Cellular internalization and intracellular biotransformation of silver nanoparticles in <i>Chlamydomonas reinhardtii</i>

<p>It is necessary to elucidate cellular internalization and intracellular biotransformation in order to accurately assess the toxicity and fate of nanoparticles after interaction with organisms. Therefore, this work employed a combination of high resolution imaging and <i>in situ</i> detection spectroscopic techniques to systematically investigate the intracellular localization, morphology and chemical speciation of silver in the cells of <i>Chlamydomonas reinhardtii</i>, a unicellular freshwater green alga, after exposure to AgNPs coated with polyvinylpyrrolidone at a concentration of 2.0 mg/L. High resolution secondary ion mass spectrometry and high-angle annular dark field scanning transmission electron microscopy together with energy dispersive spectroscopy and selected area electron diffraction collectively confirmed that after 48 h of exposure, AgNPs entered the periplasmic space after cellular internalization into the algal cells. Silver was also found to coexist with sulfur inside the cytoplasm in both crystalline and amorphous forms, which were further identified as β-Ag₂S and silver thiolates with synchrotron X-ray absorption spectroscopy. In combination, these analyses demonstrated that silver inside algae could be attributed to the uptake and sequestration of Ag⁺ ion released from AgNPs, which was further sequestrated into cellular compartments. This study provides solid evidence for particle internalization and biotransformation of AgNPs after interaction with algae.</p

Uptake, Translocation, and Biotransformation of Organophosphorus Esters in Wheat (<i>Triticum aestivum</i> L.)

The uptake, translocation and biotransformation of organophosphate esters (OPEs) by wheat (<i>Triticum aestivum</i> L.) were investigated by a hydroponic experiment. The results demonstrated that OPEs with higher hydrophobicity were more easily taken up by roots, and OPEs with lower hydrophobicity were more liable to be translocated acropetally. A total of 43 metabolites including dealkylated, oxidatively dechlorinated, hydroxylated, methoxylated, and glutathione-, and glucuronide- conjugated products were detected derived from eight OPEs, with diesters formed by direct dealkylation from the parent triesters as the major products, followed with hydroxylated triesters. Molecular interactions of OPEs with plant biomacromolecules were further characterized by homology modeling combined with molecular docking. OPEs with higher hydrophobicity were more liable to bind with <i>Ta</i>LTP1.1, the most important wheat nonspecific lipid transfer protein, consistent with the experimental observation that OPEs with higher hydrophobicity were more easily taken up by wheat roots. Characterization of molecular interactions between OPEs and wheat enzymes suggested that OPEs were selectively bound to <i>Ta</i>GST4–4 and CYP71C6v1 with different binding affinities, which determined their abilities to be metabolized and form metabolite products in wheat. This study provides both experimental and theoretical evidence for the uptake, accumulation and biotransformation of OPEs in plants

Experimental and Theoretical Evidence for Diastereomer- and Enantiomer-Specific Accumulation and Biotransformation of HBCD in Maize Roots

Diastereomer- and enantiomer-specific accumulation and biotransformation of hexabromocyclododecane (HBCD) in maize (<i>Zea mays</i> L.) were investigated. Molecular interactions of HBCD with plant enzymes were further characterized by homology modeling combined with molecular docking. The (−)­α-, (−)­β-, and (+)­γ-HBCD enantiomers accumulated to levels in maize significantly higher than those of their corresponding enantiomers. Bioisomerization from (+)/(−)-β- and γ-HBCDs to (−)­α-HBCD was frequently observed, and (−)­γ-HBCD was most easily converted, with bioisomerization efficiency of 90.5 ± 8.2%. Mono- and dihydroxyl HBCDs, debrominated metabolites including pentabromocyclododecene (PBCDe) and tetrabromocyclododecene (TBCDe), and HBCD-GSH adducts were detected in maize roots. Patterns of hydroxylated and debrominated metabolites were significantly different among HBCD diastereomers and enantiomers. Three pairs of HBCD enantiomers were selectively bound into the active sites and interacted with specific residues of maize enzymes CYP71C3v2 and GST31. (+)­α-, (−)­β-, and (−)­γ-HBCDs preferentially bound to CYP71C3v2, whereas (−)­α-, (−)­β-, and (+)­γ-HBCDs had strong affinities to GST31, consistent with experimental observations that (+)­α-, (−)­β-, and (−)­γ-HBCDs were more easily hydroxylated, and (−)­α-, (−)­β-, and (+)­γ-HBCDs were more easily isomerized and debrominated in maize compared to their corresponding enantiomers. This study for the first time provided both experimental and theoretical evidence for stereospecific behaviors of HBCD in plants

Molecular-Scale Investigation with ESI-FT-ICR-MS on Fractionation of Dissolved Organic Matter Induced by Adsorption on Iron Oxyhydroxides

Adsorption by minerals is a common geochemical process of dissolved organic matter (DOM) which may induce fractionation of DOM at the mineral-water interface. Here, we examine the molecular fractionation of DOM induced by adsorption onto three common iron oxyhydroxides using electrospray ionization coupled with Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS). Ferrihydrite exhibited higher affinity to DOM and induced more pronounced molecular fractionation of DOM than did goethite or lepidocrocite. High molecular weight (>500 Da) compounds and compounds high in unsaturation or rich in oxygen including polycyclic aromatics, polyphenols and carboxylic compounds had higher affinity to iron oxyhydroxides and especially to ferrihydrite. Low molecular weight compounds and compounds low in unsaturation or containing few oxygenated groups (mainly alcohols and ethers) were preferentially maintained in solution. This study confirms that the double bond equivalence and the number of oxygen atoms are valuable parameters indicating the selective fractionation of DOM at mineral and water interfaces. The results of this study provide important information for further understanding the behavior of DOM in the natural environment

