Concentrations Of Low Molecular Weight Organic Acids Research Articles

Distinct photochemistry of adsorbed and coprecipitated dicarboxylates with ferrihydrite: Implications for iron reductive dissolution and carbon stabilization

Although ligand-promoted photodissolution of ferrihydrite (FH) has long been known for low molecular weight organic acids (LMWOAs), such as oxalate (Oxa) and malonate (Mal), photochemistry of coprecipitated FH with Oxa and Mal remains unknown, despite the importance of these mineral-organic associations in carbon retention has been acknowledged recently. In this study, ferrihydrite-LMWOAs associations (FLAs) were synthesized under circumneutral conditions. Photo-dissolution kinetics of FLAs were compared with those of adsorbed LMWOAs on FH surface and dissolved Fe-LMWOAs complexes through monitoring Fe(II) formation and organic carbon decay. For aqueous Fe(III)-LMWOAs complexes, Fe(II) yield was controlled by the initial concentration of LMWOAs and nature of photochemically generated carbon-centered radicals. Inner-sphere mononuclear bidentate (MB) configuration dominated while LMWOAs were adsorbed on the FH surface. MB complex of FH-Oxa was more photoreactive, leading to the rapid depletion of Oxa. Oxa can be readsorbed but in the form of binuclear bidentate and outer-sphere complexation, with much lower photoreactivity. While LMWOAs was coprecipitated with FH, the combination mode of LMWOAs with FH includes surface adsorption with a mononuclear bidentate structure and internal physical inclusion. Higher content of LMWOAs in the FLAs promoted the photo-production of Fe(II) as compared to pure FH, while it was not the case for FLAs containing moderate amounts of LMWOAs. The distinct photochemistry of adsorbed and coprecipitated Fe-LMWOAs complexes is attributed to ligand availability and configuration patterns of LMWOAs on the surface or entrapped in the interior structure. The present findings have significant implications for understanding the photochemical redox cycling of iron across the interface of Fe-organic mineral associates.

Decreased transport of nano- and micro-plastics in the presence of low-molecular-weight organic acids in saturated quartz sand

Low-molecular-weight organic acids (LMWOAs) and nano- and micro-plastics (NPs and MPs) are both widely distributed in terrestrial systems. To better understand the influence of LMWOAs on the transport of NPs and MPs, the effects of 0.5 mM citric- (CA), malic- (MA), and tartaric- (TA) acid on the transport of nano- (0.51 μm, PS NPs) and micro- (1.1 μm, PS MPs) polystyrene particles (2 mg L−1) in saturated quartz sand were investigated. All three LMWOAs decreased the transport of PS NPs and MPs, regardless of ionic composition or strength (0.1–10 mM NaCl and 0.1–1 mM CaCl2). Further investigation revealed that the interfacial interactions between PS-quartz sand surfaces and PS-PS were altered by LMWOAs. LMWOAs adsorbed to quartz sand surfaces could serve as new deposition sites, as evidenced by the decreased transport of PS NPs and MPs in quartz sand that was subjected to pre-equilibration with selected MA, the low inhibition of PS transport with low concentrations of LMWOAs (0.1 mM), and also the adsorption of LMWOAs onto quartz sand surfaces by batch experiments. Meanwhile, the adsorption of LMWOAs on PS, hydrodynamic measurement and visual TEM observation together clarified the slight aggregation of PS NPs and MPs in suspensions, inducing the subsequent decrease in transport. Among them, the adsorption of LMWOAs onto quartz sand surfaces was found to be the main factor dominating the decreased transport of both PS NPs and MPs in saturated quartz sand.

