Colonic temperature was simultaneously obtained. Observing-response performance was impaired at increasingly higher power densities as frequency increased from the near-resonance 225 MHz to the above-resonance 5.8 GHz. The threshold power density of disrupted response rate at 225 MHz was 8.1 mW/cm²; at 1.3 GHz it was 57 mW/cm², and at 5.8 GHz it was 140 mW/cm². These power densities were associated with reliable increases in colonic temperatures above sham-exposure levels. The mean increase was typically in the range of 1°C, and response-rate changes were not observed in the absence of concomitant temperature increases. In these experiments increase of colonic temperature was a much better predictor of behavioral disruption than was either the power density of the incident field or estimates of whole-body-averaged rates of energy absorption.     

Investigation of New Microstrip Bandpass Filter Based on Patch Resonator with Geometrical Fractal Slot

A compact dual-mode microstrip bandpass filter using geometrical slot is presented in this paper. The adopted geometrical slot is based on first iteration of Cantor square fractal curve. This filter has the benefits of possessing narrower and sharper frequency responses as compared to microstrip filters that use single mode resonators and traditional dual-mode square patch resonators. The filter has been modeled and demonstrated by Microwave Office EM simulator designed at a resonant frequency of 2 GHz using a substrate of ε_r = 10.8 and thickness of h = 1.27 mm. The output simulated results of the proposed filter exhibit 22 dB return loss, 0.1678 dB insertion loss and 12 MHz bandwidth in the passband region. In addition to the narrow band gained, miniaturization properties as well as weakened spurious frequency responses and blocked second harmonic frequency in out of band regions have been acquired. Filter parameters including insertion loss, return loss, bandwidth, coupling coefficient and external quality factor have been compared with different values of perturbation dimension (d). Also, a full comparative study of this filter as compared with traditional square patch filter has been considered.     

Passive multivariable temperature and conductivity RFID sensors for single-use biopharmaceutical manufacturing components

Single-use biopharmaceutical manufacturing requires monitoring of critical manufacturing parameters. We have developed an approach for passive radio-frequency identification (RFID)-based sensing that converts ubiquitous passive 13.56 MHz RFID tags into inductively coupled sensors. We combine several measured parameters from the resonant sensor antenna with multivariate data analysis and deliver unique capability of multiparameter sensing and rejection of environmental interferences with a single sensor. We demonstrate here the integration of these RFID sensors into single-use biopharmaceutical manufacturing components. We have tested these sensors for over 500 h for measurements of temperature and solution conductivity with the accuracy of 0.1°C (32-48°C range) and accuracy of 0.3-2.9 mS/cm (0.5-230 mS/cm range). We further demonstrate simultaneous temperature and conductivity measurements with an individual RFID sensor with the accuracy of 0.2°C (5-60°C range) and accuracy of 0.9 mS/cm (0.5-183 mS/cm range). Developed RFID sensors provide several important features previously unavailable from other single-use sensing technologies such as the same sensor platform for measurements of physical, chemical, and biological parameters; multi-parameter monitoring with individual sensors; and simultaneous digital identification.     

Dielectric studies on water in solutions of purified lecithin vesicles

The complex permittivity of sonicated aqueous solutions of purified dimyristoylphosphatidylcholine has been measured as a function of frequency between 3 kHz and 40 GHz. The dielectric spectrum of the samples shows two dispersion/absorption regions, one centered at about 80 MHz the other at about 20.GHz (30°C). Otherwise than in previous studies no additional dispersion/absorption process has been found at frequencies below 10 MHz.The complex dielectric spectrum of the samples is discussed with respect to the dynamical state of solvent water in solutions of single-bilayer vesicles. The main relaxation time of the solvent water, τ₁ ((2πτ₁)^?1 ≈ 20 GHz), is smaller than that of pure water, τ_W, at the same temperature. This effect results from the action of internal depolarizing fields which obviously overcompensate and enhancement of τ₁ due to specific solute/solvent interactions (hydration) as had been previously found with micellar solutions of lysolecithins.It cannot be excluded, that some solvent water shows unusual dynamical behaviour. If there exists a substantial amount of such motionally perturbed water, however, it must be characterized by a relaxation time close to that of the phosphorylcholine zwitterions, τ₂ ((2πτ₂)^?1 ≈ 80 MHz).     

