Arsenate phytotoxicity regulation by humic acid and related metabolic mechanisms

Influence of humic acid on the bioaccumulation, elimination, and toxicity of PFOS and TBBPA co-exposure in Mytilus unguiculatus Valenciennes

Tetrabromobisphenol A (TBBPA) and Perfluorooctane sulfonate (PFOS) are emerging contaminants which coexist in marine environments, posing significant risks to ecosystems and human health. The behavior of these contaminants in the presence of dissolved organic matter (DOM), specifically the co-contamination of TBBPA and PFOS, is not well understood. The bioaccumulation, distribution, elimination, and toxic effects of TBBPA and PFOS on thick-shell mussels (Mytilus unguiculatus V.), with the absence and presence of humic acid (HA), a typical DOM, were studied. The results showed that the uptake of TBBPA decreased and the uptake of PFOS increased when exposed to 1 mg/L HA. However, at higher concentrations of HA (5 and 25 mg/L), the opposite effect was observed. Combined exposure to HA, TBBPA, and PFOS resulted in oxidative stress in the digestive gland, with the severity of stress dependent on exposure time and HA dose. Histological analysis revealed a positive correlation between HA concentration and tissue damage caused by TBBPA and PFOS. This study provides insights into the influence of HA on the bioaccumulation-elimination patterns and toxicity of TBBPA and PFOS in marine bivalves, offering valuable data for ecological and health risk assessments of combined pollutants in aquatic environments rich in DOM.

Effects of polystyrene, polyethylene, and polypropylene microplastics on the soil-rhizosphere-plant system: Phytotoxicity, enzyme activity, and microbial community

The widespread presence of soil microplastics (MPs) has become a global environmental problem. MPs of different properties (i.e., types, sizes, and concentrations) are present in the environment, while studies about the impact of MPs having different properties are limited. Thus, this study investigated the effects of three common polymers (polystyrene, polyethylene, and polypropylene) with two concentrations (0.01% and 0.1% w/w) on growth and stress response of lettuce (Lactuca sativa L.), soil enzymes, and rhizosphere microbial community. Lettuce growth was inhibited under MPs treatments. Moreover, the antioxidant system, metabolism composition, and phyllosphere microbiome of lettuce leaves was also perturbed. MPs reduced phytase activity and significantly increased dehydrogenase activity. The diversity and structure of rhizosphere microbial community were disturbed by MPs and more sensitive to polystyrene microplastics (PSMPs) and polypropylene microplastics (PPMPs). In general, the results by partial least squares pathway models (PLS-PMs) showed that the presence of MPs influenced the soil-rhizosphere-plant system, which may have essential implications for assessing the environmental risk of MPs.

Hazardous or Advantageous: Uncovering the Roles of Heavy Metals and Humic Substances in Shilajit (Phyto-mineral) with Emphasis on Heavy Metals Toxicity and Their Detoxification Mechanisms

Shilajit is a phyto-mineral diffusion and semi-solid matter used as traditional medicine with extraordinary health benefits. This study provides a comprehensive data on Shilajit with emphasis on heavy metal profile, associated toxicities, and metal detoxification mechanisms by humic substances present in Shilajit. Data was searched across papers and traditional books using Google Scholar, PubMed, Science Direct, Medline, SciELO, Web of Science, and Scopus as key scientific databases. Findings showed that Shilajit is distributed in almost 20 regions of the world with uses against 20 health problems as traditional medicine. With various humic substances, almost 11 biological activities were reported in Shilajit. This phyto-mineral diffusion possesses around 65 heavy metals including the toxic heavy metals like Cu, Al, Pb, As, Cd, and Hg. However, humic substances in Shilajit actively detoxify around 12 heavy metals. The recommended levels of heavy metals by WHO and FDA in herbal drugs is 0.20 and 0.30 ppm for Cd, 1 ppm for Hg, 10.00 ppm for As and Pb, 20 ppm for Cu, and 50 ppm for Zn. The levels of reported metals in Shilajit were found to be lower than the permissible limits set by WHO and FDA, except in few studies where exceeded levels were reported. Shilajit consumption without knowing permissible levels of metals is not safe and could pose serious health problems. Although the humic substances and few metals in Shilajit are beneficial in terms of chelating toxic heavy metals, the data on metal detoxification still needs to be clarified.

