Germanium in the soil-plant system-a review

Phytoextraction Options

Wastewaters often contain an array of economically valuable elements, including elements considered critical raw materials and elements for fertilizer production. Plant-based treatment approaches in constructed wetlands, open ponds, or hydroponic systems represent an eco-friendly and economical way to remove potentially toxic metal(loid)s from wastewater (phytoextraction). Concomitantly, the element-enriched biomass represents an important secondary raw material for bioenergy generation and the recovery of raw materials from the harvested plant biomass (phytomining). At present, phytoextraction in constructed wetlands is still considered a nascent technology that still requires more fundamental and applied research before it can be commercially applied. This chapter discusses the different roles of plants in constructed wetlands during the phytoextraction of economically valuable elements. It sheds light on the utilization of plant biomass in the recovery of raw materials from wastewater streams. Here, we consider phytoextraction of the commonly studied water pollutants (N, P, Zn, Cd, Pb, Cr) and expand this concept to a group of rather exotic metal(loid)s (Ge, REE, PGM) highlighting the role of phytoextraction in the face of climate change and finite resources of high-tech metals.

Stability Constants of Tetravalent Germanium Complexes in Aqueous Solution: Small Organic Oxygen and Nitrogen Donor Ligands

The formation constants of about 100 different complexes of Ge(IV) with some 50 organic, low-molecular-weight ligands, principally bidentate oxygen and/or nitrogen donors, are determined by potentiometric titrations at 25 °C and an ionic strength of 0.1 mol L-1 (KNO3). In the few cases for which literature data are available, these older data are critically discussed and completed in light of our new experimental data. The overall picture emerging from our work is that such complexes are comparatively weak, except when a chelate effect comes into play, as is observed with aminopolyols, α-hydroxocarboxylic acids, and carbohydrate derivatives, where even quantitative chelation of Ge is possible. No binding to amino acids or peptides is detected. An important finding is our evidence for the existence of a bridged dinuclear germanium(IV) tartrate complex in the aqueous phase, whose occurrence in solids was confirmed by X-ray diffraction beforehand. Our work has implications for the general understanding of the aqueous coordination chemistry of Ge, its geochemical cycling, and its behavior in environmental and biological systems. Potentiometric raw data for use by other researchers are publicly accessible in the repository Zenodo at CERN.

Physiological Activity of Trace Element Germanium including Anticancer Properties

Germanium is an essential microelement, and its deficiency can result in numerous diseases, particularly oncogenic conditions. Consequently, water-soluble germanium compounds, including inorganic and coordination compounds, have attracted significant attention due to their biological activity. The review analyzes the primary research from the last decade related to the anticancer activity of germanium compounds. Furthermore, the review clarifies their actual toxicity, identifies errors and misconceptions that have contributed to the discrediting of their biological activity, and briefly suggests a putative mechanism of germanium-mediated protection from oxidative stress. Finally, the review provides clarifications on the discovery history of water-soluble organic germanium compounds, which was distorted and suppressed for a long time.

Potential Ecological Risk Assessment of Critical Raw Materials: Gallium, Gadolinium, and Germanium

In recent years, the demand for critical raw materials such as gallium, gadolinium and germanium (G(s)) has steadily increased in various industries. However, treatment or recycling rates of these elements are extremely low, which can lead to environmental pollution. An assessment of the ecological risks was also not possible until now, as there were no calculated toxicity coefficients for G(s). In this study, a well-known method, the so-called potential ecological risk index (PERI), was used for the first time to calculate the toxicity coefficients of these elements using data from recent literature studies on G(s) elements. The toxicity coefficient of each of the three elements was determined as five (5). The results show that G(s) have the same toxicity coefficient as Cu and Pb and are higher than that of Cr. The ecological risk index results varied from 4 to 414, 0.98 to 25.98 and 2.50 to 284.64 for Ga, Gd and Ge, respectively. The results show that Ga and Ge pose high ecological risk while the Er_i of Gd is low. The toxicity coefficients of these elements have been calculated for the first time in the literature and provide a practical use for calculating the potential ecological risk index.

