不同粒徑之二氧化鉛對青鱂魚成魚的生物可及性、生物累積及毒性效應評估

Abstract

奈米二氧化鉛 (nPbO2(s)) 為氯氣消毒之自來水系統中鉛管腐蝕產物的組成之一，其可能透過水質的改變 (pH、Eh及有機質含量等) 進而釋放出可溶性鉛離子 (Pb2+ (aq))。此外，nPbO2(s) 可能透過水流速度改變進而脫附，並直接提高自來水中的鉛總濃度。前人研究指出nPbO2(s) 在水溶液中的動力變化 (顆粒聚集及沉降行為) 會影響青鱂魚 (Oryzias latipes) 之生物有效性 (Bioavailability)。此外nPbO2(s) 會抑制青鱂魚幼魚的乙醯膽鹼酯酶 (Acetylcholinesterase, AChE) 活性。然而nPbO2(s) 進入青鱂魚體內的途徑、不同鉛物種對青鱂魚的生物可及性 (Bioaccessibility)、生物累積 (Bioaccumulation) 之情形，以及顆粒粒徑是否影響其毒性行為等尚待釐清。 本研究將青鱂魚分別暴露於nPbO2(s) (20-40 mg/L)、大顆粒二氧化鉛 (bPbO2(s)) (20-40 mg/L) 及硝酸鉛 (Pb2+(aq)) (200-325 µ/L) 三種材料3天至14天，並藉由X光吸收近邊緣結構 (X-ray Absorption Near-Edge Structures，XANES) 以及伏安法 (Voltammetry) 分析其暴露途徑器官 (鰓、消化道) 中鉛物種的變化，進而探討不同粒徑材料的生物可及性差異。此外，透過分析魚腦及魚肝內的鉛累積含量，以比較不同材料間所造成的鉛元素生物累積之差異情形。最後，評估不同粒徑材料對其神經毒性、氧化壓力以及鈉鉀幫浦活性影響之程度。 研究結果顯示，nPbO2(s) 及bPbO2(s) 在鰓上具有比消化道較高的解離程度。這可能表示在水中解離性低的nPbO2(s) 會在鰓的表面發生還原溶解的現象，其中又以nPbO2(s) 較bPbO2(s) 有較明顯的溶解現象。雖然消化道的鉛解離程度較低，其總鉛濃度卻較鰓高，因此消化道應仍是青鱂魚攝入PbO2(s) 顆粒之主要途徑之一。XANES圖譜也顯示鰓上的鉛物種型態較接近於Pb2+(aq)，消化道內的鉛物種變化則較不明顯。魚肝的鉛累積濃度結果，三種材料間無明顯差異，魚腦的鉛累積濃度則為bPbO2(s) 最高，nPbO2(s) 與Pb2+(aq) 累積濃度較低且近似。神經毒性方面，暴露7天三種材料皆會抑制青鱂魚成魚腦部AChE活性，但14天暴露下，硝酸鉛 (325 µ/L)、nPbO2(s) (40 mg/L) 及 bPbO2(s) (40 mg/L) 暴露之青鱂魚腦部之AChE活性卻沒有受到抑制，並與控制組間沒有顯著差異。可能是14天較高鉛濃度暴露下，誘導了青鱂魚腦部AChE活性的上升以及回復，氧化壓力的結果顯示，暴露14天的處理之下，以硝酸鉛 (325 µ/L) 的處理組青鱂魚肝臟中的脂質過氧化產物MDA含量最高，nPbO2(s) 及 bPbO2(s) 的MDA含量則較少，顯示鉛離子可能是主要導致脂質過氧化的因子。魚鰓之NKA活性則在第14天才開始有顯著的變化，其中以硝酸鉛 (325 µ/L) 的處理組有較明顯的抑制現象，nPbO2(s) 及 bPbO2(s) 雖然有抑制的趨勢，但是較不明顯。綜合上述的結果推測nPbO2(s)、bPbO2(s) 皆可能會造成水生生物的神經毒性、氧化壓力以及鈉鉀幫浦的干擾，然而詳細的作用機制仍待後續研究進一步探討。

Previous study indicates that nPbO2(s) causes inhibition of acetylcholinesterase (AChE) activity in larvae of medaka fish (Oryzias latipes). The bioavailability of nPbO2(s) is affected by particle aggregation and precipitation in water. However the uptake mechanism of nPbO2(s) in medaka fish and its bioaccessibility, bioaccumulation, and toxicity potency to aquatic organisms remain unclear. The objectives of this study is to understand the uptake mechanisms, bioaccumulation and toxic effects (on nervous system disruption) of 3 lead species [nPbO2(s), bulky lead dioxide bPbO2(s), and Pb2+(aq)] in medaka fish. Particle diameters of nPbO2(s) and bPbO2(s) measured with Transmission Electron Microscopy (TEM) were 34.5 ± 11.4 and 132.4 ± 54.2 nm respectively. The hydrodynamic diameters of nPbO2(s) and bPbO2(s) meausred with Dynamic light scattering (DLS) were 132.6 ± 36.9 and 217.3 ± 57.0 nm respectively. Results showed that both of nPbO2(s) and bPbO2(s) revealed low water solubility in dosing solutions, but nPbO2(s) had higher solubility than bPbO2(s). X-ray absorption near-edge structure (XANES) spectra also showed that both lead dioxide particles were stable in dosing solutions. However, XANES results and quantification of lead speciation showed that both nPbO2(s) and bPbO2(s) can be reductively dissolved into Pb(II) in gill and intestine with higher extend in gill tissues. As well, the nPbO2(s) has higher dissolution than bPbO2(s) in fish intestine. In addition, lead bioconcentration in liver of treated fish have no difference among lead groups; however, bPbO2(s) treatment appeared to result in higher lead bioaccumulation in fish brain, as compared with nPbO2(s) and Pb2+(aq) treatment. We observed dose-dependent inhibition of AChE activity in the brain of treated fish with 7-day exposures to three lead solutions; however, such inhibition appeared to be restored with 14-day exposure at higher concentrations. Both of nPbO2(s) and bPbO2(s) can increase the content of MDA in liver dose-dependently and the highest MDA content was found in Pb2+(aq) treated fish liver. Finally, the inhibition of NKA activity in gill was observed in nPbO2(s) and bPbO2(s) treated fish with 14-day exposure, and greatest inhibition was found in Pb2+(aq) treated fish with 14-day exposure.