Effect of low molecular weight organic acids on the lead and chromium release from widely-used lead chromate pigments under sunlight irradiation

Lead chromate pigments are commonly used yellow inorganic pigments. They can pose environmental risks as they contain toxic heavy metals lead and chromium. Low molecular weight organic acids (LMWOAs), as widespread dissolved organic matter (DOM), affect the lead and chromium release from the pigment in water. In this work, the role of LMWOAs in the photodissolution of commercial lead chromate pigment was investigated. The pigment underwent significant photodissolution under simulated sunlight exposure with LMWOAs, and subsequently released Cr(III) and Pb(II). The photodissolution process is caused by the reduction of Cr(VI) by photogenerated electrons of the lead chromate pigment. The LMWOAs promoted photodissolution of the pigment by improving the electron-hole separation. The formation of Cr(III)-contained compounds leads to a slower release of chromium than lead. The photodissolution kinetics increase with decreasing pH and increasing LMWOAs concentration. The photodissolution of lead chromate pigment was basically positively related to the total number of hydroxyl and carboxyl groups in LMWOAs. The LMWOAs with stronger affinity to lead chromate pigment, lower adiabatic ionization potential (AIP) and higher energy of the highest occupied molecular orbital (EHOMO) are favorable to Cr(VI) reduction by photogenerated electrons and pigment photodissolution. 2.39% of chromium and 10.34% of lead released from the lead chromate pigment in natural conditions during a 6-h sunlight exposure. This study revealed the photodissolution mechanism of lead chromate pigment mediated by LMWOAs with different molecular structures, which helps understand the environmental photochemical behavior of the pigment. The present results emphasize the important role of DOM in the heavy metals release from commercial inorganic pigments.

The Hormetic Response of Soil P Extraction Induced by Low-Molecular-Weight Organic Acids

The hormetic effect is a toxicological phenomenon in the soil ecosystem. The influence of low-molecular-weight organic acids (LMWOAs) on the release and activation of soil phosphorus (P) has become the focus of toxicological research. To what extent LMWOAs can regulate the hormetic effect of P release and then influence soil P nutrients is worth attention. This study aimed to investigate the effects of different types and concentrations of exogenous LMWOAs on P extraction, establish the relationship between the concentration of LMWOAs and P extraction efficiency, and calculate the hormetic parameters to understand the mechanism of types and concentrations of LMWOAs in P extraction efficiency. Four organic acids, i.e., citric, oxalic, tartaric, and malic acids, induced hormetic effects on P extraction that were concentration dependent. The relationship between LMWOAs and P extraction efficiency was explained by a quadratic polynomial equation. The critical threshold of citric acid concentration was similar to that of oxalic acid, whereas that of tartaric acid was similar to that of malic acid. The critical thresholds of the P concentration extracted by malic acid and citric acid were higher than those extracted by oxalic acid and tartaric acid due to the differences in the structure and properties of LMWOAs. The critical thresholds of P extraction efficiency of oxalic acid were lower than those of the other three organic acid types. These results provide evidence for the use of citric acid and malic acid to increase soil P.

Generation and decomposition of low-molecular-weight organic acids in hydrous pyrolysis experiments with Opalinus Clay rock

Increasing temperature in claystone formations accelerates geochemical reactions between organic matter, water and mineral phases, promoting the generation and decomposition of organic compounds such as low-molecular-weight organic acids (LMWOA). LMWOA presence in claystone formations can enhance mineral dissolution, stimulate gas generation through thermal decomposition reactions, affect metal-ion complexation, alter sorption processes, and provide feedstock for microbial life. Hydrous pyrolysis experiments (80–200 °C and 20 MPa) using Dickson-type flexible gold-titanium reaction cells were conducted to investigate the thermal generation and decomposition of LMWOA in organic matter poor (0.64 wt%) Opalinus Clay. Maximum concentrations of dominant species were 44 µmol/kg clay formate (120 °C), 310 µmol/kg clay acetate (200 °C), and 74 µmol/kg clay oxalate (120 °C). Calculated half-live ranges of the dominant compounds were between 8.88 × 1010 and 2.81 × 10−3 years for the experimental conditions. Comparison of observed and calculated reaction rates suggests that the thermal generation and decomposition of LMWOA proceed simultaneously. Therefore, the observed generation rates in these and other previously published experiments are minimum estimates that need to be corrected for parallel decomposition reactions. The new quantitative reaction rate data presented here facilitate modelling of production/decomposition and local pore water concentrations of LMWOA by the extrapolation of kinetic data to temperatures encountered, for example, in natural heating during subsidence or by thermal intrusions.

