微塑料在海洋中的分布、生態(tài)效應(yīng)及載體作用

陳孟玲, 高 菲, 2, 王新元, 魏一凡, 許 強, 2, 劉春勝, 2

微塑料在海洋中的分布、生態(tài)效應(yīng)及載體作用

陳孟玲1, 高 菲1, 2, 王新元1, 魏一凡1, 許 強1, 2, 劉春勝1, 2

(1. 海南大學(xué) 海洋學(xué)院, 海南 ?？?570228; 2. 海南大學(xué) 南海海洋資源利用國家重點實驗室, 海南 ?？?570228)

微塑料通常被定義為最大尺寸小于5 mm的塑料碎片。受人類活動的影響, 微塑料在海洋環(huán)境中廣泛存在, 引起了人們對其潛在影響的關(guān)注。由于粒徑較小, 微塑料可以通過多種途徑進(jìn)入水生生物體內(nèi), 沿著食物鏈遷移、傳遞, 影響海洋生態(tài)系統(tǒng)的健康與穩(wěn)定。在海洋中長期停留的微塑料會吸附環(huán)境中的重金屬、有機污染物和微生物等, 加劇微塑料對海洋生物的毒性作用。本文綜述了海洋環(huán)境中微塑料的污染特征, 微塑料對海洋生物行為、生理等的影響, 以及微塑料與微生物、其他污染物的相互作用和復(fù)合效應(yīng), 并對微塑料對海洋環(huán)境及生物影響的研究進(jìn)行了展望。

微塑料; 分布; 特征; 生態(tài)效應(yīng); 載體作用

塑料是一類由單體經(jīng)加聚或縮聚反應(yīng)而形成的高分子有機化合物[1]。從20世紀(jì)70年代, 塑料已被廣泛應(yīng)用于多種領(lǐng)域[2], 世界塑料產(chǎn)量在2017年達(dá)到近3.5億噸[3]。由于全球塑料的總產(chǎn)量逐年增加, 且塑料降解速率慢, 導(dǎo)致塑料在土壤、海洋、淡水中隨處可見[4-6]。中國海洋微塑料污染形勢也不容樂觀, 塑料碎片不僅大量存在于沿海水域, 而且在南海陸坡及深海生物體內(nèi)也有發(fā)現(xiàn)[7]。

海洋中常見的塑料碎片包括聚氯乙烯(PVC)、聚酰胺(PA)、聚對苯二甲酸乙二醇酯(PET)、聚乙烯(PE)、聚丙烯(PP)和聚苯乙烯(PS)等[8](表1)。粒徑小于5 mm的塑料碎片被定義為微塑料[9], 按照其形成方式可分為初生微塑料和次生微塑料。初生微塑料是指工業(yè)生產(chǎn)過程中原初就被制備成為微米級的小粒徑塑料顆粒, 如化妝品中的塑料微珠, 由于體積小, 容易逃過水處理系統(tǒng), 這使它們能夠進(jìn)入自然排水系統(tǒng), 并最終進(jìn)入海洋中。次生塑料微粒則指大型塑料在環(huán)境中分裂或分解而成的塑料微粒或碎片。陸源塑料廢棄物在雨水、風(fēng)浪的驅(qū)動下進(jìn)入海洋中, 海上漁業(yè)活動可直接產(chǎn)生塑料廢棄物, 塑料廢棄物因光降解、氧化和水解降解作用而碎化產(chǎn)生次生微塑料[10-11]。微塑料可以吸附微生物、微藻、重金屬、抗生素等, 使其密度和表面電荷發(fā)生變化, 黏附的物質(zhì)不僅改變微塑料的毒性和生物利用度, 還會引起微塑料浮力的變化, 促使其發(fā)生沉降[12-13]。

表1 常見微塑料種類及密度

受到生物和非生物因素的影響, 微塑料在水生生態(tài)系統(tǒng)中廣泛存在, 對水環(huán)境中生物的生存造成威脅[14]。微塑料可以進(jìn)入生物的器官、組織[15], 隨著生物在食物網(wǎng)中遷移、積累[16], 對生物的生存、遺傳產(chǎn)生負(fù)面影響[17]。此外, 微塑料與生物、污染物的相互作用會威脅海洋生態(tài)系統(tǒng)的健康與穩(wěn)定[18]。本文主要綜述微塑料在海洋環(huán)境中的分布、生態(tài)效應(yīng)及載體作用, 以期促進(jìn)海洋環(huán)境中微塑料的研究工作, 為海洋環(huán)境的保護(hù)提供參考。

