Xiaoqin Nie

Nano-Scale Spatial Assessment of Calcium Distribution in Coccolithophores Using Synchrotron-Based Nano-CT and STXM-NEXAFS

Calcified coccolithophores generate calcium carbonate scales around their cell surface. In light of predicted climate change and the global carbon cycle, the biomineralization ability of coccoliths has received growing interest. However, the underlying biomineralization mechanism is not yet well understood; the lack of non-invasive characterizing tools to obtain molecular level information involving biogenic processes and biomineral components remain significant challenges. In the present study, synchrotron-based Nano-computed Tomography (Nano-CT) and Scanning Transmission X-ray Microscopy-Near-edge X-ray Absorption Fine Structure Spectromicroscopy (STXM-NEXAFS) techniques were employed to identify Ca spatial distribution and investigate the compositional chemistry and distinctive features of the association between biomacromolecules and mineral components of calcite present in coccoliths. The Nano-CT results show that the coccolith scale vesicle is similar as a continuous single channel. The mature coccoliths were intracellularly distributed and immediately ejected and located at the exterior surface to form a coccoshpere. The NEXAFS spectromicroscopy results of the Ca L edge clearly demonstrate the existence of two levels of gradients spatially, indicating two distinctive forms of Ca in coccoliths: a crystalline-poor layer surrounded by a relatively crystalline-rich layer. The results show that Sr is absorbed by the coccoliths and that Sr/Ca substitution is rather homogeneous within the coccoliths. Our findings indicate that synchrotron-based STXM-NEXAFS and Nano-CT are excellent tools for the study of biominerals and provide information to clarify biomineralization mechanism.

Contribution of surface functional groups and interface interaction to biosorption of strontium ions by Saccharomyces cerevisiae under culture conditions

Contribution of surface functional groups and detailed interface interaction for biosorption of strontium ions by Saccharomyces cerevisiae under culture conditions was investigated through chemical modification, in addition to spectroscopic and mesoscopic methods. The results showed that the biosorption ratio decreased approximately 10%, 60%, and 70% for ester group, carboxyl group, and amino group modified yeast cells, respectively. Fourier transform infrared spectroscopy and surface functional group potentiometric titration results revealed that –NH2, –COOH, and –OH were the major binding groups. The amino group displayed the greatest contribution to biosorption of strontium ions, followed by the carboxyl group and, finally, the ester group. Electrostatic interaction was the initial role and establishment of a coordination complex was the most common mechanism of interface interaction between strontium ions and the yeast cell surface. Mesoscopic analysis suggested that strontium ions may be first adsorbed on the cell surface and then transported into the cytoplasm. Transmembrane transport and the bioaccumulation model revealed that yeast cells may regulate the distribution of strontium ions through a transportation mechanism. A detailed interface interaction was discussed for S. cerevisiae biosorption of low concentration strontium ions under culture conditions. The results suggested that optimal biosorption for a microorganism relies upon enrichment of proteins and polysaccharides on the cell surface.

Calcification response of Pleurochrysis carterae to iron concentrations in batch incubations: implication for the marine biogeochemical cycle

Calcified coccolithophores, a diverse and widely distributed group of marine microalgae, produce biogenic calcite in the form of coccoliths located on the cell surface. Using batch incubations of the coccolithophorid Pleurochrysis carterae, we investigated the responses of this calcification process to iron concentrations by changing the iron supply in the initial culture media from a normal concentration to 1 ppm (parts per million), 5 ppm, and 10 ppm. Time-dependent measurements of cell population, production of inorganic carbon (coccoliths), and organic carbon (organic cellular components) showed that elevated iron supply in the growth medium of P. carterae stimulates carbon sequestration by increasing growth along enhanced photosynthetic activity and calcification. In addition, the acquired time-dependent UV-Vis and FT-IR spectra revealed that iron fertilization-enhanced coccolith calcification is accompanied by a crystalline phase transition from calcite to aragonite or amorphous phase. Our results suggest that iron concentration has a significant influence on the marine carbon cycle of coccolithophores.