Distribution characteristics of low molecular weight organic acids in seawater of the Changjiang Estuary and its adjacent East China Sea: Implications for regional environmental conditions

In this study, components, concentrations, distribution characteristics and sources of low molecular weight organic acids (LMWOAs) in seawater of the Changjiang Estuary and its adjacent East China Sea were investigated in March 2015. Lactic, acetic and formic acids were identified with their concentration range of 0–16.7, 0–42.7 and 0–6.7 μmol·L−1, respectively. In the surface seawater, high concentrations of LMWOAs appeared in the sea area close to the estuary and along the coast. LMWOAs were important fractions of dissolved organic carbon and acetic acid was dominant component of LMWOAs. Riverine, terrestrial input, phytoplankton and sediment release were important sources for the LMWOAs, and human activities were considered as dominant sources for them in sampling period. The consistency of regions with high concentrations of LMWOAs, eutrophication, seasonal hypoxia and frequent red tide occurrence suggested LMWOAs as potential indicators for evaluating pollution status in coastal areas.

Impact of a long-term cultivation on low molecular weight organic acids in greenhouse soil and their influence on vegetable uptake heavy metals

ABSTRACT Low-molecular-weight organic acids (LMWOAs), a type of small organic molecules with one or more carboxyl groups present in soil, play an important role in the soil ecosystem. Greenhouse soil could be as a key example to understand LMWOA distribution and its ecological effects on heavy metals. In this study, 54 soil samples and 24 vegetable samples were collected from Wuqing, Tianjin, China, to investigate the distribution of LMWOAs in greenhouse soil and its effect on the transfer of heavy metals from soil to vegetables. The results showed that the mean concentrations of lactic acid, acetic acid, formic acid, propionic acid, butyric acid, malic acid, and oxalic acid detected in the greenhouse soil were 5.00, 20.57, 0.62, 23.38, 0.64, 4.67, and 29.03 mg/kg. Except for formic acid, the concentrations of LMWOAs in greenhouse soils were higher than LMWOAs in field soils. The content of oxalic acid, acetic acid, lactic acid, malic acid, propionic acid, and butyric acid in greenhouse soils is 2.3 times, 2.25 times, 1.82 times, 2.38 times, 1.38 times, 1.93 times that of LMWOAs in field soil. LMWOAs in the soils had obvious surface cohesion characteristics. The correlation between LMWOAs indicates that propionic acid has the same origin and/or similar variation characteristics as malic acid, oxalic acid, acetic acid, butyric acid, and lactic acid. Oxalic acid, formic acid, acetic acid, and lactic acid have a significant contribution to soil acidification. Accumulation of available phosphorus (AP) and available potassium (AK) in the soils may result in increased acetic acid, butyric acid, oxalic acid, and lactic acid. The above results indicate that LMTOA has different effects on the transfer of heavy metals from soil to vegetable. In addition, the accumulation of LMWOAs could stimulate the transfer of Cd, Zn, and Cu from soil to vegetable, but inhibit the absorption of Pb by vegetables.

Introducing N2-fixing trees (Acacia mangium) in eucalypt plantations rapidly modifies the pools of organic P and low molecular weight organic acids in tropical soils