Regulation and mechanism of pyrite and humic acid on the toxicity of arsenate in lettuce

Pyrite and humic acid are common substances in nature, and the combined effects of pyrite and humic acid on arsenic phytotoxicity are more widespread in the actual environments than that of a single substance, but have received less attention. In this study, the interaction between pyrite and humic acid in arsenate solution was studied, and the effects of pyrite and humic acid on plant toxicity of arsenate were evaluated. The results showed that arsenate + pyrite + fulvic acid (V-PF) treatment immobilized more arsenic by forming chemical bonds such as AsS and Fe-As-O and reduced the migration of arsenic to plants. Compared to the arsenate + fulvic acid (VF), arsenate + pyrite (VP) and arsenate (V) group, the inorganic arsenic content of lettuce leaves in the V- PF group was reduced by 19.8 %, 13.4 % and 13.4 %, respectively. In addition, the V-PF group increased the absorption of Ca, Fe and Cu in plant roots, and improved the activity of superoxide dismutase (SOD) in plant leaves. Compared to the VF group, SOD and MDA in the V-PF group increased by 34.1 % in 30 days and decreased by 47.3 % in 40 days, respectively. The biomass of lettuce in V-PF group was increased by 29.3 % compared with that in VF group on day 50. The protein content of the V-PF group was 58.3 % higher than that of the VF group and 23.1 % higher than that of the VP group. Furthermore, metabolomics analysis showed that the V-PF group promoted glycolysis by up-regulating glyoxylic acid and dicarboxylic acid metabolism, thus reducing carbohydrate accumulation. Phosphocreatine metabolism was also up-regulated, which decreased the oxidative damage in lettuce induced by arsenic. This study will provide new ideas for scientifically and rationally assessing the ecological environmental risks of arsenic and regulating its toxicity.

Metabolomic profiling reveals key factors and associated pathways regulating the differential behavior of rice (Oryza sativa L.) genotypes exposed to geogenic arsenic

Arsenic (As) toxicity is an escalating problem; however, information about the metabolic events controlling the varied pattern of As accumulation in rice genotypes within their natural environment is still lacking. The present study is thus an advancement in unravelling the response of such rice genotypes. Soil-water-rice samples were analyzed for As accumulation using ICP-MS. Furthermore, we implemented metabolomics through LC-MS/MS and UHPLC to identify metabolic signatures regulating As content by observing the metalloid's composition in rice agrosystem. Results showed that rice genotypes differed significantly in their levels of metabolites, with Mini mansoori and Pioneer having the highest levels. Mini mansoori contained least As which might have been regulated by Ala, Ser, Glu, Phe, Asn, His, Ile, Lys, Gln, Trp, Tyr, chlorogenic, p-coumaric, trans-ferulic, rutin, morin, naringenin, kampferol, and myricetin, while Asp, Arg, Met, syringic, epigalocatechin, and apigenin contributed to the greater As acclimatization ability of Pioneer. Multivariate tools separated the rice genotypes into two major clusters: Pioneer-Mini mansoori and Damini-Sampoorna-Chintu. KEGG identified three major metabolic pathways (aminoacyl-tRNA, phenylpropanoid, and secondary metabolites biosynthesis route) linked with As tolerance and adaptation mechanisms in rice. Overall, these two genotypes symbolize their As hostile and accommodating attitudes probably due to the accumulated metabolites and the physicochemical attributes of the soil-water. Thus, thorough understanding of the metabolic reactions to As may facilitate the emergence of As tolerant/resilient genotypes. This will aid in the selection of molecular markers to cultivate healthier rice genotypes in As-contaminated areas.

