Impact of phenanthrene on organic acids secretion and accumulation by perennial ryegrass, Lolium perenne L., root

The dissipation of organophosphate esters mediated by ryegrass root exudate oxalic acid in soil: Analysis of enzymes activities, microorganism

Complex rhizoremediation is the main mechanism of phytoremediation in organic-contaminated soil. Low molecular weight organic acids (LMWOAs) in root exudates have been shown to increase the bioavailability of contaminants and are essential for promoting the dissipation of contaminants. The effects of root exudates on the dissipation of organophosphate esters (OPEs) in soil are unclear. Consequently, we studied the combined effects of root exudates, soil enzymes and microorganisms on OPEs (tri (1-chloro-2-propyl) phosphate (TCPP) and triphenyl phosphate (TPP)) dissipation through pot experiments. Oxalic acid (OA) was confirmed to be the main component of LMWOAs in root exudates of ryegrass. The existence of OA increased the dissipation rate of OPEs by 6.04%-25.50%. Catalase and dehydrogenase activities were firstly activated and then inhibited in soil. While, urease activity was activated and alkaline phosphatase activity was inhibited during the exposure period. More bacteria enrichment (e.g., Sphingomonas, Pseudomonas, Flavisolibacter, Pontibacter, Methylophilus and Massilia) improved the biodegradation of OPEs. In addition, the transformation paths of OPEs hydrolysis and methylation under the action of root exudates were observed. This study provided theoretical insights into reducing the pollution risk of OPEs in the soil.

Combined maize straw-biochar and oxalic acids induced a relay activity of abundant specific degraders for efficient phenanthrene degradation: Evidence based on the DNA-SIP technology

Biochar-oxalic acid composite application (BCOA) have shown to be efficient in the remediation of polycyclic aromatic hydrocarbon (PAH)-contaminated soil, but the functional degraders and the mechanism of improving biodegradation remains unclear. In this study, with the help of stable isotope probing technology of phenanthrene (Phe), we determined that BCOA significantly improved Phe mineralization by 2.1 times, which was ascribed to the increased numbers and abundances of functional degraders. The BCOA increased contents of dissolved organic carbon and available nutrients and decreased pH values in soil, thus promoting the activity, diversity and close cooperation of the functional Phe-degraders, and stimulating their functions associated with Phe degradation. In addition, there is a relay activity among more and diverse functional Phe-degraders in the soil with BCOA. Specifically, Pullulanibacillus persistently participated in Phe-degradation in the soil with BCOA throughout the incubation period. Moreover, Pullulanibacillus, Blastococcus, Alsobacter, Ramlibacter, and Mizugakiibacter were proved to be potential Phe-degraders in soil for the first time. The specific Phe degraders and their relay and cooperation activity in soils as impacted by BCOA were first identified with DNA-stable isotope probing technology. Our findings provided a novel perspective to understand the efficient degradation of PAH in the BCOA treatments, revealed the potential of soil native microbes in the efficient bioremediation of PAH-contaminated natural soil, and provided a basis for the development of in-situ phytoremediation technologies to remediate PAH pollution in future.

Exogenous phosphorus treatment facilitates chelation-mediated cadmium detoxification in perennial ryegrass (Lolium perenne L.)

Cadmium (Cd) is an on-going environmental pollutant associated with hindered plant growth. In response, plants possess various strategies to alleviate Cd stress, including reactive oxygen species (ROS) scavenging and chelation-mediated Cd detoxification. The present study examined the Cd defense mechanism of perennial ryegrass (Lolium perenne L.), taking into account the effect of exogenous phosphorus (P) input. It was found that despite triggering antioxidant enzyme activity, Cd stress heightened lipid peroxidation levels. Exogenous P input partially mitigated the lipid peroxidation impact and decreased the levels of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) antioxidant enzymes, revealing reduced ROS-scavenging activity. Importantly, notable relationships were determined between the amount of Cd uptake in the root and the amount of non-protein thiols (R² = 0.914), glutathione (R² = 0.805) and phytochelatins (R² = 0.904) in proportion to the amount of exogenous P applied. The levels of amino acids proline and cysteine were also enhanced by exogenous P input showing their influence in alleviating Cd stress. Overall, it is reported that Cd detoxification in ryegrass plants can be stimulated by exogenous P input, which facilitates chelation-mediated Cd detoxification processes.