Many studies have shown that introducing N2-fixing trees (e.g. Acacia mangium) in eucalypt plantations can increase soil N availability as a result of biological N2 fixation and faster N cycling. Some studies have also shown improved eucalypt P nutrition. However, the effects of N2-fixing trees on P cycling in tropical soils remain poorly understood and site-dependent. Our study aimed to assess the effects of planting A. mangium trees in areas managed over several decades with eucalypt plantations on soil organic P (Po) forms and low molecular weight organic acids (LMWOAs). Soil samples were collected from two tropical sites, one in Brazil and one in the Congo. Five different treatments were sampled at each site: monospecific acacia, monospecific eucalypt, below acacias in mixed-species, below eucalypts in mixed-species as well as native vegetation. Po forms and LMWOAs were identified in sodium hydroxide soil extracts using ion chromatography and relationships between these data and available P were determined. At both sites, the concentrations of most Po forms and LMWOAs were different between native ecosystems and monospecific eucalypt and acacia plots. Also, patterns of Po and LMWOAs were clearly separated, with glucose-6-P found mainly under acacia and phytate and oxalate mainly under eucalypt. Despite the strongest changes occurred at site with a higher N2 fixation and root development, acacia introduction was able to change the profile of organic P and LMWOAs in <10 years. The variations between available Pi, Po and LMWOA forms showed that P cycling was dominated by different processes at each site, that are rather physicochemical (via Pi desorption after LMWOAs release) at Itatinga and biological (via organic P mineralization) at Kissoko. Specific patterns of Po and LMWOAs forms found in soil sampled under acacia or eucalypt would therefore explain the effect of acacia introduction in both sites.

Selective metal leaching from technosols based on synthetic root exudate composition

This study focused on metal release from technosols induced by synthetic root exudate (SRE). The effect of SRE composition on metal release was studied using six technosols. This was done by treating the technosols with SRE solutions having varying concentrations of low molecular weight organic acids (LMWOAs), namely oxalic, citric, and malic acids. Consequently, the physico-chemical parameters (pH and electric conductivity), Ca, Mg, Fe, Zn, and Cu release (by atomic absorption spectroscopy, AAS), chemical changes (by Fourier transform infrared, FT-IR), and organic parameters (by fluorescence) were investigated. Metal release showed to be dependent on the SRE composition and technosol characteristics. Citric acid selectively released Ca, Mg, Zn, and Cu from technosols in a concentration-dependent manner; oxalic acid showed a significant role in the release of Mg and Fe. Under relatively high LMWOA concentrations, particulate organo-mineral complexes precipitated. Additionally, technosol weathering was seen by the dissolution of humic substances and ferriallophanes, which in turn caused metal release. However, re-precipitation of these phases showed to re-sorb metals, thus underestimating the role of LMWOAs in metal release. Therefore, the selective metal leaching was highly dependent on the SRE composition and LMWOA concentrations on one hand, and on the mineral, organic, and organo-mineral components of the technosols on the other. The understanding of such processes is crucial for proposing and implementing environmental management strategies to reduce metal leaching or for the beneficial re-usage of metals (e.g., for agromining) from technosols.

Variations of dissolved organic matter and Cu fractions in rhizosphere soil induced by the root activities of castor bean

The root soil interaction affects metal bioavailability in the rhizosphere, thus impacting the uptake and accumulation of metals by plants. In this study, a greenhouse experiment using a root-bag technique for castor bean plants was conducted to determine the i) rhizosphere effect on the fractions of Cu, and ii) the characteristics of dissolved organic matter (DOM) in the rhizosphere soil. Results showed that the Cu concentration in the leaves, stems, and roots was 15.41, 6.71, and 47.85 mg kg−1, respectively, in the control and reached up to 96.5, 254.9, and 3204 mg kg−1 in Cu400 treatment, respectively. After cultivating castor bean plants, the concentration of acid exchangeable Cu in rhizosphere soil was higher than that in the bulk soil for the same Cu addition, whereas the concentrations of reducible Cu, oxidizable Cu, and residual Cu in the rhizosphere soil were all lower than those in the bulk soil, respectively. In comparison to the bulk soil, the pH decreased while the total nitrogen and total carbon concentrations both increased in the rhizosphere soil. Moreover, the concentrations of total low molecular weight organic acids (LMWOAs) and total amino acids in the rhizosphere soil of the Cu treatments increased by between 15.18% to 47.17% and 36.35%–200%, respectively with respect to the control. The less complex DOM with a high LMWOAs concentration in the rhizosphere soil shifted the soil Cu from a relative stable fraction to available fractions.