Application of biochar and humic acid improves the physiological and biochemical processes of rice (Oryza sativa L.) in conferring plant tolerance to arsenic-induced oxidative stress

Biochar (BC) and humic acid (HA) are well-documented in metal/metalloid detoxification, but their regulatory role in conferring plant oxidative stress under arsenic (As) stress is poorly understood. Therefore, we aimed at investigating the role of BC and HA (0.2 and 0.4 g kg-1 soil) in the detoxification of As (0.25 mM sodium arsenate) toxicity in rice (Oryza sativa L. cv. BRRI dhan75). Arsenic exhibited an increased lipid peroxidation, hydrogen peroxide, electrolyte leakage, and proline content which were 32, 30, 9, and 89% higher compared to control. In addition, the antioxidant defense system of rice consisting of non-enzyme antioxidants (18 and 43% decrease in ascorbate and glutathione content) and enzyme activities (23-50% reduction over control) was decreased as a result of As toxicity. The damaging effect of As was prominent in plant height, biomass acquisition, tiller number, and relative water content. Furthermore, chlorophyll and leaf area also exhibited a decreasing trend due to toxicity. Arsenic exposure also disrupted the glyoxalase system (23 and 33% decrease in glyoxalase I and glyoxalase II activities). However, the application of BC and HA recovered the reactive oxygen species-induced damages in plants, upregulated the effectiveness of the ascorbate-glutathione pool, and accelerated the activities of antioxidant defense and glyoxalase enzymes. These positive roles of BC and HA ultimately resulted in improved plant characteristics with better plant-water status and regulated proline content that conferred As stress tolerance in rice. So, it can be concluded that BC and HA effectively mitigated As-induced physiology and oxidative damage in rice plants. Therefore, BC and HA could be used as potential soil amendments in As-contaminated rice fields.

Microplastics meet invasive plants: Unraveling the ecological hazards to agroecosystems

The objective of this study was to assess the combined impact of environmental microplastic pollution and biological invasion which represent critical global eco-environmental challenges. The invasion of Solidago canadensis L. and soil microplastic contamination in the agroecosystem pose severe hazards to soil and plant ecology and human health. Oryza sativa L. (rice) was examined after individual and combined exposure to Solidago canadensis L. invasion (SI) and soil polyethylene microplastic contamination (MPc). Comparing the individual and combination treatments to the control, leaf biomass decreased, with varying changes in carbon, nitrogen, and phosphorus. Antioxidant enzyme activity and reactive oxygen species levels were significantly reduced following SI exposure and increased following the combined treatment (SI × MP). In contrast, ascorbate peroxidase and catalase activities were reduced after the combined treatment. Due to the confluence of various abiotic stressors, the combined treatment had a higher impact on leaf metabolites than the singular SI and MPc treatments. However, in comparison, the combined treatment significantly influenced the metabolic profile. In conclusion, the interaction between SI and MPc resulted in significant metabolic alterations. These changes were characterized by shifts in metabolite pools influenced by antioxidant enzyme activities and nutrient content, ultimately enhancing defense mechanisms within rice crops. Consequently, these stressors threaten the food safety, sustainability, and agricultural output of crops. The co-exposure of invasive plants and microplastics sheds light on the bio-ecological risks associated with microplastics in staple foods and offers valuable insights into the phytotoxicity of invasive plants in the presence of polyethylene microplastics.

The crucial elements for lettuce (Lactuca sativa L.) growth under DMA stress and the linkage with DMA behavior: A new application of ionome

Dimethylarsinic acid (DMA) is one of the common arsenic (As) species present in soil and is more toxic to plants than others. Identifying the crucial elements for plant growth under DMA stress is essential to enhance plant tolerance to DMA. Herein, we provided for the first time an ionome-based approach to address this issue. The phenotype, As species and concentrations of 11 essential elements in lettuce tissues were monitored under exposures of 0.1, 0.5, 1, 2, 5 mg L-1 DMA in hydroponic culture for 32 days. Lettuces remained normal (no significant difference in phenotype from the control) under 0.1-2 mg L-1 DMA stress, and were inhibited with fresh weights of leaf and root under 5 mg L-1 DMA stress. Integrating the difference in ionome profiles between the two growth states (normal and inhibited) and the responses of the individual element, Mg and S were clarified as the most possible candidates for the crucial elements for lettuce growth under DMA stress. Under 5 mg L-1 DMA stress, the accumulation of Mg and S declined, yet their BCF values were significantly increased, which was consistent with the change in BCF of DMA. Based on the physiological functions of Mg and S and the toxicity of DMA, it could be inferred that the enhanced transfer of Mg and S to leaves should be induced by the potential damage caused by the increased DMA accumulation in leaves, and would result in a shortage of both elements in roots as well as the growth inhibition.