Use of Microbial Consortia in Bioremediation of Metalloid Polluted Environments

Metalloids are released into the environment due to the erosion of the rocks or anthropogenic activities, causing problems for human health in different world regions. Meanwhile, microorganisms with different mechanisms to tolerate and detoxify metalloid contaminants have an essential role in reducing risks. In this review, we first define metalloids and bioremediation methods and examine the ecology and biodiversity of microorganisms in areas contaminated with these metalloids. Then we studied the genes and proteins involved in the tolerance, transport, uptake, and reduction of these metalloids. Most of these studies focused on a single metalloid and co-contamination of multiple pollutants were poorly discussed in the literature. Furthermore, microbial communication within consortia was rarely explored. Finally, we summarized the microbial relationships between microorganisms in consortia and biofilms to remove one or more contaminants. Therefore, this review article contains valuable information about microbial consortia and their mechanisms in the bioremediation of metalloids.

Distribution and ecological risk assessment of trace elements in the paddy soil-rice ecosystem of Punjab, Pakistan

Trace elements (TEs) contamination of agricultural soils requires suitable criteria for regulating their toxicity limits in soil and food crops, which depends on their potential ecological risk spanning regional to global scales. However, no comprehensive study is available that links TE concentrations in paddy soil with ecological and human health risks in less developed regions like Pakistan. Here we evaluated the data set to establish standard guidelines for defining the hazard levels of various potentially toxic TEs (such as As, Cd, Co, Cu, Cr, Fe, Mn, Ni, Pb, Se, Zn) in agricultural paddy soils of Punjab, Pakistan. In total, 100 topsoils (at 0-15 cm depth) and 204 rice plant (shoot and grain) samples were collected from five ecological zones of Punjab (Gujranwala, Hafizabad, Vehari, Mailsi, and Burewala), representing the major rice growing regions in Pakistan. The degree of contamination (C_d) and potential ecological risk index (PERI) established from ecological risk models were substantially higher in 100% and 97% of samples, respectively. The positive matrix factorization (PMF) model revealed that the elevated TEs concentration, notably Cd, As, Cr, Ni, and Pb, in the agricultural paddy soil was attributed to the anthropogenic activities and groundwater irrigation. Moreover, the concentration of these TEs in rice grains was higher than the FAO/WHO's safe limits. This study provided a baseline, albeit critical knowledge, on the impact of TE-allied ecological and human health risks in the paddy soil-rice system in Pakistan; and it opens new avenues for setting TEs guidelines in agro-ecological zones globally, especially in underdeveloped regions.

Assessment of the Ecotoxicity of Pollution by Potentially Toxic Elements by Biological Indicators of Haplic Chernozem of Southern Russia (Rostov region)

The content of various chemical elements such as metals, metalloids, and nonmetals in the environment is associated with natural and anthropogenic sources. It is necessary to normalize the content of metals, metalloids, and nonmetals as potentially toxic elements (PTE) in the Haplic Chernozem. The soils of the Southern Russia are of high quality and fertility. However, this type of soil, like Haplic Chernozem, is subject to contamination with a wide range of PTE. The aim of the work was to rank metals, metalloids, and nonmetals by ecotoxicity in Haplic Chernozem. To assess the ecotoxicity of chernozem, data for 15 years (2005-2020) were used. Biological indicators used to assess the ecotoxicity of Haplic Chernozem: catalase activity, cellulolytic activity, number of bacteria, Azotobacter spp. abundance, to change of length of radish's roots. Based on these biological indicators, an integral indicator of the state of Haplic Chernozem was calculated. The ecotoxicity of 23 metals (Cd, Hg, Pb, Cr, Cu, Zn, Ni, Co, Mo, Mn, Ba, Sr, Sn, V, W, Ag, Bi, Ga, Nb, Sc, Tl, Y, Yb), 5 metalloids (B, As, Ge, Sb, Te) and 2 nonmetals (F, Se) as priority pollutants. It is proposed to distinguish three hazard classes of metals, metalloids, and nonmetals to Haplic Chernozem: I class - Te, Ag, Se, Cr, Bi, Ge, Sn, Tl, Hg, Yb, W, Cd; II class - As, Co, Sc, Sb, Cu, Ni, B, Nb, Pb, Ga; III class - Sr, Y, Mo, Zn, V, Ba, Mn, F. It is advisable to use the results of the study for predictive assessment of the impact of metals, metalloids, and nonmetals on the ecological state of the soil during pollution.