1 海洋環(huán)境中的微塑料

近年來, 關(guān)于微塑料污染特征的研究很多, 且大都集中在海洋環(huán)境中。海洋中的微塑料80%來源于陸地, 其次是海上生產(chǎn)活動[19]。陸地來源主要有道路運輸中的輪胎磨損和車輛風(fēng)化、工業(yè)生產(chǎn)中的污水排放和塑料制造、日常生活中的洗滌和清潔產(chǎn)品的使用等。捕撈、運輸、養(yǎng)殖和旅游等海上生產(chǎn)活動也是微塑料的來源。由此產(chǎn)生的微塑料或塑料廢物直接進(jìn)入海洋或通過參與海陸間水循環(huán)過程而間接地進(jìn)入海洋生態(tài)系統(tǒng)[20], 具體途徑主要有(圖1): 1)河流輸入; 2) 大氣沉降; 3) 污水排放; 4) 海上生產(chǎn)活動[21]。

圖1 微塑料進(jìn)入海洋生態(tài)系統(tǒng)的途徑

1.1 海洋中微塑料的分布

從極地[22]到赤道, 從人口稠密的地區(qū)到偏遠(yuǎn)無人居住的島嶼, 微塑料已經(jīng)入侵了多數(shù)海洋生物的棲息地(表1和表2), 這些塑料碎片漂浮于海洋表層水中[23]、滯留于海岸線上或沉入海底。

表2 不同海洋區(qū)域沉積物中微塑料的豐度及粒徑

注:“*”表示該區(qū)域的最大豐度

表3 不同海洋區(qū)域水體中微塑料的豐度及粒徑

注:“*”表示該區(qū)域的最大豐度”;“a”表示該區(qū)域的平均豐度, 原文中未給出標(biāo)準(zhǔn)差

1.1.1 近海微塑料污染

近岸海域是海洋系統(tǒng)和陸地系統(tǒng)的交匯區(qū)域, 與人類活動息息相關(guān)[24]。目前, 有關(guān)世界各沿海地區(qū)微塑料分布的研究較多, 主要包括潮間帶微塑料的污染特征、近岸海水和近岸沉積物中微塑料的污染特征等[25-27]。微塑料進(jìn)入海洋后隨著風(fēng)浪、懸浮沙及底質(zhì)運動而遷移, 最終匯集在海灘和近岸沉積物中[28]。

海灘和近岸沉積物中微塑料的來源復(fù)雜, 其豐度高于深海沉積物中微塑料的豐度[29-30]。Juan等[31]發(fā)現(xiàn)大西洋沿岸加那利群島某些區(qū)域的海岸沉積物中微塑料豐度大于100 g·L–1, 而Cauwenberghe等[32]對采自大西洋和地中海11個站位的深海沉積物進(jìn)行微塑料分離, 平均豐度僅為0.5個·25 cm–3。如果將沉積物的單位都換算為cm3, 微塑料豐度也存在數(shù)量級的差異。

受人類活動影響程度不同的海灘和近岸沉積物中, 微塑料的豐度也存在較大差異[33]。葡萄牙沿岸沉積物中的微塑料豐度范圍為1.5～362個·m–2, 港口及工業(yè)區(qū)附近海灘沉積物中的微塑料豐度明顯更高[34]。黃渤海沿岸的微塑料豐度變化范圍也較大(1.3～ 14 712.5個·kg–1), 且未開發(fā)海灘(缺乏管理)>旅游海灘>漁港附近海灘[35]。此外, 季風(fēng)、洋流等也會影響微塑料在海灘和近岸沉積物中分布。因季風(fēng)和洋流的影響, 韓國蘇爺島(soya)海灘不同區(qū)域沉積物中的微塑料豐度差異高達(dá)100倍, 其中南岸海灘的微塑料豐度相對較高[36]。