Interpretable machine learning-accelerated seed treatment using nanomaterials for environmental stress alleviation

Crops are constantly challenged by different environmental conditions. Seed treatment using nanomaterials is a cost-effective and environmentally friendly solution for environmental stress mitigation in crop plants. Here, 56 seed nanopriming treatments are used to alleviate environmental stresses in maize. Seven selected nanopriming treatments significantly increase the stress resistance index (SRI) by 13.9% and 12.6% under salinity stress and combined heat-drought stress, respectively. Metabolomics data reveal that ZnO nanopriming treatment, with the highest SRI value, mainly regulates the pathways of amino acid metabolism, secondary metabolite synthesis, carbohydrate metabolism, and translation. Understanding the mechanism of seed nanopriming is still difficult due to the variety of nanomaterials and the complexity of interactions between nanomaterials and plants. Using the nanopriming data, we present an interpretable structure-activity relationship (ISAR) approach based on interpretable machine learning for predicting and understanding its stress mitigation effects. The post hoc and model-based interpretation approaches of machine learning are integrated to provide complementary advantages and may yield more illuminating or trustworthy results for researchers or policymakers. The concentration, size, and zeta potential of nanoparticles are identified as dominant factors for correlating root dry weight under salinity stress, and their effects and interactions are explained. Additionally, a web-based interactive tool is developed for offering prediction-level interpretation and gathering more details about a specific nanopriming treatment. This work offers a promising framework for accelerating the agricultural applications of nanomaterials and may contribute to nanosafety assessment.

Regulation of arsenite toxicity in lettuce by pyrite and glutamic acid and the related mechanism

Compared with the effect of a single substance on arsenic plant toxicity, the effect of coexisting pyrite and natural organic matter can better reflect actual environmental conditions. In this study, the interaction between pyrite and glutamic acid in arsenite solution was explored, the influence of pyrite and glutamic acid on arsenite plant toxicity was evaluated, and the metabolic regulation mechanism of pyrite and glutamic acid on the arsenite phytotoxic effect was clarified by metabolomics analysis. Combined pyrite and glutamic acid treatment fixed more arsenic by forming chemical bonds such as AsS, AsO, and As-O-OH in culture solution and reduced inorganic arsenic levels in plants. Compared with glutamic acid alone and pyrite alone, the combined treatment reduced the inorganic arsenic concentration in plants by 4.7 % and 40.0 %, respectively. The combined treatment limited plant ROS accumulation and maintained the leaf chlorophyll content by increasing SOD synthesis. Compared with the effect of As(III) alone, the chlorophyll content increased by 15.1-21.0 % on average under the combined treatment. The combined treatment promoted the absorption of Ca, Cu, Fe, Mo and Zn in lettuce, enhanced plant adaptation to As(III) and significantly improved plant nutritional quality. Compared with glutamic acid alone, the combined treatment increased the VC, fiber and protein contents by 128.9 %, 202.8 % and 36.7 %, respectively. Metabolomics analysis indicated that in the combined treatment group, the upregulation of tyrosine, pyruvate and N metabolism increased the plant chlorophyll content. The upregulation of S metabolism increases VC synthesis in plants and inhibits ROS accumulation, thus maintaining normal plant growth and development. The upregulation of glutathione and glycine metabolism enhances plant stress resistance. This study will provide a new way to scientifically and rationally evaluate the ecological risk of arsenic and regulate its toxicity.