Influence of alkyl polyglucoside, citric acid, and nitrilotriacetic acid on phytoremediation in pyrene-Pb co-contaminated soils

At present, the remediation for organic pollutants and heavy metals co-contaminated soils is a challenge which needs to be broken through. In this study, alkyl polyglucoside (APG), citric acid (CA), and nitrilotriacetic acid (NTA) were chosen to enhance the phytoremediation of pyrene and Pb co-contaminated soils by perennial ryegrass. Through the comparison of the results with different applications, it could be found that the application of NTA was beneficial to the growth of perennial ryegrass, the underground and aboveground biomass were increased by 172.9-236.1% and 61.9-142.8%, respectively, meanwhile, photosynthetic activity of perennial ryegrass was affected positively. More importantly, the combined application of APG and NTA maximally promoted the accumulation and translocation of Pb (BF/TF, 0.44/0.61) and dissipation of pyrene (71.6%). These results indicated that the combined application of APG and NTA could be promising for future practical application of phytoremediation. However, the optimal dosage ratio of APG and NTA for phytoremediation needs to be further researched.

Combined effects of dissolved humic acids and tourmaline on the accumulation of 2, 2', 4, 4', 5, 5'- hexabrominated diphenyl ether (BDE-153) in Lactuca sativa

In order to investigate the effects of dissolved humic acid (DHA) and tourmaline on uptake of 2, 2', 4, 4', 5, 5'- hexabrominated diphenyl ether (BDE-153) by Lactuca sativa, different fractions of DHA, including DHA₁ and DHA₄, as well as different doses of tourmaline were introduced into BDE-153 contaminated solutions for plant growth. The levels of BDE-153 in L. sativa tissues were positively correlated with the Fe levels (R² = 0.9264) in seedings of the treatments with different doses of tourmaline. However, when adding DHA₁ and DHA₄ into the system, the correlation coefficients (R²) decreased to 0.6976 and 0.5451 from 0.9264, respectively. In contrast with the Fe contents, the presence of DHAs didn't affect the R² between the levels of BDE-153 and the lipid contents in plant tissues. Our results indicated that both DHA₁ and DHA₄ could severely alter the BDE-153 uptake by L. sativa through reducing the Fe uptake instead of the lipid contents. Additionally, DHA₄ exhibited much stronger abilities to alter the BDE-153 accumulation than DHA₁. Transmission electron microscopy (TEM) observations indicated that either DHA₁ or tourmaline or co-treatment with DHA and tourmaline had no negative impact on L. sativa at the cellular level. The present study provides important information for the impacts of different fractions of DHA extracted from soil on the BDE-153 migration in plant systems. Moreover, we elucidated the importance of the iron in tourmaline for migration of the polybrominated diphenyl ethers (PBDEs) in plant systems.

Response of low-molecular-weight organic acids in mangrove root exudates to exposure of polycyclic aromatic hydrocarbons

Low-molecular-weight organic acids (LMWOAs) represent an important component of root exudates. They play a pivotal role in the degradation of polycyclic aromatic hydrocarbons (PAHs) in sediments as they influence PAH bioavailability and degrader colonization. This study examined variations of LMWOAs in mangrove root exudates (Aegiceras corniculatum and Kandelia obovata) after exposure in phenanthrene and pyrene solution for 7 and 40 days, respectively. After 7 days of treatment, total root exudates and six types of LMWOA in root exudates from both mangrove species were enhanced. The largest increase was found in oxalic acid, i.e., the dominant component in determined LMWOAs. Coupled with the enhancement in LMWOA exudation rates, root metabolism intensities, measured as the dehydrogenase activity, increased. In contrast, after 40 days of exposure, the exudation rate of total LMWOAs had dropped markedly in PAH-contaminated groups compared to the control, indicating that PAHs negatively impacted root metabolism and activities due to their toxicity. The largest decrease was also found in oxalic acid, suggesting that the biological reactions related with oxalic acid are vulnerable under PAH stresses.

Phytoavailability and mechanism of bound PAH residues in filed contaminated soils

Understanding the phytoavailability of bound residues of polycyclic aromatic hydrocarbons (PAHs) in soils is essential to assessing their environmental fate and risks. This study investigated the release and plant uptake of bound PAH residues (reference to parent compounds) in field contaminated soils after the removal of extractable PAH fractions. Plant pot experiments were performed in a greenhouse using ryegrass (Lolium multiflorum Lam.) to examine the phytoavailablility of bound PAH residues, and microcosm incubation experiments with and without the addition of artificial root exudates (AREs) or oxalic acid were conducted to examine the effect of root exudates on the release of bound PAH residues. PAH accumulation in the ryegrass after a 50-day growth period indicated that bound PAH residues were significantly phytoavailable. The extractable fractions, including the desorbing and non-desorbing fractions, dominated the total PAH concentrations in vegetated soils after 50 days, indicating the transfer of bound PAH residues to the extractable fractions. This transfer was facilitated by root exudates. The addition of AREs and oxalic acid to test soils enhanced the release of bound PAH residues into their extractable fractions, resulting in enhanced phytoavailability of bound PAH residues in soils. This study provided important information regarding environmental fate and risks of bound PAH residues in soils.