1.1.2 遠(yuǎn)洋微塑料污染

遠(yuǎn)洋海域存在大量的微塑料, 據(jù)探險科學(xué)組織報道稱, 南極半島海水樣品中的微塑料平均豐度為22個·L–1, 最大濃度高達(dá)117個·L–1[37]。與近岸相比, 遠(yuǎn)海水體中微塑料的豐度與洋流、波浪、風(fēng)力湍流等關(guān)系更為密切。其中, 海洋環(huán)流在微塑料的運輸和積累方面可發(fā)揮重要作用, 對北太平洋副熱帶環(huán)流和南太平洋環(huán)流的研究表明, 微塑料的豐度分別為334 271和26 898個·km–2[38-39]。Cózar等[40]發(fā)現(xiàn)開放式海域大部分的微塑料主要積聚在5個副熱帶環(huán)流的輻合帶。在夏威夷與加利福尼亞之間的海域中, 微塑料豐度高達(dá)6.8×105個·km–2[41]。在大西洋上升流和非上升流區(qū)域, 受上升流影響, 微塑料豐度較低且沒有顯著性差異[42]。

遠(yuǎn)海微塑料豐度相對近岸海水較低。西北太平洋沿同一維度從西向東微塑料豐度呈遞減趨勢, 從1.5×104個·km–2遞減到6.6×102個·km–2[43]。塑料生產(chǎn)量、消費量的激增導(dǎo)致遠(yuǎn)海微塑料積累量越來越多, 遠(yuǎn)海海水樣品微塑料的豐度隨著取樣時間的推移而增大。研究者對同一海域的樣品進(jìn)行分析, 2018年微塑料平均豐度相對2017年增長5.2×104個·km–2[43-44]。盡管遠(yuǎn)海微塑料的豐度存在空間異質(zhì)性, 但是海水中微塑料檢出率也有隨時間推移而增加的趨勢[45]。20世紀(jì)70年代, 在東北太平洋約64%的調(diào)查地點(21個)發(fā)現(xiàn)了塑料碎片[46]。然而, 最近兩次巡航中的所有調(diào)查地點的微塑料檢出率為100%[43-44]。

1.1.3 深海微塑料污染

探究水體和沉積物中微塑料的報道較多[47-48], 但是大多數(shù)樣品來自近岸水環(huán)境。關(guān)于微塑料在深海中污染特征的研究較少, 大多集中在探究深海沉積物中微塑料的豐度[13, 32, 49]。海底被認(rèn)為是海洋塑料垃圾的一個“匯”[50], 粒徑接近或與其他有機物聚集形成“海洋雪”到達(dá)深海。但由于“海洋雪”的沉降速率為1～ 368 m·d–1[51], 且塑料顆粒在表面張力和海洋洋流的作用下有較長時間停留在海面和懸浮在水體中[13], 深海沉積物中的微塑料豐度相對近岸沉積物較低[52]。

早在2013年, Cauwenberghe等[32]就調(diào)查了南冰洋以及北大西洋深海(水深>4 000 m)沉積物中微塑料的分布, 結(jié)果表明該區(qū)域微塑料豐度較低, 為0.5～1個·25 cm–3; 同樣在地中海(水深3 500 m)、太平洋(水深2 200 m)、印度洋(水深1 000 m)深海沉積物中也檢測到較低的微塑料豐度, 分別為10～35個·50 mL–1、6～40個·50 mL–1和1.5～3.5個·50 mL–1[13, 49]。極地哈斯加藤天文臺附近的深海(水深5 570 m)垃圾在2002年到2014年期間不斷增加[53], 2015年采集的沉積物樣品中微塑料豐度最高可達(dá)6 595個·kg–1。有研究認(rèn)為較高的微塑料來源于海冰的溶化[54]。此外, 也有少數(shù)研究探索了深海水體中的微塑料豐度。例如, Courtene-Jones等[55]在大西洋東北部的羅卡爾海槽深海(水深2 227 m)水體中檢測到微塑料的濃度為70.8個·m–3。