Transport of Cd2+ through saturated porous media: Insight into the effects of low-molecular-weight organic acids

Low-molecular-weight organic acids (LMWOAs) are ubiquitous in the aquatic environment and consequently may affect the heavy metal transport in aquifer systems. In this study, the influences of LMWOAs on the transport of Cd2+ under different pH conditions in saturated porous media were evaluated. For this, three LMWOAs such as acetic acid, tartaric acid, and citric acid were employed. A two-site nonequilibrium transport model was applied to simulate the transport data. Under acidic conditions (pH 5.0), the results indicated that LMWOAs inhibited the transport of Cd2+ even at the low concentrations of organic acids (i.e., 0.05 and 0.1 mM). The inhibition effects might be attributed to the complexation role of the sand surface-bound organic acids and also electrostatic interaction. Meanwhile, the inhibition effects of LMWOAs on Cd2+ transport in the following order of citric acid > tartaric acid > acetic acid, which was also in agreement with the decreasing complex stability constants between Cd2+ and LMWOAs. This order may be dependent on their molecular structures (i.e., amount and type of functional groups) and complexing strength. Interestingly, when the LMWOA concentrations 0.5 mM, tartaric acid and citric acid still inhibited Cd2+ transport, while acetic acid slightly enhanced the Cd2+ mobility due to its weaker complexing strength. However, under neutral conditions (pH 7.0), LMWOAs generally enhanced the transport of Cd2+. The transport-enhancement of LMWOAs was ascribed to the formation of stable aqueous non-adsorbing Cd–organic acid complexes. In addition, citric acid could obviously inhibit the transport of Cd2+ under competitive transport conditions (i.e., with competing cations), which is mainly due to different complex affinities of citric acid to Pb2+ and Cd2+. These findings demonstrate that LMWOAs may inhibit or facilitate Cd2+ transport under different environmental conditions. Thus, environmental assessment concerning the transport of heavy metals should consider the roles of organic acids.

Biological enhancement of mineral weathering by &amp;lt;i&amp;gt;Pinus&amp;lt;/i&amp;gt; &amp;lt;i&amp;gt;sylvestris&amp;lt;/i&amp;gt; seedlings – effects of plants, ectomycorrhizal fungi, and elevated CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;

Abstract. Better understanding and quantifying the relative influence of plants, associated mycorrhizal fungi, and abiotic factors such as elevated CO2 on biotic weathering is essential to constraining weathering estimates. We employed a column microcosm system to examine the effects of elevated CO2 and Pinus sylvestris seedlings, with or without the ectomycorrhizal fungi Piloderma fallax and Suillus variegatus, on rhizosphere soil solution concentrations of low-molecular-weight organic acids (LMWOAs) and on the weathering of primary minerals. Seedlings significantly increased mineral weathering, as estimated from elemental budgets of Ca, K, Mg, and Si. Elevated CO2 increased plant growth and LMWOA concentrations but had no effect on weathering. Colonization by ectomycorrhizal fungi, particularly P. fallax, showed some tendency to increase weathering. LMWOA concentrations correlated with seedling biomass across both CO2 and mycorrhizal treatments but not with total weathering. We conclude that nutrient uptake, which reduces transport limitation to weathering, is the primary mechanism by which plants enhanced weathering in this system. While the experimental system used departs from conditions in forest soils in a number of ways, these results are in line with weathering studies performed at the ecosystem, macrocosm, and microcosm scale, indicating that nutrient uptake by plants and microbes is an important biological mechanism by which mineral weathering is enhanced.

Aggregation, Sedimentation, and Dissolution of Copper Oxide Nanoparticles: Influence of Low-Molecular-Weight Organic Acids from Root Exudates.

