Effect of root exudates on sorption, desorption, and transport of phenanthrene in mangrove sediments

Enhanced biodegradation of benzo[a]pyrene with Trametes versicolor stimulated by citric acid

Polycyclic aromatic hydrocarbons (PAHs) are a class of persistent organic pollutants with carcinogenic, mutagenic and teratogenic effects. The white-rot fungi in the fungal group have significant degradation ability for high molecular weight organic pollutants. However, exogenous fungi are easily antagonized by indigenous microorganisms. Low molecular weight organic acids, a small molecular organic matter secreted by plants, can provide carbon sources for soil microorganisms. Combining organic acids with white rot fungi may improve the nutritional environment of fungi. In this study, immobilized Trametes versicolor was used to degrade benzo[a]pyrene in soil, and its effect on removing benzo[a]pyrene in soil mediated by different low molecular weight organic acids was investigated. The results showed that when the degradation was 35 days, the removal effect of the experimental group with citric acid was the best, reaching 43.7%. The degradation effect of Trametes versicolor on benzo[a]pyrene was further investigated in the liquid medium when citric acid was added, and the effects of citric acid on the biomass, extracellular protein concentration and laccase activity of Trametes versicolor were investigated by controlling different concentrations of citric acid. In general, citric acid can act as a carbon source for Trametes versicolor and promote its extracellular protein secretion and laccase activity, thereby accelerating the mineralization of benzo[a]pyrene by Trametes versicolor. Therefore, citric acid can be used as a biostimulant in the remediation of PAHs contaminated soil with Trametes versicolor.

Role of Root Exudates on the Soil Microbial Diversity and Biogeochemistry of Heavy Metals

Due to extensive industrialization and escalation in pollution, the world is facing problems related to soil heavy metal pollution. The traditional ways of soil remediation are neither feasible nor cost-effective in most of the real-world scenarios, where metal concentration is relatively low in soil. Therefore, phytoremediation using plants and plant secretions to remediate heavy metal-contaminated soil is recently getting more attention. The plant root exudates act as an ecological driver in the rhizospheric region where they influence and guide the microbial community to function in such a way that can be advantageous for plant growth. They also promote phytoremediation process by altering the bioavailability of pollutants in soil. Root exudates affect the biogeochemical properties of heavy metals as well. In this review, existing literature on the role of root exudates (natural as well as artificial) on the phytoremediation of heavy metal-contaminated (particularly lead) soil is reviewed. The effect of root exudates on the biogeochemistry of lead in soil is also discussed.

Evaluation of negative effect of Naphthenic acids (NAs) on physiological metabolism and polycyclic aromatic hydrocarbons adsorption of Phragmites australis

Toxic substances in the environment disturb the adsorption of pollutants in plants but little is known about the underlying mechanisms of these processes. This study evaluated the PAH adsorption by Phragmites australis under NAs stress. Results showed that Naphthenic acids (NAs) significantly decreased the adsorption of PAHs and had higher selectivity for type and structure. P. australis root cell growth and mitosis were significantly affected by NAs, which was accompanied by serious disturbances in mitochondrial function. The physiological evaluation showed the NAs could increase Reactive Oxygen Species (ROS) accumulation by around 16-fold and cause damage to the root cell normal redox equilibrium. The levels of three key related antioxidants, PLA, CAT and POD, decreased significantly to 35-50% under NAs stress and were dependent upon NAs concentration. Furthermore, NAs could significantly change the concentration and species of root exudates of P. ausralis. Autotoxic substances, including alcohol and amines, increased by 28.63% and 23.96, respectively. Sixteen compounds were identified and assumed as potential biomarkers. Galactonic, glyceric, and octadecanoic acid had the general effect of activating PAH in soil. The global view of the metabolic pathway suggests that NAs influenced the citric acid cycle, fatty acid synthesis, amino acid metabolism and the phenylpropanoid pathway. Detection data results indicated that the energy products cause hypoxia and oxidative stress, which are the main processes under the NAs. Furthermore, verification of these processes was fulfilled through gene expression and biomarkers quantification. Our results provide novel metabolic insights into the mechanisms of PAHs adsorption by P. australis under NAs disturbance, suggesting that monitoring NAs in phytoremediation applications is necessary.