Toxicity effects of polystyrene nanoplastics and arsenite on Microcystis aeruginosa

Despite the increasing research on the fate of nanoplastics (NPs, <100 nm) in freshwater systems, little is known about the joint toxic effects of metal(loid)s and NPs modified with different functional groups on microalgae. Here, we explored the joint toxic effects of two types of polystyrene NPs [one modified with a sulfonic acid group (PSNPs-SO₃H), and one without this functional group (PSNPs)] and arsenic (As) on the microalgae Microcystis aeruginosa. The results highlighted that PSNPs-SO₃H showed a smaller hydrodynamic diameter and greater potential to adsorb positively charged ions than PSNPs, contributing to the more severe growth inhibition, while both of them produced oxidative stress. Metabolomics further revealed that the fatty acid metabolism of the microalgae was significantly up-regulated under both NPs exposure, while PSNPs-SO₃H down-regulated the tricarboxylic acid cycle (TCA cycle) of the microalgae. As uptake by algae was significantly reduced by 82.58 % and 59.65 % in the presence of 100 mg/L PSNPs and PSNPs-SO₃H, respectively. The independent action model showed that the joint toxicity of both NPs with As was assessed as antagonistic. In addition, PSNPs and PSNPs-SO₃H had dissimilar effects on the composition of the microalgae extracellular polymeric substances (EPS), resulting in different uptake and adsorption of As, thereby affecting the physiology and biochemistry of algae. Overall, our findings propose that the specific properties of NPs should be considered in future environmental risk assessments.

Bioaugmentation on humification during co-composting of corn straw and biogas slurry

In order to increase the nutrients and humic acid (HA) contents of corn straw (CS) derived organic fertilizer and recover resources from biogas slurry (BS) simultaneously, the co-composting of CS and BS was carried out with the addition of biochar and microbial agents including lignocellulose degrading and ammonia assimilating bacteria. The results showed that 1 kg straw could treat 2.5 L BS by recovering nutrients and bio-heat introduced evaporation. The bioaugmentation strengthened both the polyphenol and Maillard humification pathways by promoting the polycondensation of precursors (reducing sugars, polyphenols, and amino acids). HA obtained in the microbial-enhanced group (20.83 g/kg), biochar-enhanced group (19.34 g/kg), and combined-enhanced group (21.66 g/kg) were significantly higher than that in the control group (16.26 g/kg). The bioaugmentation achieved directional humification and reduced the loss of C and N by promoting the CN formation of HA. The humified co-compost had nutrient slow-release effect in agricultural production.

Potential mechanism of humic acid attenuating toxicity of Pb2+ and Cd2+ in Vallisneria natans

Humic substances are widely present in aquatic environments. Due to the high affinity of humic substances for metals, the interactions have been particularly studied. To assess the effect of humic acid (HA) on submerged macrophytes and biofilms exposed to heavy metal stress, Vallisneria natans was exposed to solutions containing different concentrations of HA (0.5-2.0 mg·L^-1), Pb²⁺ (1 mg·L^-1) and Cd²⁺ (1 mg·L^-1). Results suggested that HA positively affected the plant growth and alleviated toxicity by complexing with metals. HA increased the accumulation of metals in plant tissues and effectively induced antioxidant responses and protein synthesis. It was also noted that the exposure of HA and metals promoted the abundance and altered the structure of microbial communities in biofilms. Moreover, the positive effects of HA were considered to be related to the expression of related genes resulting from altered DNA methylation levels, which were mainly reflected in the altered type of demethylation. These results demonstrate that HA has a protective effect against heavy metal stress in Vallisneria natans by inducing effective defense mechanisms, altering biofilms and DNA methylation patterns in aquatic ecosystems.

Fulvic acid mitigates cadmium toxicity-induced damage in cucumber seedlings through the coordinated interaction of antioxidant enzymes, organic acid, and amino acid