Secondary metabolites released by the rhizosphere bacteria Arthrobacter oxydans and Kocuria rosea enhance plant availability and soil-plant transfer of germanium (Ge) and rare earth elements (REEs)

Here, we explore effects of metallophore-producing rhizobacteria on the plant availability of germanium (Ge) and rare earth elements (REEs). Five isolates of the four species Rhodococcus erythropolis, Arthrobacter oxydans, Kocuria rosea and Chryseobacterium koreense were characterized regarding their production of element-chelators using genome-mining, LC-MS/MS analysis and solid CAS-assay. Additionally, a soil elution experiment was conducted in order to identify isolates that increase solubility of Ge and REEs in soil solution. A. oxydans ATW2 and K. rosea ATW4 released desferrioxamine-, bacillibactin- and surfactin-like compounds that mobilized Ge and REEs as well as P, Fe, Si and Ca in soil. Subsequently, oat, rapeseed and reed canary grass were cultivated on soil and sand and treated with cells and iron depleted culture supernatants of A. oxydans ATW2 and K. rosea ATW4. Inoculation increased plant yield and shoot phosphorus (P), manganese (Mn), Ge and REE concentrations. However, effects of the inoculation varied substantially between the growth substrates and plant species. On sand, A. oxydans ATW2 increased accumulation of REEs in all plant species and root-shoot translocation in rapeseed, while K. rosea ATW4 enhanced REE accumulation in rapeseed only, without effects on other plant species. Sand-cultured oat plants showed increased Ge accumulation and root-shoot translocation in presence of A. oxydans ATW2 cells and K. rosea ATW4 supernatant; however, there was no effect on other plant species, irrespective the growth substrate used. In contrast, soil-cultured rapeseed showed enhanced REE accumulation in presence of cells of A. oxydans ATW2 while there were no effects on other plant species and Ge. The processes involved are not yet fully understood. Nevertheless, we demonstrated that chemical microbe-soil-plant relationships influence plant availability of nutrients together with Ge and REEs, which has major implications on our understanding of biogeochemical element cycling and development of sustainable bioremediation and biomining technologies.

Physiological responses, tolerance, and remediation strategies in plants exposed to metalloids

Metalloids are a subset of particular concern to risk assessors and toxicologists because of their well-documented potential hazards to plant system. Most of the metalloids are major environmental contaminants which affect crop productivity when present in high concentrations in soil. Metalloids are coupled with carrier proteins of the plasma membrane and translocated to various organs causing changes in key metabolic processes, damages cell biomolecules, and finally inhibit its growth. Phytoremediation-based approaches help in understanding the molecular and biochemical mechanisms for prerequisite recombinant genetic approaches. Recent advancements in proteomics and plant genomics help in understanding the role of transcription factors, metabolites, and genes in plant system which confers metal tolerance. The present review summarizes our current status of knowledge in this direction related to various physiological responses, detoxification mechanisms, and remediation strategies of metalloids in crop plants in relation to plant-metalloid tolerance. Further, the role of various transcription factors and miRNAs in conferring metal tolerance is also briefed. Hence, the present review mainly focused on the alterations in the physiological activities of plants due to metalloid toxicity and the various mechanisms which get activated inside the plants to mitigate their toxic effects.

Application of Fourier transform ion cyclotron resonance mass spectrometry in deciphering molecular composition of soil organic matter: A review

Swiftly deciphering soil organic matter (SOM) composition is critical for research on soil degradation and restoration. Recent advances in analytical techniques (e.g., optical methods and mass spectrometry) have expanded our understanding of the composition, origin, and evolution of SOM. In particular, the use of Fourier transform ion cyclotron resonance mass spectrometers (FTICR-MS) makes it possible to interpret SOM compositions at the molecular level. In this review, we discuss extraction, enrichment, and purification methods for SOM using FTICR-MS analysis; summarize ionization techniques, FTICR-MS mechanisms, data analysis methods, and molecular compositions of SOM in different environments (providing new insights into its origin and evolution); and discuss factors affecting its molecular diversity. Our results show that digenesis, combustion, pyrolysis, and biological metabolisms jointly contribute to the molecular diversity of SOM molecules. The SOM thus formed can further undergo photodegradation during transportation from land to fresh water (and subsequently oceans), resulting in the formation of dissolved organic matter (DOM). Better understanding the molecular features of DOM therefore accelerates our understanding of SOM evolution. In addition, we assess the degradation potential of SOM in different environments to better inform soil remediation methods. Finally, we discuss the merits and drawbacks of applying FTICR-MS on the analysis of SOM molecules, along with existing gaps in knowledge, challenges, and new opportunities for research in FTICR-MS applications and SOM identification.