The root signals in rhizospheric inter-organismal communications

Root exudates play a key role in mediating plant-plant and plant-rhizomicrobiome interactions, including regulating biochemical/physiological aspects of plant-associated microorganisms, to enhance host plant growth and resilience. Root exudates can act as signals to reduce the competition from neighboring plants and recruiting/choreographing a wide range of diverse rhizomicrobiome members to make the host plant a good fit with its immediate environment. Root exudate production is a dynamic and key process, but there is a limited understanding of the metabolites or metabolic pathways involved in the inter-organismal communications facilitated by them. Given the well-known symbiotic relationships between plants and associated rhizomicrobiome members, adding root exudates to microbial isolation media may allow some of the large segments of rhizomicrobiome members that are not currently culturable to be grown in vitro. This will provide new insights into how root signals orchestrate associated microbes, will benefit agricultural production in the face of challenges posed by climate change, and will help to sustainably provide food for a growing global human population.

Effect of dissolved organic matter and heavy metals ions on sorption of phenanthrene at sedimentary particle scale

Human activities significantly increase the input of offshore heavy metals and organic pollutants. Although particle-scale and heterogeneous organic matters are fundamentally important to the fate of hydrophobic organic compounds (HOCs), deep understanding of the adsorption mechanism of HOCs on soil/sediment particles under the influence of heavy metal and organic pollution input is needed. This study investigates the effects of exotic DOM and heavy metals ions on the phenanthrene adsorption on sediment fractions. The adsorption experiments demonstrated that exotic DOM increased phenanthrene adsorption amount of sediment, with the greatest enhancement on clay particles (<2 µm). Nevertheless, the mechanism was differentiated accordingly to particle dimensions in terms of increased binding coefficients and mobility of phenanthrene. Furthermore, the introduction of heavy metals considerably enhanced the nonlinear sorption of phenanthrene. The Freundlich exponent N reduced by 0.01-0.24 when adding Cu²⁺, Zn²⁺ and Pb²⁺, especially for coarse particles (31-63 µm) fraction. In comparison, the enhancement of nonlinearity adsorption by Cu²⁺ and Zn²⁺ is significantly lower than Pb²⁺ ions. To our knowledge, the particle-scale study broadens the horizon of environmental fate and ecological risk of HOCs in intertidal regions, which is significantly affected by tidal action.

A comprehensive review of recent research concerning the role of low molecular weight organic acids on the fate of organic pollutants in soil

Plants exude through the roots different compounds, including, among others, low-molecular weight organic acids (LMWOAs), with a relevant effect on multiple metabolic activities. Numerous studies have revealed their role in improving soil mineral acquisition and tolerance against inorganic pollutants. However, less information is available on how they may alter the fate of organic pollutants in soil, which may cause environmental problems, compromise soil quality and have a detrimental effect on animal and human health. This review intends to cover recent studies (from 2015 onwards) and provide up-to-date information on how LMWOAs influence environmental key processes of organic pollutants in soil, like adsorption/desorption, degradation and transport, without forgetting plant uptake, with obvious environmental and health repercussions. Critical knowledge gaps and future research needs are also discussed, because understanding these processes will help searching effective strategies for pollutant reduction and control in soil.