The rhizosphere is an essential pathway for the uptake of metal-based nanoparticles (MNPs) by plant roots. However, the interaction between root exudates and MNPs is still unclear. In this study, we initially identified the major low-molecular-weight organic acids (LMWOAs) in the rice root exudates using hydroponics. Then, the individual LMWOAs were added to CuO nanoparticle suspensions to investigate their effects on the environmental behavior of the MNPs. The results showed that both the variety and the concentration of LMWOAs impacted the aggregation, sedimentation, and dissolution of CuO nanoparticles (NPs). Almost all LMWOAs except succinic acid inhibited the aggregation of CuO NPs by enhancing the electrostatic repulsive force between NPs. The presence of citric and oxalic acids rather than lactic acid greatly improved the stability of CuO NP suspensions, but other acids showed a low promoting and high inhibiting effect on NP sedimentation. Moreover, all the LMWOAs from root exudates facilitated the dissolution of CuO NPs with a positive dose-dependent correlation, especially formic acid. Notably, citric acid, as the most abundant LMWOAs in rice root exudates, largely determined the aggregation, sedimentation, and dissolution of CuO NPs. This study provides a better understanding on NP–plant interactions in the rhizosphere.

Bacterial Isolate Inhabiting Spitsbergen Soil Modifies the Physiological Response of Phaseolus coccineus in Control Conditions and under Exogenous Application of Methyl Jasmonate and Copper Excess.

The aim of the study was to demonstrate the potential of the promotion and regulation of plant physiology and growth under control and copper stress conditions, and the impact of the exogenous application of methyl jasmonate on this potential. Runner bean plants were treated with methyl jasmonate (1 or 10 µM) (J; J1 or J10) and Cu (50 µM), and inoculated with a bacterial isolate (S17) originating from Spitsbergen soil, and identified as Pseudomonas luteola using the analytical profile index (API) test. Above- and under-ground plant parts were analyzed. The growth parameters; the concentration of the photosynthetic pigments, elements, flavonoids (FLAVO), phenolics (TPC), allantoin (ALLA), and low molecular weight organic acids (LMWOAs); the activity of antioxidant enzymes and enzymes of resistance induction pathways (e.g., superoxide dismutase (SOD), catalase (CAT), ascorbate (APX) and guaiacol (GPX) peroxidase, glucanase (GLU), and phenylalanine (PAL) and tyrosine ammonia-lyase (TAL)), and the antioxidant capacity (AC) were studied. The leaves exhibited substantially higher ALLA and LMWOA concentrations as well as PAL and TAL activities, whereas the roots mostly had higher activities for a majority of the enzymes tested (i.e., SOD, CAT, APX, GPX, and GLU). The inoculation with S17 mitigated the effect of the Cu stress. Under the Cu stress and in the presence of J10, isolate S17 caused an elevation of the shoot fresh weight, K concentration, and TAL activity in the leaves, and APX and GPX (also at J1) activities in the roots. In the absence of Cu, isolate S17 increased the root length and the shoot-to-root ratio, but without statistical significance. In these conditions, S17 contributed to a 236% and 34% enhancement of P and Mn, respectively, in the roots, and a 19% rise of N in the leaves. Under the Cu stress, S17 caused a significant increase in FLAVO and TPC in the leaves. Similarly, the levels of FLAVO, TPC, and AC were enhanced after inoculation with Cu and J1. Regardless of the presence of J, inoculation at Cu excess caused a reduction of SOD and CAT activities, and an elevation of GPX. The effects of inoculation were associated with the application of Cu and J, which modified plant response mainly in a concentration-dependent manner (e.g., PAL, TAL, and LMWOA levels). The conducted studies demonstrated the potential for isolate S17 in the promotion of plant growth.