深海微塑料的粒徑普遍較小。在南極海深海沉積物中的微塑料有65%粒徑小于1 mm[32], 西北太平洋千島海溝深海(水深5 766 m)沉積物中微塑料粒徑均小于1 mm[49]。大多數(shù)深海生物直接或間接地以海洋有機碎屑為食[56], 容易攝入粒徑接近“海洋雪”的微塑料。Taylor等[56]在大西洋、印度洋海域包括刺胞動物、棘皮動物和節(jié)肢動物等在內(nèi)的深海底棲無脊椎動物體內(nèi)分離出微塑料; 大西洋洛克爾海槽底棲無脊椎動物海蛇尾、海星和峨螺體內(nèi)也發(fā)現(xiàn)了微塑料, 檢出率為48%[55]。然而, 大規(guī)模調(diào)查深海微塑料污染的研究較少, 微塑料對底棲生物的生態(tài)風(fēng)險也需要深入研究。

綜上所述, 微塑料廣泛分布于世界各海域中[57-58], 且具有空間異質(zhì)性, 其差異大小取決于人類活動強度、海岸開發(fā)程度、水動力學(xué)、河流排水和航運交通等因素[59]。另外, 取樣方法可能影響結(jié)果的準(zhǔn)確性, 導(dǎo)致某些觀測值存在偏差, 例如, 有研究表明利用拖網(wǎng)收集海洋表層水體中的微塑料時, 網(wǎng)目的大小和微塑料的豐度值也存在負(fù)相關(guān)[60], 使用網(wǎng)目較大的浮游生物網(wǎng)可能會低估水體中微塑料的豐度; 也有研究認(rèn)為網(wǎng)目大小和微塑料的豐度值之間缺乏相關(guān)性, 例如南大西洋中的公?；蚰戏情_普省附近的沿海水域[61-62]。此外, 描述微塑料豐度的單位很多, 導(dǎo)致不同研究之間的結(jié)果難以比較。整體而言, 海岸沉積物的微塑料豐度總體上高于深海沉積物; 近岸水域微塑料來源較多、海陸環(huán)境之間存在復(fù)雜的相互作用, 微塑料豐度的異質(zhì)性更高; 遠(yuǎn)海和深海中微塑料的豐度較低, 且前者大于后者。

1.2 海洋中微塑料的特征

由于來源的多樣性, 海洋中微塑料的種類繁多, 有不同的密度、粒徑、形狀和顏色。微塑料的特征會影響其在海洋中的分布和生態(tài)效應(yīng)。例如, 粒徑較小或顏色與浮游生物相近的微塑料易被海洋生物攝入[63], 密度則直接影響微塑料的分布。

1.2.1 海洋中微塑料的粒徑

微塑料的最大粒徑為5 mm[9], 最小粒徑因采樣和實驗處理方法不同而存在較大差異(如圖2)。目前多采用浮選法分離沉積物中的微塑料[22], 檢測出的微塑料的最小粒徑與使用的篩孔和濾膜的孔徑大小相關(guān)。除少數(shù)研究使用采水器外, 通常使用浮游生物網(wǎng)或bongo網(wǎng)采集海水中的微塑料[2, 60]。海水中微塑料的最小尺寸因拖網(wǎng)使用的網(wǎng)目不同在1～500 μm范圍內(nèi)變化, 333 μm為最常見[64]。通過篩分和測量可以將微塑料的尺寸分類, 用以分析微塑料粒徑的頻數(shù)分布。與沉積物中微塑料的粒徑相比, 海水中微塑料的粒徑頻數(shù)分布較離散。

1.2.2 海洋中微塑料的顏色

微塑料的顏色可以分為白色、無色透明、黃色、黑色、藍(lán)色等, 透明微塑料在海洋環(huán)境中最為常見。在微塑料研究中, 通常通過鏡檢進(jìn)行分離, 鏡檢過程中有色微塑料容易辨別, 而透明和白色等較淺顏色的微塑料卻易被忽略, 因而環(huán)境中的透明或白色微塑料數(shù)量可能被低估[60]。

圖2 海洋環(huán)境中微塑料的粒徑范圍

微塑料的顏色分類可以為后期溯源提供線索。有研究認(rèn)為透明微塑料主要來源于一次性塑料制品, 如塑料袋[65], 而藍(lán)色微塑料被認(rèn)為與研究區(qū)域的漁業(yè)活動(漁具損壞等)有關(guān)[66]。但塑料碎片的風(fēng)化以及在海洋中停留的時間長短都會影響微塑料的顏色, 許多有色塑料可能在進(jìn)入海洋后逐漸褪去顏色[67]。因此, 顏色僅可用作微塑料來源的初步判斷, 準(zhǔn)確鑒定尚需借助其他手段。