Uptake and transport of steroid estrogens in soil-plant systems and their dissipation in rhizosphere: Influence factors and mechanisms

Residual steroid estrogens (SEs) in soil may be absorbed by plants, and subsequently threaten human health via food chains. However, the environmental behavior of SEs in soil-plant systems remains unclear. In this study, a wheat pot experiment using rhizosphere bags was performed to investigate the uptake and dissipation of target SEs (17 beta-estradiol (E2) and estrone (E1)) in different soils. The results indicated that soils with higher organic matter and silt and clay reduced the plant uptake of estrogens. Compared with E1, E2 was less accumulated in plants, which was mainly correlated with its higher hydrophobicity and shorter half-life. Estrogens tended to concentrate in the plant roots instead of translocating to the shoots. In addition, plant cultivation enhanced estrogen dissipation in the rhizosphere with an improvement of 10-21%. This improvement mainly resulted from stimulating the activities of estrogen-degrading enzymes, increasing the total bacterial populations, and promoting the development of estrogen degraders. Furthermore, this promotion effect will increase with plant growth. These findings will help us understand the characteristics of SEs taken up by plants and the role of the rhizosphere in SEs elimination, and provide theoretical insights into reducing the pollution risk of SEs in agricultural soils.

Assessment of dynamic microbial community structure and rhizosphere interactions during bioaugmented phytoremediation of petroleum contaminated soil by a newly designed rhizobox system

To understand the plant (Vigna unguiculata) and plant-growth promoting bacteria (PGPB) (Microcococcus luteus WN01) interactions in crude oil contaminated soil, experiments were conducted based on the newly designed rhizobox system. The rhizobox was divided into three main compartments namely the rhizosphere zone, the mid-zone, and the bulk soil zone, in accordance with the distance from the plant. Plants were grown in these three-chambered pots for 30 days under natural conditions. The plant root exudates were determined by analyzing for carbohydrates, amino acids, and phenolic compounds. The degradation of alkane, polycyclic aromatic hydrocarbons (PAHs), and total petroleum hydrocarbons (TPHs) were quantified by GC-FID. Soil catalase, dehydrogenase, and invertase activities were determined. The microbial community structure was assessed using denaturing gradient gel electrophoresis (DGGE). Results showed that the inoculation of M. luteus WN01 significantly enhanced cowpea root biomass and exudates, especially the phenolic compounds. Bioaugmented phytoremediation by cowpea and M. luteus promoted rhizodegradation of TPH. Cowpea stimulated microbial growth, soil dehydrogenase, and invertase activities and enhanced bacterial community diversity in oil contaminated soil. The rhizosphere zone of cowpea inoculated with M. luteus showed the highest removal efficiency, microbial activities, microbial population, and bacterial community diversity indicating the strong synergic interactions between M. luteus and cowpea.

Organophosphate pesticides sequestered in tissues of a seagrass species - Zostera capensis from a polluted watershed

Organophosphate pesticides (OPPs) are persistent in the environment, but little information is available on their bioaccumulation in seagrass. In this study, the seagrass - Zostera capensis was collected from Swartkops Estuary in South Africa to investigate the bioaccumulation of OPPs from contaminated sediments and the water column. This plant was chosen because it grows abundantly in the estuary's intertidal zone, making it a viable phytoremediator in the urban environment. Extraction was performed by the QuEChERS method followed by GC-MS analysis. The mean concentration of ∑OPPs ranged from 0.01 to 0.03 μg/L for surface water; 6.20-13.35 μg/kg dw for deep-rooted sediments; 18.79-37.75 μg/kg dw for leaf tissues and 12.14-39.80 μg/kg dw for root tissues of Z. capensis. The biota-sediment accumulation factors (BSAFs) were greater than one, indicating the potential for Z. capensis to bioaccumulate and intercept the targeted pesticides. A weak insignificant correlation observed between log BSAFs and log Kow indicates that the bioaccumulation of OPPs in tissues of Z. capensis were not dependent on the Kow. Eight of the selected pesticides had root-leaf translocation factors (TFr-l) greater than 1, indicating that Z. capensis can transport these chemicals from roots to leaves. The results from this study implies that this plant species can clean up OPP contamination in the environment.