Relationship between Molecular Components and Reducing Capacities of Humic Substances

Humic substances (HSs) are collections of diverse organic compounds with broad redox capacities, which directly or indirectly affect the biogeochemical behaviors and fates of almost all the pollutants in the environment. The present study investigates the relationships between the molecular characteristics of HSs and their reducing capacities or electron-donating capacities (EDCs) by electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS), total phenolic assay, and mediated electrochemical oxidation analysis. For decreasing the molecular heterogeneity of bulk HSs, HSs were first separated into three fractions according to their polarities. The results demonstrated that compounds in HS fractions with moderate polarity possessed a high content of total phenols and consistently had high EDCs. A strong linear correlation (<i>R</i>² = 0.97) existed between EDCs and the total phenolic content, which confirmed that phenols contributed to the EDCs of HSs. Further analysis of molecular components confirmed that polyphenol-like compounds with medium oxygen content were the major moieties acting as electron donors in HSs. This study provides a linkage between the molecular components of HSs and their EDCs, which will help us to understand the molecular-dependent reducing properties of HSs or other dissolved organic matters under oxic conditions

Dissolution and Microstructural Transformation of ZnO Nanoparticles under the Influence of Phosphate

The toxicity and fate of nanoparticles (NPs) have been reported to be highly dependent on the chemistry of the medium, and the effects of phosphate have tended to be ignored despite the wide existence of phosphate contamination in aqueous environments. In the present study the influence of phosphate on the dissolution and microstructural transformation of ZnO NPs was investigated. Phosphate at a low concentration rapidly and substantially reduced the release of Zn²⁺ into aqueous solution. Synchrotron X-ray absorption spectroscopy and X-ray diffraction analysis reveal that interaction between ZnO NPs and phosphate induced the transformation of ZnO into zinc phosphate. Transmission electronic microscopy observation shows that the morphology of the particles changed from structurally uniform nanosized spherical to anomalous and porous material containing mixed amorphous and crystalline phases of ZnO and zinc phosphate in the presence of phosphate. To our knowledge, this is the first study in which the detailed process of phosphate-induced speciation and microstructural transformation of ZnO NPs has been analyzed. In view of the wide existence of phosphate contamination in water and its strong metal-complexation capability, phosphate-induced transformations may play an important role in the behaviors, fate, and toxicity of many other metal-based nanomaterials in the environment

Molecular-Scale Investigation on the Formation of Brown Carbon Aerosol via Iron-Phenolic Compound Reactions in the Dark

Brown carbon (BrC) is one of the most mysterious aerosol components responsible for global warming and air pollution. Iron (Fe)-induced catalytic oxidation of ubiquitous phenolic compounds has been considered as a potential pathway for BrC formation in the dark. However, the reaction mechanism and product composition are still poorly understood. Herein, 13 phenolic precursors were employed to react with Fe under environmentally relevant conditions. Using Fourier transform ion cyclotron resonance mass spectrometry, a total of 764 unique molecular formulas were identified, and over 85% of them can be found in atmospheric aerosols. In particular, products derived from precursors with catechol-, guaiacol-, and syringol-like-based structures can be distinguished by their optical and molecular characteristics, indicating the structure-dependent formation of BrC from phenolic precursors. Multiple pieces of evidence indicate that under acidic conditions, the contribution of either autoxidation or oxygen-induced free radical oxidation to BrC formation is extremely limited. Ligand-to-Fe charge transfer and subsequent phenoxy radical coupling reactions were the main mechanism for the formation of polymerization products with high molecular diversity, and the efficiency of BrC generation was linearly correlated with the ionization potential of phenolic precursors. The present study uncovered how chemically diverse BrC products were formed by the Fe-phenolic compound reactions at the molecular level and also provide a new paradigm for the study of the atmospheric aerosol formation mechanism

Soil’s Hidden Power: The Stable Soil Organic Carbon Pool Controls the Burden of Persistent Organic Pollutants in Background Soils

Persistent organic pollutants (POPs) tend to accumulate in cold regions by cold condensation and global distillation. Soil organic matter is the main storage compartment for POPs in terrestrial ecosystems due to deposition and repeated air–surface exchange processes. Here, physicochemical properties and environmental factors were investigated for their role in influencing POPs accumulation in soils of the Tibetan Plateau and Antarctic and Arctic regions. The results showed that the soil burden of most POPs was closely coupled to stable mineral-associated organic carbon (MAOC). Combining the proportion of MAOC and physicochemical properties can explain much of the soil distribution characteristics of the POPs. The background levels of POPs were estimated in conjunction with the global soil database. It led to the proposition that the stable soil carbon pools are key controlling factors affecting the ultimate global distribution of POPs, so that the dynamic cycling of soil carbon acts to counteract the cold-trapping effects. In the future, soil carbon pool composition should be fully considered in a multimedia environmental model of POPs, and the risk of secondary release of POPs in soils under conditions such as climate change can be further assessed with soil organic carbon models