Molecular mechanism of dissolvable metal nanoparticles-enhanced CO2 fixation by algae: Metal-chlorophyll synthesis.

Algae-driven photosynthetic CO2 fixation is a promising strategy to mitigate global climate changes and energy crises. Yet, the presence of metal nanoparticles (NPs), particularly dissolvable NPs, in aquatic ecosystems introduces new complexities due to their tendency to release metal ions that may perturb metabolic processes related to algal CO2 fixation. This study selected six representative metal NPs (Fe3O4, ZnO, CuO, NiO, MgO, and Ag) to investigate their impacts on CO2 fixation by algae (Chlorella vulgaris). We discovered an intriguing phenomenon that bivalent metal ions released from the metal NPs, especially from ZnO NPs, substituted Mg2+ within the porphyrin ring. This interaction led to 81.8% and 76.1% increases in Zinc-chlorophyll and Magnesium-chlorophyll contents within algal cells at 0.01 mM ZnO NPs, respectively. Integrating metabolomics and transcriptomics analyses revealed that ZnO NPs mainly promoted the photosynthesis-antenna protein pathway, porphyrin and chlorophyll metabolism, and carbon fixation pathway, thereby mitigating the adverse effects of Zn2+ substitution in light harvesting and energy transfer for CO2 fixation. Ultimately, the genes encoding Rubisco large subunit (rbcL) responsible for CO2 fixation were upregulated to 2.60-fold, resulting in a 76.3% increase in carbon fixation capacity. Similar upregulations of rbcL expression (1.13-fold) and carbon fixation capacity (76.1%) were observed in algal cells even at 0.001 mM ZnO NPs, accompanied by valuable lipid accumulation. This study offers novel insights into the molecular mechanism underlying NPs on CO2 fixation by algae and potentially introduces strategies for global carbon sequestration.

High salinity promotes the photoaging of polystyrene microplastics with humic acid in seawater.

The photoaging of microplastics (MPs) accumulated in the sea can be influenced by humic acid (HA). However, the role of salinity cannot be ignored, as it may potentially disrupt the interaction between MPs and HA, thereby altering the photoaging of MPs. Herein, this study investigated how salinity influences the effect of humic acid (HA, derived from lignite) on the photoaging of polystyrene microplastics (PS MPs) in artificial and natural seawater. The results revealed that HA promoted the photoaging of PS MPs under both low (5 PSU) and high salinity (35 PSU) in light conditions (L), reflected in the formation of fragments, the production of oxygen-containing functional groups (OH, CO, and OCO), and the increase in hydrophilicity of PS MPs. Furthermore, high salinity promoted the photoaging of PS MPs with HA more significantly, as evidenced by the similar indicators and the order of oxygen/carbon atom ratio (O/C): L-HA-High (0.15) > L-HA-Low (0.10) > Unaged (0.02). Interestingly, due to the reduction of electrostatic repulsion, the adsorption of HA on photoaged PS MPs in natural and artificial high salinity seawater was 1.77 mg/g and 0.39 mg/g, respectively, which was significantly higher than those PS MPs photoaged in the low salinity seawater. Furthermore, the electron spin resonance (ESR) results confirmed that more hydroxyl radicals (OH) were generated after adsorbing HA under high salinity conditions, thus promoting the fragmentation and oxidation of PS MPs. Overall, our findings highlight the crucial role of salinity in influencing the photoaging of MPs with HA and help to assess the marine risk of MPs accurately.

Blockage of ATPase-mediated energy supply inducing metabolic disturbances in algal cells under silver nanoparticles stress.

Adenosine triphosphate (ATP) generation of aquatic organisms is often subject to nanoparticles (NPs) stress, involving extensive reprogramming of gene expression and changes in enzyme activity accompanied by metabolic disturbances. However, little is known about the mechanism of energy supply by ATP to regulate the metabolism of aquatic organisms under NPs stress. Here, we selected extensively existing silver nanoparticles (AgNPs) to investigate their implications on ATP generation and relevant metabolic pathways in alga (Chlorella vulgaris). Results showed that ATP content significantly decreased by 94.2% of the control (without AgNPs) in the algal cells at 0.20 mg/L AgNPs, which was mainly attributed to the reduction of chloroplast ATPase activity (81.4%) and the downregulation of ATPase-coding genes atpB and atpH (74.5%-82.8%) in chloroplast. Molecular dynamics simulations demonstrated that AgNPs competed with the binding sites of substrates adenosine diphosphate and inorganic phosphate by forming a stable complex with ATPase subunit beta, potentially resulting in the reduced binding efficiency of substrates. Furthermore, metabolomics analysis proved that the ATP content positively correlated with the content of most differential metabolites such as D-talose, myo-inositol, and L-allothreonine. AgNPs remarkably inhibited ATP-involving metabolic pathways, including inositol phosphate metabolism, phosphatidylinositol signaling system, glycerophospholipid metabolism, aminoacyl-tRNA biosynthesis, and glutathione metabolism. These results could provide a deep understanding of energy supply in regulating metabolic disturbances under NPs stress.