The Source Parameters of Echolocation Clicks from Captive and Free-Ranging Yangtze Finless Porpoises (Neophocaena asiaeorientalis asiaeorientalis)

The clicks of Yangtze finless porpoises (Neophocaena asiaeorientalis asiaeorientalis) from 7 individuals in the tank of Baiji aquarium, 2 individuals in a netted pen at Shishou Tian-e-zhou Reserve and 4 free-ranging individuals at Tianxingzhou were recorded using a broadband digital recording system with four element hydrophones. The peak-to-peak apparent source level (ASL__pp) of clicks from individuals at the Baiji aquarium was 167 dB re 1 μPa with mean center frequency of 133 kHz, -3dB bandwidth of 18 kHz and -10 dB duration of 58 μs. The ASL__pp of clicks from individuals at the Shishou Tian-e-zhou Reserve was 180 dB re 1 μPa with mean center frequency of 128 kHz, -3dB bandwidth of 20 kHz and -10 dB duration of 39 μs. The ASL__pp of clicks from individuals at Tianxingzhou was 176 dB re 1 μPa with mean center frequency of 129 kHz, -3dB bandwidth of 15 kHz and -10 dB duration of 48 μs. Differences between the source parameters of clicks among the three groups of finless porpoises suggest these animals adapt to their echolocation signals depending on their surroundings.     

Design and dosimetric analysis of a 385 MHz TETRA head exposure system for use in human provocation studies

A new head exposure system for double‐blind provocation studies investigating possible effects of terrestrial trunked radio (TETRA)‐like exposure (385 MHz) on central nervous processes was developed and dosimetrically analyzed. The exposure system allows localized exposure in the temporal brain, similar to the case of operating a TETRA handset at the ear. The system and antenna concept enables exposure during wake and sleep states while an electroencephalogram (EEG) is recorded. The dosimetric assessment and uncertainty analysis yield high efficiency of 14 W/kg per Watt of accepted antenna input power due to an optimized antenna directly worn on the subject's head. Beside sham exposure, high and low exposure at 6 and 1.5 W/kg (in terms of maxSAR10g in the head) were implemented. Double‐blind control and monitoring of exposure is enabled by easy‐to‐use control software. Exposure uncertainty was rigorously evaluated using finite‐difference time‐domain (FDTD)‐based computations, taking into account anatomical differences of the head, the physiological range of the dielectric tissue properties including effects of sweating on the antenna, possible influences of the EEG electrodes and cables, variations in antenna input reflection coefficients, and effects on the specific absorption rate (SAR) distribution due to unavoidable small variations in the antenna position. This analysis yielded a reasonable uncertainty of <±45% (max to min ratio of 4.2 dB) in terms of maxSAR10g in the head and a variability of <±60% (max to min ratio of 6 dB) in terms of mass‐averaged SAR in different brain regions, as demonstrated by a brain region‐specific absorption analysis. Bioelectromagnetics 33:594–603, 2012. © 2012 Wiley Periodicals, Inc.     

Circularly Polarized Plasma Curl Antenna for 2.45 GHz Portable RFID Reader

In this paper, a broadband circularly polarized plasma curl antenna for portable RFID reader at 2.45 GHz is proposed. The antenna consists of a glass tube filled with argon gas and of about 1.4 wavelengths curled into one turn. A parametric study for the plasma curl antenna parameters is investigated. A simulated compact model for the portable RFID reader is proposed. The radiation characteristics of the plasma curl antenna enclosed inside a portable reader device are calculated. The behavior of plasma curl antenna is like a metal antenna when the signal is transmitted and received. Plasma antennas have many advantages as their design permit electrical, rather than mechanical control for their radiation characteristics. They are light weight, reconfigurable, can be energized, and de-energized in seconds, which prevent signal degradation. When a particular plasma antenna is not energized, its radiation does not affect the nearby elements and allowing other antennas to transmit or receive without interference. The radiation characteristics of the plasma curl antennas are investigated and analyzed using a finite integral technique.     