First Characterization of the Formation of Anthocyanin-Ge and Anthocyanin-B Complexes through UV-Vis Spectroscopy and Density Functional Theory Quantum Chemical Calculations

The occurrence of anthocyanin (ACN) and metal (Me) complexes has been widely supported by many research works while the possibility that ACNs bind to metalloids (Mds) is yet to be proven. Here, metalloids (H₃BO₃ for B; GeO₂ for Ge) were added to cyanidin-based solutions at pH 5, 6, and 7 and ACN-Md stoichiometric ratios of 1:1, 1:10, 1:100, and 1:500, and UV-vis transmittance spectroscopy as well as density functional theory (DFT) calculations were performed to test this hypothesis. Ge and B addition caused bathochromic and hyperchromic shifts on ACN UV-vis spectra, particularly pronounced at pH 5 and a 1:500 (ACN:Md) ratio. ACN-Me complexation reactions have been evaluated where Ge showed a higher capability to bind to ACNs than B. Among the complexes envisioned, those labeled as b1, b2, and b3 feature UV-vis spectra compatible with experiments. The combination of experimental and computational data offers for the first time evidence of the formation of ACN-Md complexes.

Germanium fractions in typical paddy soil and its interaction with humic substances

Ge and Si differ strongly in their biogeochemical behavior due to the differences in binding capacity to organic matter. The mechanisms of soil organic matter affecting the mobility and bioavailability of Ge in soil-plant system remain unclear. This work aimed to investigate the soil Ge fractions and Ge binding to humic substances in paddy soil. Paddy soil samples taken from Changxing County, Zhejiang Province, China, were investigated by the sequential extraction method. Humic acid (HA) and fulvic acid (FA) isolated from paddy soils were characterized by Fourier transform infrared spectrometry (FT-IR) and 3-dimensional excitation-emission matrix (3D-EEM). The effect of humic substances on the binding of Ge was studied by fluorescence-quenching titration. Results showed that residual Ge was the dominant fraction in soil (up to 85%). The mobile Ge, organic matter bound Ge and easily reduceable compounds bound Ge accounted for approximately 10% of soil TGe and may represented critical labile pools of soil Ge. Organic matter bound Ge significantly correlated (r = 0.56, p < 0.01) with rice Ge concentrations. The fluorescence of HA and FA was markedly quenched by the addition of Ge. The conditional stability constant of HA-Ge complexes was larger than that of FA-Ge complexes, and the complexation capacity of HA-Ge complexes was lower than that of FA-Ge complexes. Humic substances played a dual role in affecting the behavior of dissolved Ge in paddy soil. HA formed stable complexes with Ge and tended to sequester Ge, while FA formed soluble and unstable complexes with Ge and tended to act as a Ge carrier in soil-plant system.

Germanium Reduces Inflammatory Damage in Mammary Glands During Lipopolysaccharide-Induced Mastitis in Mice

Ge is a trace element needed for good nutrition and health protection in animals and humans. Ge can be consumed by drinking or eating or administered by injection and transferred with the blood to exert pharmacological activities. The blood is important in the formation of milk. Mastitis is a serious health hazard in animals and humans. The present study explored the effect of Ge on mastitis and the potential underlying mechanism. A mastitis mouse model was established with LPS. mMECs were prepared for study in vitro. Histopathological changes showed that Ge had a protective effect on mammary gland tissues. Ge inhibited MPO activity to reduce inflammatory cell infiltration during mastitis. ELISA and qPCR results for tissues and cells showed that the expression of TNF-α, IL-1β, and IL-6 was decreased and that of IL-10 was increased by Ge in a dose-dependent manner in mastitis. An analysis of protein phosphorylation was performed with sandwich ELISAs for both tissues and mMECs. The results showed that Ge significantly inhibited the phosphorylation of IκB, NF-κB p65, p38, ERK, and JNK, which was dramatically increased by LPS. These results demonstrate that Ge has an inhibitory effect on inflammation that protects mammary gland tissues by inhibiting NF-κB and MAPK pathway activation and reducing TNF-α, IL-1β, and IL-6 expression. Ge may be an effective clinical treatment for mastitis and other inflammatory diseases.