Metagenomic analysis of microbial communities continuously exposed to Bisphenol A in mangrove rhizosphere and non-rhizosphere soils

Bisphenol A (BPA) is widely distributed in littoral zones and may cause adverse impacts on mangrove ecosystem. Biodegradation and phytoremediation are two primary processes for BPA dissipation in mangrove soils. However, the rhizosphere effects of different mangrove species on BPA elimination are still unresolved. In this study, three typical mangrove seedlings, namely Avicennia marina, Bruguiera gymnorrhiza (L.) and Aegiceras corniculatum, were cultivated in soil microcosms for four months and then subjected to 28-day continuous BPA amendment. Un-planted soil microcosms (as control) were also set up. The BPA residual rates and root exudates were monitored, and the metabolic pathways as well as functional microbial communities were also investigated to decipher the rhizosphere effects based on metagenomic analysis. The BPA residual rates in all planted soils were significantly lower than that in un-planted soil on day 7. Both plantation and BPA dosage had significant effects on bacterial abundance. A distinct separation of microbial structure was found between planted and un-planted soil microcosms. Genera Pseudomonas and Lutibacter got enriched with BPA addition and may play important roles in BPA biodegradation. The shifts in bacterial community structure upon BPA addition were different among the microcosms with different mangrove species. Genus Novosphingobium increased in Avicennia marina and Bruguiera gymnorrhiza (L.) rhizosphere soils but decreased in Aegiceras corniculatum rhizosphere soil. Based on KEGG annotation and binning analysis, the proposal of BPA degradation pathways and the quantification of relevant functional genes were achieved. The roles of Pseudomonas and Novosphingobium may differ in lower BPA degradation pathways. The quantity variation patterns of functional genes during the 28-day BPA amendment were different among soil microcosms and bacterial genera.

Responses of oil degrader enzyme activities, metabolism and degradation kinetics to bean root exudates during rhizoremediation of crude oil contaminated soil

During rhizoremediation process, plant roots secrete the specific exudates which enhance or stimulate growth and activity of microbial community in the rhizosphere resulting in effective degradation of pollutants. The present study characterized cowpea (CP) and mung bean (MB) root exudates and examined their influences on the degradation of total petroleum hydrocarbons (TPHs) and polycyclic aromatic hydrocarbons (PAHs) by the two oil degraders Micrococcus luteus WN01 and Bacillus cereus W2301. The effects of root exudates on soil microbial population dynamic and their enzymes dehydrogenase (DHA), and catechol 2,3 dioxygenase (C23O) activities were assessed. Both root exudates enhanced the degradation by both oil degraders. Cowpea root exudates maximized the removal of TPHs and PAHs by M. luteus WN01. Both bacterial population and DHA increased significantly in the presence of both root exudates. However, the C23O activities were significantly higher in WN01 treated. No significant influence of root exudates was observed on the C23O activities of W2301 treated. By using gas chromatography -mass spectroscopy, the dominant compounds found in cowpea and mung bean root exudates were 4-methoxy-cinnamic acid and terephthalic acid. Found in lower amount were propionic, malonic acid, and citric acid which were associated with enhanced PAHs desorption from soil and subsequent degradation. Novelty statement This is the first study to characterize the low molecular weight organic acids from root exudates of cowpea and mung bean and their influences on hydrocarbon desorption and hence enhancing the biodegradation process. The findings of the present study will greatly contribute to a better understanding of plant-microbe interaction in total petroleum hydrocarbons contaminated soil.