A new software of calculating the pH values of coastal seawater: Considering the effects of low molecular weight organic acids

Effects of low molecular weight organic acids (LMWOAs) on the pH value of seawater were investigated in the surface seawater of the Jiaozhou Bay, China. The new software of Org·TCO2TA was developed to calculate the pH values of seawater based on the alkalinity (Alk) equation where organic acid Alk (Org-Alk) was separated into LMWOA Alk (LMWOA-Alk) and humic acid Alk (HA-Alk). In the calculations, all dissociation constants of organic acids were from previous literature. In our study, the average concentration of total LMWOAs was 14.5 ± 11.2 μmol·kg−1 SW. pH values from the Org·TCO2TA software were closer to the pH values from spectrophotometric measurement than those from the CO2SYS program, indicating pH values can be influenced by high concentrations of LMWOAs in coastal seawater of the Jiaozhou Bay. Although the differences still existed between the pH values from the spectrophotometric method and the calculated pH values from the Org·TCO2TA software due to the influence of various factors, including the analytical errors of dissolved inorganic carbon and nutrients, the new software can calculate the pH values of coastal seawater more accurately by considering the effects of LMWOAs.

Differentiation in low molecular weight organic acids exudation into rhizosphere and their creation in Ulmus laevis Pall organs treated by As – pot experiment

ABSTRACTTwo-year old Ulmus laevis Pall (U. laevis) seedlings were cultivated in a three-month hydroponic experiment with inorganic (aresenite – As(III) and arsenate – As(V)) and organic (dimethylarsenic acid – DMA(V)) arsenic forms, at 0.06 and/or 0.6 mM concentrations. Further, the profile and content of total low molecular weight organic acids (LMWOAs) were investigated in the rhizosphere, roots and leaves of U. laevis. Obtained results showed that the addition of As(III) or As(V) individually or in a mixture led to increased LMWOAs concentration in the rhizosphere, especially of oxalic and malonic acids, in comparison to the control, while in roots the overall content of the profiled LMWOAs decreased. In both rhizosphere and roots, addition of the DMA(V) form resulted in the inhibition of LMWOAs exudation into the rhizosphere and their creation in plant roots. Leaves were characterised by a higher content of LMWOAs than in the rhizosphere and roots for all experimental systems, where the profile and content of LMWOAs was strictly correlated with the analysed As forms. Our study indicated that creation of LMWOAs in U. laevis organs and their exudation to the rhizosphere could be responsible for the As toxicity tolerance of the plants.

Low-molecular-weight organic acids correlate with cultivar variation in ciprofloxacin accumulation in Brassica parachinensis L.

To understand the mechanism controlling cultivar differences in the accumulation of ciprofloxacin (CIP) in Chinese flowering cabbage (Brassica parachinensis L.), low-molecular-weight organic acids (LMWOAs) secreted from the roots of high- and low-CIP cultivars (Sijiu and Cutai, respectively) and their effects on the bioavailability of CIP in soil were investigated. Significant differences in the content of LMWOAs (especially maleic acid) between the two cultivars played a key role in the variation in CIP accumulation. Based on the Freundlich sorption coefficient (Kf) and distribution coefficient (Kd), the presence of LMWOAs reduced the CIP sorption onto soil particles, and higher concentrations of LMWOAs led to less CIP sorption onto soil. On the other hand, LMWOAs enhanced CIP desorption by lowering the solution pH, which changed the surface charge of soil particles and the degree of CIP ionization. LMWOAs promoted CIP desorption from soil by breaking cation bridges and dissolving metal cations, particularly Cu2+. These results implied that the LMWOAs (mainly maleic acid) secreted from Sijiu inhibited CIP sorption onto soil and improved CIP desorption from soil to a greater extent than those secreted from Cutai, resulting in higher bioavailability of CIP and more uptake and accumulation of CIP in the former.

Simulated Desorption Kinetics of Lead by the Dominant Plant Roots Released Low Molecular Weight Organic Acids from the Water-Level-Fluctuating Zone in the Three Gorges Reservoir