此外, 調(diào)查發(fā)現(xiàn)水生生物對微塑料的攝食也受到其顏色的影響。棲息在沉積物中的水生動物傾向于攝食有色微塑料[68], 一些魚類的幼魚則會攝取更多的與浮游生物顏色相近的微塑料[63, 69]。

1.2.3 海洋中微塑料的形狀

海水、沉積物樣品和海洋生物體內(nèi)均發(fā)現(xiàn)了不同形狀的微塑料[70-71], 主要包括纖維、球形、薄膜以及泡沫微塑料等。纖維微塑料在海洋中最為常見, 在部分研究區(qū)域內(nèi)的豐度在90%以上[72]。

微塑料的形狀與塑料碎片在海洋中停留的時間長短和降解過程有關(guān), 并間接影響微塑料在海洋中的分布。大粒徑塑料碎片的形狀不規(guī)則, 而老化微塑料的邊緣較光滑[73]。Zhang等[28]研究認(rèn)為塑料纖維和薄膜比塑料微球的浮力更大、沉降速度更低。微塑料的形狀還可用于分析微塑料的潛在來源。例如, 纖維微塑料主要源于洗滌和漁業(yè)活動[74], 微塑料薄膜被認(rèn)為源自一次性塑料袋或農(nóng)用薄膜[75], 而球形微塑料主要為清潔類化妝品或工業(yè)原料中的塑料顆粒。

2 微塑料對海洋生物的生態(tài)效應(yīng)

微塑料一旦進(jìn)入水生生態(tài)系統(tǒng)后, 由于聚合物形狀、粒徑和密度的不同, 能夠廣泛地分布在淡水、海水以及沉積物中[76], 從而對不同生境或營養(yǎng)級的生物產(chǎn)生影響[77]。密度較低的塑料碎片在浮力和海水表面張力的作用下聚集在海洋微表層中[78], 影響藻類的光合作用以及浮游動物的行為、攝食和排糞[79-80]。粒徑、形狀等特征則決定了微塑料在生物體內(nèi)的滯留時間以及產(chǎn)生的損傷程度[81-84]。

近年來, 有關(guān)微塑料對水生生物毒性效應(yīng)的室內(nèi)研究快速增加。據(jù)報道, 超過690種水生動物攝入了塑料或微塑料[85]。小粒徑微塑料可穿過細(xì)胞膜直接進(jìn)入小型浮游植物體內(nèi), 對其光合作用等產(chǎn)生影響; 水生生物直接或間接攝入的微塑料顆粒不僅會對生物機體造成機械損傷和應(yīng)激反應(yīng)[86], 還可能為某些有害物質(zhì)(包括塑料添加劑、從周圍環(huán)境吸收的污染物和病原微生物)進(jìn)入水生食物網(wǎng)提供載體作用[14]。此外, 微塑料會隨著食物鏈由低營養(yǎng)級向高營養(yǎng)級傳遞,對人類食品安全帶來潛在威脅[87]。

2.1 微塑料對海洋初級生產(chǎn)者的毒性效應(yīng)

到目前為止, 關(guān)于微塑料對海洋初級生產(chǎn)者生態(tài)毒理學(xué)影響的研究僅限于浮游植物中的微藻, 且大多集中于浮游植物暴露于微塑料后的生長動態(tài)。

浮游植物在海洋生態(tài)系統(tǒng)中扮演著重要角色。微塑料與微藻之間存在吸附作用[88], 可能會抑制藻細(xì)胞的生長[89]、降低藻類的光合效率等[88], 這種吸附作用取決于微塑料的粒徑。粒徑較大的微塑料可能成為微藻附著、生長的載體[90], 而小粒徑微塑料可以吸附在微塑料表面, 對藻體與環(huán)境之間能量和物質(zhì)的轉(zhuǎn)移產(chǎn)生影響。研究表明, 0.05 μm的微塑料暴露能夠抑制杜氏鹽藻()的生長, 但粒徑為6 μm的微塑料對杜氏鹽藻的生長無顯著影響[67]。Canniff和Hoang[90]發(fā)現(xiàn)粒徑為63～75 μm的微塑料暴露組月牙藻()的濃度增加了56%, 其原因可能是粒徑較大的微塑料為月牙藻()的生長提供了附著基。