In Situ Assay of Interfacial Interaction between ZnO Nanoparticles and Live Cell Disturbed by Surfactants.

The interfacial interaction between pollutants and organisms is a critical process in controlling the environmental fates of pollutants; however, in situ assay of the interaction is still a great challenge. Here, in situ determination of dissociation constants (Kd) for ZnO nanoparticles (ZnO NPs) from live algal cells disturbed by different-charged surfactants was established using microscale thermophoresis (MST). Moreover, in situ measurement of the adhesion force between the ZnO NPs probe and live single cell was performed using an atomic force microscope (AFM). Results showed that the cationic cetyltrimethylammonium chloride (CTAC) and anionic sodium dodecylbenzenesulfonate (SDBS) increased but nonionic Triton X-100 (TX-100) decreased the adhesion of ZnO NPs on cells. However, the force signature exhibited a smooth single retracted peak at short distances in the SDBS- and TX-100-treated groups, distinguished from the "see-saw" pattern peak in the CTAC-treated groups. The extended Derjaguin-Landau-Verway-Overbeek (XDLVO) calculation further confirmed that SDBS and TX-100 mainly disturbed the short-range hydration on the NP-cell interface, while CTAC reduced the long-range electrostatic repulsion. Furthermore, an excellent linear correlation between Zn bioaccumulation and two parameters (Kd and adhesion force) indicated that NP-cell interfacial interactions affected Zn bioaccumulation. Thus, in situ assay provides a quantitative basis for the pollutant-organism interfacial interaction to evaluate the environmental fate and ecological risk of pollutants.

Metabolomics reveals the inhibition on phosphorus assimilation in Chlorella vulgaris F1068 exposed to AgNPs.

Phosphorus removal by algae-based biotechnology can be achieved through algal assimilation, surface adsorption, or abiotic precipitation. However, there are still unavailable how these phosphorus removal processes were affected by nanoparticles in wastewater. Here, we employed a non-targeted metabolomic approach to reveal the impact of silver nanoparticles (AgNPs) on the phosphorus assimilation by a unicellular green alga Chlorella vulgaris F1068 (C. vulgaris F1068). Results showed that AgNPs mostly inhibited total phosphorus (TP) removal by the algal assimilation, with TP removal efficiency being reduced by 66.2% (with 0.20 mg/L AgNPs) of the control (without AgNPs). Metabolomics analysis also indicated that AgNPs disturbed metabolic responses related to phosphorus assimilation. AgNPs inhibited phospholipid metabolism which included inositol phosphate metabolism and phosphatidylinositol signaling system (downregulation of glycerol-3-phosphate and myo-inositol, as well as upregulation of serine). Metabolites related to phosphorus assimilation products were impacted through downregulation of guanine, glutamine, alanine, and aspartic acid, as well as upregulation of succinic acid, thereby impeding the algal assimilation of phosphorus. Moreover, perturbation of glutathione metabolism induced by oxidative stress stimulated the alteration of membrane state (upregulation of glycine). These findings contribute to a molecular-scale perspective of nanoparticles on algae-based biotechnology in phosphorus removal.

The complexation with proteins in extracellular polymeric substances alleviates the toxicity of Cd (II) to Chlorella vulgaris.

The complexation with extracellular polymeric substances (EPS) greatly reduces the toxicity of heavy metals towards organisms in the environment. However, the molecular mechanism of EPS-metal complexation remains unclear owing to the limitation of precise analysis for key fractions and functionalities in EPS that associate with metals. Herein, we explored the EPS-Cd (II) complexation by fluorescence excitation emission matrix coupled with parallel factor (EEM-PARAFAC), two-dimensional Fourier transform infrared correlation spectroscopy (2D-FTIR-COS) and X-ray photoelectron spectroscopy (XPS), attempting to explain the mechanisms of EPS in alleviating Cd (II) toxicity toward a green alga Chlorella vulgaris (C. vulgaris). When the algal EPS were removed, the cell internalizations of Cd (II), growth inhibition rate and chlorophyll autofluorescence increased, but the surface adsorption and esterase activities decreased, indicating that the sorption of Cd (II) by EPS was crucial in alleviating the algal toxicity. Moreover, the complexation with proteins in EPS controlled the sorption of Cd (II) to algal EPS, resulting in the chemical static quenching of the proteins fluorescence by 47.69 ± 2.37%. Additionally, the complexing capability of the main functionalities, COO- and C-OH in proteins with Cd (II) was stronger than that of C-O(H) and C-O-C in polysaccharides or C-OH in the humus-related substances. Oxygen atom in protein carboxyl C-O might be the key site of EPS-Cd (II) complexation, supported by the modified Ryan-Weber complexation model and the obvious shift of oxygen valence-electron signal. These findings provide deep insights into understanding the interaction of EPS with heavy metals in aquatic environment.