Effects of riboflavin and AQS as electron shuttles on U(VI) reduction and precipitation by Shewanella putrefaciens

Understanding the mechanisms for electron shuttles (ESs) in microbial extracellular electron transfer (EET) is important in biogeochemical cycles, bioremediation applications, as well as bioenergy strategies. In this work, we adapted electrochemical techniques to probe electrochemically active and redox-active Shewanella putrefaciens. This approach detected flavins and humic-like substances of Shewanella putrefaciens, which were involved in electron transfer to the electrode. A combination of three-dimensional excitation-emission (EEM) florescence spectroscopy methods identified a mixture of riboflavin and humic-like substances in supernatants during sustained incubations. The reductive behaviour of U(VI) by Shewanella putrefaciens in the presence of riboflavin (RF) and anthraquinone-2-sulfonate (AQS) was also investigated in this study. The results indicated that RF and AQS significantly accelerated electron transfer from cells to U(VI), thus enhancing reductive U(VI). The precipitate was further evidenced by SEM, FTIR, XPS and XRD, which demonstrated that chernikovite [H2(UO2)2(PO4)2·8H2O] became the main product on the cell surface of S. putrefaciens. In a contrast, U(IV) mainly existed amorphously on the cell surface of S. putrefaciens with added RF and AQS. This work has significant implications in elucidating RF and AQS as electron shuttles that are efficient in reduction of uranium in geological environments.

Characteristics and mechanism of uranium photocatalytic removal enhanced by chelating hole scavenger citric acid in a TiO2 suspension system

Polycarboxylic acid acts as hole scavenger and chelating agent, which is essential for the photocatalytic removal of multivalent metal ions. The photocatalytic uranium removal, role of chelating hole scavenger citric acid (CA), and removal mechanism were investigated in a TiO2 suspension system. The results show that chelating agent CA is an efficient hole scavenger. The maximum removal efficiency of U(VI) reaches up to 98.6%. The uranium-bearing precipitates contains Na[(UO2)(Cit)], UO2, or UO4·2H2O. The mechanisms for the photocatalytic removal of U(VI) and the role of CA are discussed. These results suggest that proper chelating hole scavengers can promote and regulate the photocatalytic removal of multivalent metal ions.

Meta-analysis of experimental warming on soil invertase and urease activities

ABSTRACT Soil enzymes are regulate terrestrial carbon (C) and nitrogen (N) cycles, and how the activity of these enzymes are affected by soil warming duration is unclear. In the study, the effect of experimental soil warming duration (<2 years and >2 years) on invertase and urease activities were examined by meta-analysis. Soil warming increased invertase activity by 22% (<2 years) and 16% (>2 years), respectively, while they increased urease by 29% and 9%. Meta-analysis of soil warming experiments found that warming by less than 1.5°C increased invertase by 22%, while warming it by more than 1.5°C increased invertase by 29%; the corresponding increases in urease were 16% and 8%. These effects of experiment warming differed among ecosystem types, with warming-induced increases being greatest in forest ecosystem. The results of this meta-analysis suggest that invertase and urease become less responsive to warming over longer periods and invertase with greater warming-responsiveness than urease activity.

Laboratory assessment of bioleaching of shallow eutrophic sediment by immobilized photosynthetic bacteria

Eutrophic sediment is a serious problem in ecosystem restoration, especially in shallow lake ecosystems. We present a novel bioleaching approach to treat shallow eutrophic sediment with the objective of preventing the release of nitrate, phosphate, and organic compounds from the sediment to the water column, using porous mineral-immobilized photosynthetic bacteria (PSB). Bioactivity of bacteria was maintained during the immobilization process. Immobilized PSB beads were directly deposited on the sediment surface. The deposited PSB utilized pollutants diffused from the sediment as a nutritive matrix for growth. We evaluated the effects of light condition, temperature, initial pH, amount of PSB beads, and frequency of addition of PSB beads for contaminant removal efficiency during bioleaching operations. The presented study indicated that immobilized PSB beads using porous minerals as substrates have considerable application potential in bioremediation of shallow eutrophic lakes.