Vegetations grow vigorously in the water-level-fluctuating zone (WLFZ) of Three Gorges Reservoir (TGR) during the non-flooded period. Low molecular weight organic acids (LMWOAs) excreted from the roots of these plants may active the heavy metals in soil. In order to investigate the desorption kinetics of Pb in the vegetation covered soil of TGR, the LMWOAs released from Cynodon dactylon and Echinochloa crusgalli roots, which are the dominant species in WLFZ, were analyzed both qualitatively and quantitatively. Furthermore, single and mixed species of LMWOAs were also used in the simulation. The results showed that the variety and content of LMWOAs secreted from Cynodon dactylon roots were larger and higher than those of Echinochloa crusgalli. Citric acid, malonic acid, acetic acid and malic acid were the same LMWOAs released from the two types of plants. The contents of acetic acid were the highest among all LMWOAs, which were 0.765 and 0.261 mmol·kg-1 in Cynodon dactylon and Echinochloa crusgalli roots, respectively.Citric acid, malonic acid and acetic acid were the same LMWOAs detected from their rhizosphere soils. In the citric acid, malonic acid and acetic acid desorption test, the desorption captivities of Pb were inhibited when the concentration of LMWOAs was lower than 1 mmol·L-1. The desorption ability displayed a decreasing order of malonic acid > citric acid > acetic acid. However, the three LMWOAs promoted the desorption of Pb in soil when their concentrations were higher than 1 mmol·L-1, respectively. The desorption ability displayed a decreasing order of citric acid > malonic acid > acetic acid. During the entire process of desorption, 0-240 min was the fast reaction stage, and the remaining time was the slow reaction stage. The pseudo-second order kinetics equation could fit best for the kinetic process.

Effect of difference between day and night temperature on tomato (Lycopersicon esculentum Mill.) root activity and low molecular weight organic acid secretion

ABSTRACTThe difference between day and night temperature (DIF) is a major environmental factor affecting crop growth, but the mechanisms are not fully understood. We investigated crop performance, root activity and concentrations of low molecular weight organic acids (LMWOAs) secreted by tomato (Lycopersicon esculentum Mill.) root under different DIF conditions. A fixed daily temperature of 25°C and five DIF treatments (−12, −6, 0, 6 and 12°C) were used to grow tomato in a climate chamber. Root/shoot ratio; leaf maximum photosynthetic rate (Pmax); root activity; total nitrogen (N), phosphorus (P) and potassium (K) concentrations in roots; and types and concentrations of LMWOAs were measured at different growth stages. Results showed that positive and negative DIFs inhibited the dry matter accumulation of aerial parts, while 0°C DIF was conducive to the accumulation. Compared to 0°C DIF, positive DIFs significantly increased root dry weight, Pmax, root activity and total N, P and K concentrations in roots, while negative DIFs had contrary effects. During the whole growth period, tomato root activity decreased in the order of fruit setting stage, mature stage and flowering stage. Tomato roots secreted oxalic acid, formic acid, malic acid, malonic acid, lactic acid, acetic acid, citric acid, succinic acid and propionic acid under positive DIFs, while acetic acid was not detected in the negative DIF treatments. Oxalic acid concentration was significantly higher than other LMWOAs. Furthermore, in the same growth stage, positive DIFs caused more LMWOA secretion than negative DIFs and 0°C DIF. There were significant positive correlations between the total LMWOA concentration and root activity, root/shoot ratio, Pmax and total N, P and K concentrations in roots. Based on the results, more attention should be paid to the potential effect on tomato growth posed by DIFs, positive DIFs have higher positive influence than negative DIFs, and 6°C DIF is best for greenhouse tomato growth.

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

The effect of root exudates on the environmental behaviors of phenanthrene in mangrove sediments is poorly understood. In order to evaluate their influence, comprehensive laboratory experiments were performed using batch equilibrium and thin-layer chromatography (TLC) analyses. In the presence of root exudates, sorption of phenanthrene was inhibited, whereas desorption and mobility were promoted, and were elevated as root exudate concentrations increased. Among the three representative low molecular weight organic acids (LMWOAs) (citric, oxalic, and acetic acids), citric acid promoted desorption and mobility of phenanthrene more effectively than the other two. In addition, application of artificial root exudates (AREs) enhanced phenanthrene desorption, and mobility was always lower than that with the same concentration of LMWOAs, suggesting that LMWOAs predominantly affected the fate of phenanthrene in sediments. The results of this study could enhance our understanding of the mobility of persistent organic pollutants in sediment–water system.