Oxalic Acid in Root Exudates Enhances Accumulation of Perfluorooctanoic Acid in Lettuce

Perfluorooctanoic acid (PFOA) is bioaccumulative in crops. PFOA bioaccumulation potential varies largely among crop varieties. Root exudates are found to be associated with such variations. Concentrations of low-molecular-weight organic acids (LMWOAs) in root exudates from a PFOA-high-accumulation lettuce variety are observed significantly higher than those from PFOA-low-accumulation lettuce variety (p < 0.05). Root exudates and their LMWOAs components exert great influences on the linear sorption-desorption isotherms of PFOA in soils, thus activating PFOA and enhancing its bioavailability. Among root exudate components, oxalic acid is identified to play a key role in activating PFOA uptake, with >80% attribution. Oxalic acid at rhizospheric concentrations (0.02-0.5 mM) can effectively inhibit PFOA sorption to soils by decreasing hydrophobic force, electrostatic attraction, ligand exchange, and cation-bridge effect. Oxalic acid enhances dissolution of metallic ions, iron/aluminum oxides, and organic matters from soils and forms oxalate-metal complexes, based on nuclear magnetic resonance spectra, ultraviolet spectra, and analyses of metal ions, iron/aluminum organometallic complexes, and dissolved organic carbon. The findings not only reveal the activation process of PFOA in soils by root exudates, particularly oxalic acid at rhizospheric concentrations, but also give an insight into the mechanism of enhancing PFOA accumulation in lettuce varieties.

The Role of Biochar in Reducing the Bioavailability and Migration of Persistent Organic Pollutants in Soil-Plant Systems: A Review

The amendment of biochar in soils contaminated with persistent organic pollutants (POPs) is an environmentally friendly in situ remediation measure. Numerous studies focused on the application of biochars to reduce the uptake of POPs by plants in soils. In this review, we summarized the role of biochar in reducing the migration of POPs in soil-plant systems. The mechanisms of biochar reducing the bioavailability of POPs in the soil, i.e. immobilization and promoted biodegradation, and the influencing factors are fully discussed. Especially in rhizosphere amended with biochar, the synergistic effect of POPs-root exudates-microorganisms on the reduced bioavailability of POPs is analyzed. This paper suggests that future researches should focus on the long-term environmental fate of POPs sorbed on high-temperature biochars and the long-term impacts of low-temperature biochars on the interaction of POPs-root exudates-rhizosphere microorganisms. All the above are necessary for efficient and safe use of biochar for remediating POP-contaminated farmland soils.

Adsorption of cadmium and lead ions by phosphoric acid-modified biochar generated from chicken feather: Selective adsorption and influence of dissolved organic matter

To improve the adsorption efficiency, a H₃PO₄-modified biochar (CFCP) was prepared using chicken feather and applied to Cd²⁺ and Pb²⁺ adsorption. The pseudo-second-order model could explain the Cd²⁺ and Pb²⁺ adsorption behavior. CFCP had faster adsorption rate than non-modified biochar (CFC2). The Langmuir and Freundlich isotherm could better describe the Cd²⁺ and Pb²⁺ adsorption, respectively. The value of q_m for Cd²⁺ adsorption and K_F for Pb²⁺ adsorption by CFCP was 7.84 mg·g^-1 and 24.41 mg^1-(1/n)·L^1/n·g^-1, which was 1.38 and 5.41 times of the corresponding results of CFC2. Relative to Cd²⁺, Pb²⁺ was selectively adsorbed by biochars in the binary metal system. Phosphate precipitation explained in part the selective adsorption of Pb²⁺. Proline, glucose, and pH (4-6) had little influence on Cd²⁺ and Pb²⁺ adsorption. Electrostatic interaction, precipitation, and O-H bonds were the primary adsorption mechanisms. The increased N-containing heterocycles of CFCP accounted for the increased Cd²⁺ and Pb²⁺ adsorption.

