|其他摘要||Arsenic (As) is one of the most toxic elements, and poses great risk to organisms due to its carcinogenic, teratogenic and mutagenic effects. Currently, water arsenic contamination has become a global environment problem. Severe As contamination has been observed in China. Xinjiang is the first place in mainland China where endemic arsenic poisoning event was reported. Furthermore, due to the exploitation of mineral resource in Xinjiang and its special arid saline environment, the water As contamination here is further aggravated. Therefore, it is necessary to develop a practical and efficient method for remediation of As-polluted water. In this work, a new kind of aquatic fern -Microsorium pteropus was selected to study its physiological responses and remediation for As in water. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and confocal laser scanning microscope (CLSM) were employed to investigate the effects of As on cell structure of M. pteropus at cell and subcell levels. The rapid chlorophyll fluorescence technology was also employed to study the effects of As on the photosynthetic system of M. pteropus. Effects of As on the antioxidant systems, osmotic regulation substances, and membrane lipid peroxidation of M. pteropus were studied by spectrophotometry. Effects of As on the root exudates of M. pteropus were studied complementarily by chromatography, infrared spectroscopy, and molecular fluorescence spectroscopy. Moreover, remediation efficiency of As in water M. pteropus for arsenic contamination was studied. The main conclusions of this study can be drawn as the followings:
(1) Membrane system, chloroplast, mitochondria, and stoma of M. pteropus were damaged by high concentration of As (Ⅲ). In M. pteropus cell, As was mainly distributed in cell walls, especially for root cell. SEM-EDX results showed that, in the solution existing simultaneously Fe2+ and As(Ⅲ), Fe and As contents were increased on surface of root tip of M. pteropus, which might lead to form the Fe plaque on surface of root tip.
(2) Results of the changes in chlorophyll fluorescence induction kinetics curves and fluorescence parameters showed that photosynthetic system of M. pteropus was significantly inhibited by high concentrations (25 mg L-1 and 50 mg L-1) of As (Ⅲ); while less influence was observed for As (Ⅲ) at low concentration (1 mg L-1) and medium concentration (5 mg L-1). Results of simultaneous determination of Photosystem I (PS I) and PS II showed that high concentrations (25 mg L-1 and 50 mg L-1) of As (Ⅲ) posed higher inhibitory effects on PS II of M. pteropus than PS I. Moreover, according to results of chloroplast pigment contents, it was showed that the synthesis of chloroplast pigment was hampered by high concentrations (25 mg L-1 and 50 mg L-1) of As (Ⅲ), but the synthesis of chlorophyll a and carotenoids was promoted by medium concentration (5 mg L-1) of As (Ⅲ). While, low concentration (1 mg L-1) of As (Ⅲ) presented less effect on the synthesis of chloroplast pigment.
(3) The synthesis of proteins in M. pteropus was significantly facilitated by 1mg L-1 and 5 mg L-1 As (Ⅲ), but the synthesis of proteins in root was facilitated, in leaf was inhibited by 25 mg L-1 and 50 mg L-1 As (Ⅲ). For antioxidant enzyme system, the activity of superoxide dismutase (SOD) in the root of M. pteropus was significantly changed, indicating that SOD played a main role in scavenging free radicals. The activity of SOD in M. pteropus root was also significantly enhanced by low concentration (1 mg L-1) and medium concentration (5 mg L-1) of As(Ⅲ), but dramatically decreased by high concentrations of As (25 mg L-1 and 50 mg L-1). Overall, the activity of SOD in M. pteropus rhizome showed an upward tendency with the increasement of As(Ⅲ) concentration. However, its activity in M. pteropus leaf tended to decrease at low concentration of As(Ⅲ) and then increase at higher concentration of As(Ⅲ). For catalase (CAT), its activity in M. pteropus root showed first a upward tendency and was decreased thereafter with the increment of As(Ⅲ) concentration. While its activity in M. pteropus stem was decreased with the increment of As(Ⅲ) concentration. The CAT activity in M. pteropus leaf was negatively correlated to its activity in M. pteropus root. Low or no expression of POD in M. pteropus was observed under the experimental conditions. Contents of MDA and free proline were significantly changed under the stress of As(Ⅲ), especially in the root, suggesting that both of them could be used as bioindicators for As-polluted water.
(4) The secretion capacity of fluorescent substances in M. pteropus was altered by the presence of As(Ⅲ). As(Ⅲ) mainly affected the protein secretion in M. pteropus root. As(Ⅲ) can not promote the secretion of one type of protein in M. pteropus root, but inhibit the production of several other proteins. Proteins or peptides secreted by M. pteropus showed a high binding affinity for As(Ⅲ). As(Ⅲ) also impeded the secretion of lactic acid in M. pteropus root.
(5) Results of As uptake of by M. pteropus under different experimental conditions showed that arsenic content in different organs of M. pteropus was increase with the increment of As (Ⅲ) concentration, but contents of different species of As in different organs were increased in different degree. The concentration of total As in roots, rhizomes and leaves of M. pteropus cultured in water containing 25 mg L-1 As (Ⅲ) was exceeded the threshold value of 1000 mg kg-1 DW in arsenic hyperaccumulators in terrestrial plants, indicating that M. pteropus possesses with the potential of remediating As-polluted water. Arsenic mainly existing in the root of M. pteropus is in organic forms. Arsenic mainly exists in the forms of trivalent inorganic arsenic in the rhizomes, and mainly in the forms of trivalent arsenic and organic one in the leaves of M. pteropus. Under the condition of 100 μmol photons m-2s-1, arsenic accumulation in M. pteropus are highest; while below or above this luminous flux, the arsenic accumulation is lower. To some degree, an antagonistic action between NaCl and As accumulation in M. pteropus was observed. But the inhibition of NaCl depended on its concentrations in different organs. The accumulations of As (inorganic As) in roots and rhizomes and inorganic As in rhizomes and leaves were inhibited by the presence of low salt (1.5% NaCl) and high salt (2.5% NaCl), respectively. Phosphorus concentration of in tap water of control group facilitated the Fe plaque on surface root tip. 0.5 mg L-1 phosphorus facilitated the transformation of organic arsenic in M. pteropus to inorganic arsenic and the accumulation of inorganic arsenic in rhizomes and leaves of M. pteropus. The accumulation of As in roots and rhizomes of M. pteropus was inhibited by the presence of 2.5 mg L-1 phosphorus. But the influence of 2.5 mg L-1 phosphorus on the accumulation of As in leaves was negotiable. Base on the results of pH, the accumulation and transformation of organic arsenic and pentavalent arsenic in M. pteropus were facilitated under acidic condition (pH=5.5); while the accumulation of As in M. pteropus was inhibited under alkaline conditions (pH=8.5). Moreover, the contents of trivalent As in M. pteropus were slightly changed under acidic condition (pH=5.5).
(6) M. pteropus showed high removal efficiency for As in tailings. 24.95 μg of As in the mine tailing leachates was removed by the presence of 1 g of M. pteropus after 5-day treatment. M. pteropus also presented high removal efficiency for Mn. Under the same conditions, the removal efficiency of Mn in the mine tailing leachates was up to 88.15%. In a relatively short period of time, a certain amount of the M. pteropus showed some sorption capacities for a certain concentration of As, indicating the presence of possible mechanisms controlling the equilibrium of As accumulation and excretion in M. pteropus.|