Col11a2 Deletion Reveals the Molecular Basis for Tectorial Membrane Mechanical Anisotropy

The tectorial membrane (TM) has a significantly larger stiffness in the radial direction than other directions, a prominent mechanical anisotropy that is believed to be critical for the proper functioning of the cochlea. To determine the molecular basis of this anisotropy, we measured material properties of TMs from mice with a targeted deletion of Col11a2, which encodes for collagen XI. In light micrographs, the density of TM radial collagen fibers was lower in Col11a2 -/- mice than wild-types. Tone-evoked distortion product otoacoustic emission and auditory brainstem response measurements in Col11a2 -/- mice were reduced by 30-50 dB independent of frequency as compared with wild-types, showing that the sensitivity loss is cochlear in origin. Stress-strain measurements made using osmotic pressure revealed no significant dependence of TM bulk compressibility on the presence of collagen XI. Charge measurements made by placing the TM as an electrical conduit between two baths revealed no change in the density of charge affixed to the TM matrix in Col11a2 -/- mice. Measurements of mechanical shear impedance revealed a 5.5 ± 0.8 dB decrease in radial shear impedance and a 3.3 ± 0.3 dB decrease in longitudinal shear impedance resulting from the Col11a2 deletion. The ratio of radial to longitudinal shear impedance fell from 1.8 ± 0.7 for TMs from wild-type mice to 1.0 ± 0.1 for those from Col11a2 -/- mice. These results show that the organization of collagen into radial fibrils is responsible for the mechanical anisotropy of the TM. This anisotropy can be attributed to increased mechanical coupling provided by the collagen fibrils. Mechanisms by which changes in TM material properties may contribute to the threshold elevation in Col11a2 -/- mice are discussed.     

Reconfigurable tapered coaxial slot antenna for hepatic microwave ablation

ABSTRACT

Microwave ablation is rapidly being rediscovered and developed for treating many cancers of liver, lung, kidney and bone, as well as arrhythmias and other medical conditions. The microwaves ablate tissue by heating it to cytotoxic temperatures. The microwave antenna design suffers the challenges of effective coupling and penetration into body tissues, uncontrolled power deposition due to applicator construction limitations affecting uniform heating of target region, and narrowband operation leading to mismatch for many patients and detrimental heating. To meet out the requirements of wideband operation and localized lesion reconfigurable linearly tapered slot interstitial wideband antenna has been proposed for working in the 1.38?GHz to 4.31?GHz frequency band. The performance of the antenna is evaluated by using FEM-based HFSS software. The slot height and taper height are reconfigured for parametric analysis achieving maximum impedance matching and spherical ablation zone without requiring any additional adjustable structures. The tapering of the slot in coaxial antenna generates current distribution at the edges of the slot for maximizing specific absorption rate.     

Characterization of the metabolically modified heavy metal-resistant Cupriavidus metallidurans strain MSR33 generated for mercury bioremediation

Background

Mercury-polluted environments are often contaminated with other heavy metals. Therefore, bacteria with resistance to several heavy metals may be useful for bioremediation. Cupriavidus metallidurans CH34 is a model heavy metal-resistant bacterium, but possesses a low resistance to mercury compounds.

Methodology/Principal Findings

To improve inorganic and organic mercury resistance of strain CH34, the IncP-1β plasmid pTP6 that provides novel merB, merG genes and additional other mer genes was introduced into the bacterium by biparental mating. The transconjugant Cupriavidus metallidurans strain MSR33 was genetically and biochemically characterized. Strain MSR33 maintained stably the plasmid pTP6 over 70 generations under non-selective conditions. The organomercurial lyase protein MerB and the mercuric reductase MerA of strain MSR33 were synthesized in presence of Hg²⁺. The minimum inhibitory concentrations (mM) for strain MSR33 were: Hg²⁺, 0.12 and CH₃Hg⁺, 0.08. The addition of Hg²⁺ (0.04 mM) at exponential phase had not an effect on the growth rate of strain MSR33. In contrast, after Hg²⁺ addition at exponential phase the parental strain CH34 showed an immediate cessation of cell growth. During exposure to Hg²⁺ no effects in the morphology of MSR33 cells were observed, whereas CH34 cells exposed to Hg²⁺ showed a fuzzy outer membrane. Bioremediation with strain MSR33 of two mercury-contaminated aqueous solutions was evaluated. Hg²⁺ (0.10 and 0.15 mM) was completely volatilized by strain MSR33 from the polluted waters in presence of thioglycolate (5 mM) after 2 h.

Conclusions/Significance

A broad-spectrum mercury-resistant strain MSR33 was generated by incorporation of plasmid pTP6 that was directly isolated from the environment into C. metallidurans CH34. Strain MSR33 is capable to remove mercury from polluted waters. This is the first study to use an IncP-1β plasmid directly isolated from the environment, to generate a novel and stable bacterial strain useful for mercury bioremediation.