Biological soil crusts alleviate the stress of arsenic on rice germination and the underlying immobilization mechanisms

The adaptability, distribution, ecological function and restoration application of biological soil crusts on metal tailings: A critical review

A large amount of metal tailings causes many environmental issues. Thus, the techniques for their ecological restoration have garnered extensive attention. However, they are still in the exploratory stage. Biological soil crusts (BSCs) are a coherent layer comprising photoautotrophic organisms, heterotrophic organisms and soil particles. They are crucial in global terrestrial ecosystems and play an equal importance in metal tailings. We summarized the existing knowledge on BSCs growing on metal tailings. The main photosynthetic organisms (cyanobacteria, eukaryotic algae, lichens, and mosses) of BSCs exhibit a high heavy metal(loid) (HM) tolerance. BSCs also have a strong adaptability to other adverse conditions in tailings, such as poor structure, acidification, and infertility. The literature about tailing BSCs has been rapidly increasing, particularly after 2022. The extensive literature confirms that the BSCs distributed on metal tailings, including all major types of metal tailings in different climatic regisions, are common. BSCs perform various ecological functions in tailings, including HM stress reduction, soil structure improvement, soil nutrient increase, biogeochemical cycle enhancement, and microbial community restoration. They interact and accelerate revegetation of tailings (at least in the temperate zone) and soil formation. Restoring tailings by accelerating/inducing BSC formation (e.g., resource augmentation and inoculation) has also attracted attention and achieved small-scale on-site application. However, some knowledge gaps still exist. The potential areas for further research include the relation between BSCs and HMs, large-scale quantification of tailing BSCs, application of emerging biological techniques, controlled laboratory experiments, and other restoration applications.

The dissolution characteristics of cadmium containing birnessite produced from paddy crusts

The cadmium (Cd) accumulates in birnessite as it forms on the surface of paddy crusts (PC). The stability of Cd-containing birnessite is influenced by environmental factors, and destabilized birnessite releases dissolved Cd. We report the effects of pH, oxalic acid, and light on the dissolution of Cd-containing birnessite. We found that at pH 4.0, with light and 0.20 mol/L oxalic acid, the ratio of dissolved Cd and manganese (Mn) peaked after 24 h at 2978.0 μg/g and 326.8 mg/g, respectively. The three environmental factors affected the dissolution of Cd-containing birnessite in the following order: pH > oxalic acid > light. During dissolution process, Cd and Mn did not dissolve simultaneously, and the dissolved Cd/Mn ratio in the solution was significantly lower than that of the pristine mineral (33.5 × 10-3). Compared with Mn, Cd dissolution was inhibited by strong acidity (pH 4.0-5.0), and the dissolved Cd/Mn ratio was 5-10 × 10-3. Mild acidity (pH 6.0) was weakly inhibitory, with a Cd/Mn ratio of 6-15 × 10-3. In an alkaline (pH 8.0) oxalate environment, light illumination inhibited Cd dissolution, and the Cd/Mn ratio decreased over time due to the stability of the products formed by oxalate and carbonate, with Cd being more stable than those formed by Mn. Our findings would provide insights into the migration and transformation of PC-associated Cd in paddy fields.

Influence of extracellular polymeric substances on arsenic bioaccumulation and biotransformation in biofilms

It is well recognized that biofilms can biosorb and biotransform heavy metals in aquatic environments. However, the effects of extracellular polymeric substance (EPS) on inorganic arsenic (As) bioaccumulation and biotransformation in biofilms are still unrevealed and need to be investigated. In order to explore the above scientific issues, the As accumulation and speciation in EPS-containing or EPS-free biofilms and growth medium under As(V)/As(III) exposure conditions were measured. After the removal of EPS, the amount of As uptake (Asup) and As adsorption (Asad) in biofilms were significantly reduced, no matter whether exposed to As(V) or As(III). FTIR analysis further suggested that the interaction between these functional groups with As was limited after the removal of EPS. In the EPS-containing biofilms, the Asad was mainly As(V) with low toxicity. However, after the removal of EPS, the Asad was mainly As(III) with high fluidity, and no methylated As was found. Moreover, the removal of EPS inhibited As(III) oxidation and methylation by biofilms, resulting in the decrease of As(V) and methylated As in the growth medium. The findings of this study emphasized the essential impact of EPS on the biosorption and biotransformation of As in biofilms. This study provides a unique understanding of the role of biofilms in As biogeochemical cycle, and water quality purification function in water environments.

Native microalgae and Bacillus XZM remediate arsenic-contaminated soil by forming biological soil crusts

Biological soil crusts (BSCs) are a useful tool for immobilization of metal(loid)s in mining areas. Yet, the typical functional microorganisms involved in promoting the fast development of BSCs and their impacts on arsenic(As) contaminated soil remain unverified. In this study, As-contaminated soil was inoculated with indigenous Chlorella thermophila SM01 (C. thermophila SM01), Leptolyngbya sp. XZMQ, isolated from BSCs in high As-contaminated areas and plant growth-promoting (PGP) bacteria (Bacillus XZM) to construct BSCs in different manners. After 45 days of ex-situ culture experiment, Leptolyngbya sp. XZMQ and bacteria could form obvious BSCs. Compared to single-inoculated microalgae, the co-inoculation of Leptolyngbya sp. XZMQ and Bacillus XZM increased soil pH and water content by 10% and 26%, respectively, while decreasing soil EC and density by 19% and 14%, respectively. The soil catalase, alkaline phosphatase, sucrase, and urease activities were also increased by 30.53%, 96.24%, 154.19%, and 272.17%, respectively. The co-inoculation of Leptolyngbya sp. XZMQ and Bacillus XZM drove the formation of BSCs by producing large amounts of extracellular polymeric substances (EPS). The three-dimensional fluorescence spectroscopy (3D-EEM) analysis showed that induced BSCs increased As immobilization by enhancing the contents of tryptophan and tyrosine substances, fulvic acid, and humic acid in EPS. The presence of the -NH2 and -COOH functional groups in tryptophan residues were determined using Fourier Transform Infrared Spectroscopy (FTIR). X-Ray Diffraction (XRD) analysis showed that there were iron (hydrogen) oxides in BSCs, which could form ternary complexes with humic acid and As, thereby increasing the adsorption of As. Therefore, BSCs formed by co-inoculation of Leptolyngbya sp. XZMQ and Bacillus XZM increased the immobilization of As, thereby reducing the content of soluble As in the environment. In summary, our findings innovatively provided a new method for the remediation of As-contaminated soil in mining areas.