Bioaccumulation and cycling of polybrominated diphenyl ethers (PBDEs) and dechlorane plus (DP) in three natural mangrove ecosystems of South China

Polybrominated diphenyl ethers (PBDEs) and dechlorane plus (DP) in mangrove sediments and tissues of nine species from three Mangrove Reserves of Hainan Island were studied. The average concentrations of PBDEs and DP in mangrove leaves, branches, roots and fruits were 1048, 498, 546 and 364 pg g^-1 dw, and 294, 181, 108 and 165 pg g^-1 dw, respectively. The elevated PBDEs and DP concentrations in mangrove leaves may be caused by atmospheric sedimentation. The predominant PBDE congeners in sediments were BDE-209 and those in mangrove tissues were BDE-28. The average f_anti (ratio of [anti-DP]/[DP]) of DP in sediments and tissues were 0.47 and 0.32, respectively. Sonneratia hainanensis, a fast growing mangrove plant, has a relatively high tolerance and absorptive capacity to PBDEs and DP in sediments, suggesting that it could be used as an effective plant for phytoremediation. The biota sediment accumulation factors (BSAFs) of PBDEs in mangrove branches were positively correlated with log K_OW (R² = 0.43, p < 0.05). The standing accumulation, annual absorption, annual net retention, annual return, and turnover period of PBDEs and DP in mangrove tissues of the ecosystems were estimated, and the results indicated that mangroves are playing an important role in retaining PBDEs and DP.

Bioaccumulation and Cycling of Polycyclic Aromatic Hydrocarbons (PAHs) in Typical Mangrove Wetlands of Hainan Island, South China

Mangrove wetlands are important coastal ecosystems in tropical and subtropical regions, and mangrove sediments and tissues often are the pollutant sinks due to their high organic matter contents. Polycyclic aromatic hydrocarbons (PAHs) in the mangrove sediments and tissues of nine species from three typical mangrove wetlands of Hainan Island were studied. The average concentration of PAHs in all mangrove tissues was 403 ng g^-1 dw, with PAHs concentrations in leaf, branch, root, and fruit of 566, 335, 314, and 353 ng g^-1 dw, respectively. PAHs levels were much higher in leaf than in other mangrove tissues, which may be caused partly by atmospheric deposition of PAHs. The dominant individual PAH compounds in mangrove tissues were phenanthrene (41.3%), fluoranthene (14.7%), and pyrene (11.4%), while in sediments were naphthalene (73.4%), phenanthrene (3.9%), and pyrene (3.6%), respectively. The biota-sediment accumulation factors of PAH congeners in the mangrove wetlands showed different patterns, with the most predominant of phenanthrene. The cycling of PAHs in the mangrove wetlands of Hainan Island also were estimated, and the results showed that the standing accumulation, the annual absorption, the annual net retention, the annual return, and the turnover period in all mangrove tissues of the community were 2228 µg m^-2, 869 µg m^-2 a^-1, 206 µg m^-2 a^-1, 663 µg m^-2 a^-1, and 3.4 a, respectively. These results indicated that mangroves are playing an important role in retaining PAHs.

The role of artificial root exudate components in facilitating the degradation of pyrene in soil

Root exudates play an important role in the phytoremediation of soils contaminated by organic pollutants, but how root exudate components affect the remediation process is not well understood. In this study, we explored the effects and mechanisms of the major root exudates, including glucose, organic acids, and serine, in the rhizoremediation of pyrene-contaminated soil. The results showed that glucose increased the degradation of pyrene (54.3 ± 1.7%) most significantly compared to the organic acids (45.5 ± 2.5%) and serine (43.2 ± 0.1%). Glucose could significantly facilitate the removal of pyrene in soil through promoting dehydrogenase activity indicated by a positive correlation between the removal efficiency of pyrene and the soil dehydrogenase activity (p < 0.01). Furthermore, root exudates were able to change soil microbial community, particularly the bacterial taxonomic composition, thereby affecting the biodegradation of pyrene. Glucose could alter soil microbial community and enhance the amount of Mycobacterium markedly, which is dominant in the degradation of pyrene. These findings provide insights into the mechanisms by which root exudates enhance the degradation of organic contaminants and advance our understanding of the micro-processes involved in rhizoremediation.