Exogenous malic and acetic acids reduce cadmium phytotoxicity and enhance cadmium accumulation in roots of sunflower plants

There is increasing evidence showing that low molecular weight organic acids (LMWOA) are involved in heavy metal resistance mechanisms in plants. The aim of this study was to investigate the effects of exogenous malic (MA) or acetic (AA) acids on the toxicity and accumulation of cadmium (Cd) in sunflower (Helianthus annuus L.). For this purpose, plants were grown in hydroponics under controlled conditions. Single Cd stress (5 μM Cd for 14 days) induced strong phytotoxic effects, as indicated by a decrease in all growth parameters, concentration of photosynthetic pigments, and root activity, as well as a high level of hydrogen peroxide (H2O2) accumulation. Exogenous MA or AA (250 or 500 μM) applied to the Cd-containing medium enhanced the accumulation of Cd by the roots and limited Cd translocation to the shoots. Moreover, the MA or AA applied more or less reduced Cd phytotoxicity by increasing the growth parameters, photosynthetic pigment concentrations, decreasing accumulation of H2O2, and improving the root activity. Of the studied organic acids, MA was much more efficient in mitigation of Cd toxicity than AA, probably by its antioxidant effects, which were stronger than those of AA. Plant response to Cd involved decreased production of endogenous LMWOA, probably as a consequence of severe Cd toxicity. The addition of MA or AA to the medium increased endogenous accumulation of LMWOA, especially in the roots, which could be beneficial for plant metabolism. These results imply that especially MA may be involved in the processes of Cd uptake, translocation, and tolerance in plants.

Exogenous glycinebetaine alleviates the detrimental effect of Cd stress on perennial ryegrass

Glycinebetaine (GB) is an important organic osmolyte that accumulates in many plant species in response to abiotic stresses including heavy metals. The objective of this study was to investigate whether exogenous GB would ameliorate the adverse effect of cadmium (Cd) stress on perennial ryegrass (Lolium perenne). Fifty-three days old seedlings were exposed to hydroponic culture for 7 days with six treatments: T1 (control), T2 (0 mM Cd + 20 mM GB), T3 (0 mM Cd + 50 mM GB), T4 (0.5 mM Cd + 0 mM GB), T5 (0.5 mM Cd + 20 mM GB), T6 (0.5 mM Cd + 50 mM GB). Cd stress resulted in a remarkable decrease in turf quality, vertical shoot growth rate (VSGR), normalized relative transpiration (NRT) and Chlorophyll (Chl) content; with significant increases in electric conductivity (EL), malondialdehyde (MDA) content, superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) activity, oxalic and tartaric acid content. Exogenous application of GB decreased EL and MDA content in Cd stressed plants, and increased turf quality, VSGR, NRT, Chl content, SOD, CAT, POD activity, oxalic, tartaric acid content, and the gene expression level of SOD and POD when compared with Cd stressed without GB. Perennial ryegrass with 20 mM GB application suppressed the Cd accumulation in both shoots and roots. A lower translocation factor of Cd was found in GB treated plants than non-GB treated plants, and the lowest translocation factor was observed in the 20 mM GB application. These results suggested that GB could alleviate the detrimental effect of Cd on perennial ryegrass and the amelioration was mainly related to the elevation in SOD, CAT, and POD at enzyme and gene expression levels, which reduced Cd content in shoots and improved cell membrane stability by reducing oxidation of membrane lipids. These findings lead us to conclude that application of GB with 20 mM is the best strategy to ameliorate the detrimental impacts of Cd stress on perennial ryegrass.

Low-molecular-weight organic acids influence the sorption of phenanthrene by different soil particle size fractions

The impact of low-molecular-weight organic acids (LMWOAs) on the sorption of phenanthrene (a representative polycyclic aromatic hydrocarbon) by different particle size fractions of a soil was investigated using a batch technique. Citric and malic acids were used in experiments. Four soil fractions were fractionated: fine sand, silt, coarse clay, and fine clay. Laser granulometry confirmed the suitability of the fractionation method used for the particle size distributions in this investigation. The sorption of phenanthrene by the different soil fractions was described well using a linear distribution-type model, and the distribution coefficients () followed a descending order of fine sand > fine clay > coarse clay > silt, irrespective of the addition of organic acids. This order was significantly positively correlated with the organic carbon content of test soil solids. The values for phenanthrene sorption by soil fractions initially increased but then decreased as the concentrations of citric and malic acids increased (0-1000 mmol L). The presence of citric and malic acid at lower concentrations (<100 mmol L) generally promoted the sorption of phenanthrene, while higher concentrations (>100 mmol L) inhibited sorption irrespective of the soil fraction. The mechanism of the LMWOA-influenced sorption of phenanthrene by test solids is discussed based on the observed sorption of organic acid, the dissolution of metal cations and minerals in soil, and the competition from dissolved organic matter in solution that were released from soil solids.