Indigenous cyanobacteria enhances remediation of arsenic-contaminated soils by regulating physicochemical properties, microbial community structure and function in soil microenvironment

Biocrust was widely used for the immobilization and removal of arsenic (As) in drainage systems of rice fields and mining areas. In this study, the role of an indigenous cyanobacteria (Leptolyngbya sp. XZMQ) was explored in the bioremediation of As-contaminated farmland and tailing soil. After 80 d of inoculation with cyanobacteria, total As (As(T)) accumulated in the cyanobacterial crust of farmland and tailing soil was 279.89 mg kg^-1 and 269.57 mg kg^-1, respectively, and non-EDTA exchangeable fraction was the major fraction of it. The As(T) in farmland and tailing soil of micro-environment decreased by 10.76% and 12.73%, respectively. Meanwhile, the available As (As(a)) decreased by 21.25% and 27.65%, respectively. The XRD results showed that hematite and SiO₂ existed in cyanobacterial crust of farmland and tailing soil. FTIR spectra indicated that the adsorption of As in cyanobacterial crust was mediated by OH and CO. After inoculation of Leptolyngbya sp. XZMQ, in subcrust soil, As biotransformation gene aioA was the most abundant, followed by arsM. The dominant phyla of soil biota were Proteobacteria, Cyanobacteria, Actinobacteria, and Bacteroiota, which could play critical roles in shaping aioA and arsM harboring microbe communities in soil. Redundancy analysis (RDA) showed that soil organic carbon (OC), pH, and chlorophyll a (Chl a) were the most important environmental factors in altering soil bacterial communities. Correlation analysis showed the Leptolyngbya had a positive correlation with Chl a, effective nitrogen (N(a)), electrical conductivity (EC), OC, pH in the soil, respectively, while it had a significant negative correlation with As(a), As(III) and As(T). These results emphasized on the significance of cyanobacteria in the behavior of As in mine soils and offered a promising strategy for bioremediation of As-contaminated soil in the mining area.

More than just a substrate for mites: Moss-dominated biological soil crust protected population of the oribatid mite, Oppia nitens against cadmium toxicity in soil

Metal-impacted sites often need aggressive ecorestoration strategies to restore a functional plant-soil system. The use of biological soil crusts for soil stabilization, moisture retention and C and N input in disturbed and contaminated soils is becoming a more common ecorestoration practice. Biological soil crusts comprise cyanobacteria, fungi, lichens, and bryophytes (mostly moss). Moss-dominated BSCs provide significant N mineralization rate in most terrestrial ecosystems. Oribatid mites or moss mites dominate moss-dominated BSCs and provide essential ecosystem services such as decomposition and nutrient cycling. We hypothesized that moss-dominated BSCs would create a high-quality habitat niche for O. nitens to resist Cd-induced toxicity. Adult mites were exposed to Cd for 28 days in soil with or without BSCs that were aged for eight months. Cadmium toxicity to mites in soil without BSCs was 1.7 and 5.4times greater than in soil with BSCs, respectively for the mites reproduction and instantaneous population growth rate (PGR_i). The moss-dominated BSC did not reduce Cd bioavailability in the mites but increased the mite's resilience to Cd toxicity, likely mediated by the trophic transfer of calcium from the BSC to the mites. Our work identifies a second mechanistic avenue by which BSCs are useful for ecorestoration, i.e., the improvement of soil invertebrate physiology to resist metal stress.

Immobilization on anionic metal(loid)s in soil by biochar: A meta-analysis assisted by machine learning

Metal pollution in soil has become one of the most serious environmental problems in China. Biochar is one of the most widely used remediation agents for soil metal pollution. However, the literature does not provide a consistent picture of the performance of biochar on the immobilization of anionic metal(loid)s, especially arsenic, in soil. To obtain a baseline understanding on the interactions of metals and biochar, 597 data records on four metal(loid)s (As, Cr, Sb and V) were collected from 70 publications for this meta-analysis, and the results are highlighted below. Biochar has a significant immobilization effect on anionic metal(loid)s in soil and reduces the bioavailability of these metals to plants. Subgroup analysis found that biochar could decrease the potential mobility of Cr, Sb and V, but the immobilization effect on As was not always consistent. Meanwhile, biochar pH and soil pH are the most key factors affecting the immobilization effect. To summarize, biochar can effectively immobilize Cr, Sb and V in soil, but more attention should be given to As immobilization in future applications. By regulating the properties of biochar and appropriate modification, anionic metal(loid)s in soil can be immobilized more effectively. Hence, both of the soil quality and crop quality can be improved.