Fulvic acid (FA) can significantly alleviate cadmium (Cd) stress, but the specific metabolic response of FA to Cd toxicity is still not clarified. In the present study, we used untargeted metabolomic [gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS)] analysis to profile cucumber metabolism in response to Cd stress after spray application of FA. Our results showed that 331 differentially enriched metabolites (DEMs) were identified in leaf materials. These DEMs were enriched in 21 shared pathways in comparative groups of "Cd treatment vs. the control treatment" and "FA + Cd treatment vs. the Cd treatment." Specifically, treatment with FA significantly enhanced the organic acid content (citric acid, isocitric acid, 2-oxoglutaric acid, fumaric acid, and malic acid), which would contribute to provide sufficient substrates for the tricarboxylic acid (TCA) cycle and amino acid biosynthesis, thereby ensuring the normal production of energy and amino acid. At the same time, FA significantly increased the amino acid content (aspartate, citrulline, histidine, leucine, and phenylalanine). The accumulation of organic acid and amino acid can act as chelating agents for heavy metal ions and as scavengers of reactive oxygen species (ROS), thereby reducing intracellular oxidative damage. Furthermore, the application of FA improves antioxidant enzymes and accelerates ROS clearance. The improved contents of organic acid and amino acid, and the increased activity of antioxidant enzymes both played a central role in the reduction of malondialdehyde (MDA, 14.08%), hydrogen peroxide (H₂O₂, 61.70%) contents, and superoxide anion radical (O₂^-, 30.41%) production rate in plants under Cd stress. Taken together, the present study demonstrates the effects of FA on the antioxidant capacity and carbohydrate and amino acid metabolism of cucumber seedlings exposed to Cd stress, which provides comprehensive insights into the regulation of plants' response to Cd toxicity with FA was applied in cucumber.

Enhanced generation of reactive oxygen species by pyrite for As(III) oxidation and immobilization: The vital role of Fe(II)

The migration and conversion of arsenic in the environment usually accompany by the redox of iron-bearing minerals. For instance, the oxidation of pyrite can generate reactive oxygen species (ROS) affecting the species of arsenic, but the types and roles of ROS have been unclear. This paper demonstrated the vital role of Fe(II) in the pyrite for the formation of ROS. Results showed that exogenous addition of Fe(II) significantly enhanced the removal rate of As(III) by pyrite. 2,2'-bipyridine (BPY) decreased the oxidation of As(III) by complexing with Fe²⁺ in solution, whilst EDTA enhanced the oxidation of As(III) by boosting the autoxidation of Fe²⁺. In addition, neutral pH is superior for the oxidation of As(III) and removal of total arsenic. Importantly, Methanol, SOD enzyme and PMOS inhibited 54%, 28% and 17.5% of As(III) oxidation, respectively, which indicated O₂^•- and •OH were the main contributors to As(III) oxidation, and Fe(IV) contributed a small part of As(III) oxidation. The content of As(V) in the FeS₂-Fe²⁺-As(III) system was higher than that in the FeS₂-As(III) system, further confirming the vital role of Fe(II) for As(III) oxidation. Lepidocrocite was produced in a single Fe²⁺ system, which was not detected in the FeS₂-As(III) system. Thus, the presence of mineral surfaces changed the oxidation products of Fe²⁺ and accelerated the oxidation and immobilization of As(III). FA (Fulvic Acid) and HA (Humic Acid) accelerated the oxidation of As(III), but the oxidation of As(III) by pyrite was inhibited to a certain extent, with increasing phenolic hydroxyl groups in phenolic acid. Our findings provide new insight into the oxidative species in the pyrite-Fe(II) system and will help guide the remediation of arsenic pollution in complex environmental systems.

The molecular characteristics of DOMs derived from bio-stabilized wastewater activated sludge and its effect on alleviating Cd-stress in rice seedlings (Oryza sativa L.)

To recycle fertilizing contents in wastewater activated sludge (WAS) is attracting increasing interest. Dissolved organic matters (DOMs) in WAS with high content are biologically active. In this work, the molecular composition of DOMs derived from two typical bio-stabilized WAS (DOM_BWS), aerobic composting (DOM_ACS) and anaerobic digestion (DOM_ADS), were analyzed. The mitigative effect and molecular mechanisms of DOM_BWS on rice seedlings (Oryza sativa L.) under Cd-stress were investigated. Our study indicated that DOM_BWS significantly alleviated Cd-stress and facilitated growth recovery of rice seedlings with distinct absorption mechanisms. DOM_ACS, primarily composed of CHO class with low molecular weight rich in carboxyl groups, forming labile Cd-DOM complexes, which promoted Cd-absorption of rice seedlings. While DOM_ADS comprised large molecular weight of CHON class interacted with Cd to produce stable macromolecular complexes in the form of microaggregates, consequently reducing Cd-absorption. At transcriptional level, DOM_BWS restored auxin signal transduction and phenylpropanoid biosynthesis pathways in root cells, and got the expression of glutathione S-transferase well. Besides, DOM_ACS significantly promoted the metabolism of amino acids to alleviate phytotoxicity, while DOM_ADS improved the DNA repair function of rice seedlings. These findings provided novel insights into land-use of bio-stabilized WAS for remediation of heavy metals contaminated soils and food security.

Bioaugmentation mechanism on humic acid formation during composting of food waste

In this study, microbes were added to food waste compost in order to investigate the bioaugmentation mechanism of Humic acid (HA) formation. Thermogravimetric analysis, structural equation model, Fourier transform infrared spectroscopy and statistical analysis were utilized to explain the bioaugmentation mechanism. The results showed that bioaugmentation increased humification rate and degree. Bioaugmentation not only promoted the formation of aromatic structures and CC bonds but also brought different change orders of functional groups in HA. The HA obtained in bioaugmentation group (BA, 7.51 g/kg) was significantly higher compared to the control group (CK, 2.37 g/kg). Similarly, the HA/FA of BA (1.90) was also higher than that of CK (0.62), and peaked at 2.34 on day 40. The polyphenol humification pathway played a major role regardless of the addition of inoculant. However, the exogenous microbes promoted protein and carbohydrate degradation in the initial stage, and the abundance of precursors (amino acids and reducing sugars) enhanced both Maillard and polyphenol humification pathways. When polyphenol was insufficient in later stage, bioaugmentation mainly embodied in the strengthening of Maillard humification pathway. This finding benefited the practice of directional humification process of food waste composting.

Arsenic Toxicity-Induced Physiological and Metabolic Changes in the Shoots of Pteris cretica and Spinacia oleracea

Arsenic is a ubiquitous toxic element that can be accumulated into plant parts. The present study investigated the response of Pteris cretica and Spinacia oleracea to As treatment through the analysis of selected physiological and metabolic parameters. Plants were grown in pots in As(V) spiked soil (20 and 100 mg/kg). Plants’ physiological condition was estimated through the determination of elements, gas-exchange parameters, chlorophyll fluorescence, water potential, photosynthetic pigments, and free amino acid content. The results confirmed differing As accumulation in plants, as well as in shoots and roots, which indicated that P. cretica is an As-hyperaccumulator and that S. oleracea is an As-root excluder. Variations in physiological and metabolic parameters were observed among As treatments. Overall, the results revealed a significant effect of 100 mg/kg As treatment on the analysed parameters. In both plants, this treatment affected growth, N, Mg, S, Mn, and Zn content, as well as net photosynthetic rate, chlorophyll fluorescence, and total free amino acid content. In conclusion, the results reflect the similarity between P. cretica and S. oleracea in some aspects of plants’ response to As treatment, while physiological and metabolic parameter changes related to As treatments indicate the higher sensitivity of S. oleracea.

Influence of Humic Acid on Oxidative Stress Induced by Arsenite and Arsenate Waterborne Exposure in Danio rerio

The studies on how humic acid (HA) influences the oxidative stress of arsenic in aquatic organism is limited. Using Danio rerio as case study, we explored the oxidative stress effects in aquatic organism after 96 h exposure to the HA and arsenic. Results revealed the co-exposure of HA and arsenite elevated the superoxide dismutase activities and downgraded the malondialdehyde. Thus, we speculate that HA may alleviate the oxidative stress induced by arsenite, which may be caused by the HA's coating in combination with the complexation of arsenite and HA. In addition, HA acted as the reactive oxygen species scavenger, promising to eliminate the oxygen free radicals. Contrastingly, HA may impact little on the arsenate exposure. This study can help better understand oxidative stress mechanism of co-exposure of arsenic and HA in aquatic environment.