小粒徑微塑料可以對微藻的生長、光合作用產(chǎn)生影響[91], 且存在濃度依賴效應(yīng)。Zhang等[92]發(fā)現(xiàn)1 μm PVC微球可以包裹在中肋骨條藻()表面, 對微藻的生長和光合作用產(chǎn)生了抑制, 其葉綠素含量和光合效率均顯著降低, 96 h后最大生長抑制率(IR)達(dá)39.7%。聚苯乙烯微塑料對蛋白核小球藻()的生長、光合作用具有劑量效應(yīng)的負(fù)面影響, 同研究還發(fā)現(xiàn)微塑料會造成類囊體變形和細(xì)胞膜受損等現(xiàn)象[93]。

此外, 微塑料還可以影響微藻在水體中的分布。角毛藻(e)可以通過釋放胞外黏性多糖與微塑料形成異聚體[94], 導(dǎo)致黏附在瓶壁上的異聚體隨時間增加而增多。Bhattacharya等[88]發(fā)現(xiàn)微塑料可以附著在柵藻()鞭毛上, 干擾微藻的運動、分布。值得注意的是, 微塑料還可能作為有毒微藻的載體, 對人類和動物的健康產(chǎn)生潛在影響[88]。

2.2 微塑料對海洋動物的影響

2.2.1 生長發(fā)育

一旦微塑料進(jìn)入水生生物體內(nèi), 在消化系統(tǒng)中的積累會直接影響生物的攝食和生長發(fā)育[95]。攝入的微塑料可能積聚在水生動物的消化道中, 甚至堵塞消化道, 從而產(chǎn)生虛假的飽腹感, 導(dǎo)致攝食率下降[96]。Bour等[97]觀察到暴露于微塑料(125～500 μm)中的雙殼貝類()體內(nèi)總能量儲備隨著微塑料濃度的增加而降低。海洋橈足類()在濾食含尼龍–6(5～20 μm)的藻液后, 攝食率和濾水率均降低, 且存在劑量-效應(yīng)關(guān)系[80]。攝食率的持續(xù)降低可能對水生生物產(chǎn)生各種有害影響, 例如體重減輕、生長抑制等。魚類攝食微塑料還可能導(dǎo)致腸道環(huán)境惡化, 對其營養(yǎng)吸收和生長造成影響[98]。

2.2.2 行為特征

行為特征是反映生物健康程度的重要因素[99], 然而微塑料對海洋生物行為影響的相關(guān)報道較少。微塑料可能對生物的集群、游泳、捕食和勘探等行為有負(fù)面影響。將許氏平鱸()暴露于含聚苯乙烯微球(15 μm)的水體中14 d, 其游泳速度下降、捕食勘探范圍變小而產(chǎn)生了集群行為[100]。微塑料還可附著在甲殼類浮游動物的觸角、附肢等部位, 對其游泳速度產(chǎn)生影響[83-84]。對紫貽貝[101]、牡蠣[102]等進(jìn)行微塑料暴露實驗也有類似的結(jié)果——微塑料對攝食和游泳行為產(chǎn)生了影響。微塑料除削弱動物的原有行為能力外, 還會使其產(chǎn)生異常行為。例如, 湯氏紡錘水蚤()幼體暴露在塑料微球后, 游泳時出現(xiàn)“跳躍”行為[103]。

2.2.3 生殖

微塑料可對生物的生殖產(chǎn)生干擾, 導(dǎo)致生殖細(xì)胞數(shù)量減少或質(zhì)量降低。對于體外受精的海洋生物, 其配子質(zhì)量可能會直接受到水體中微塑料的影響。González-Fernández等[104]發(fā)現(xiàn)含羧基的微塑料可增加長牡蠣()精細(xì)胞內(nèi)的活性氧。也有研究發(fā)現(xiàn)聚苯乙烯微塑料對雄性牡蠣的生殖細(xì)胞產(chǎn)生了負(fù)面影響, 使其運動水平下降[102]。

微塑料對生物繁殖的影響與微塑料的劑量和成分有關(guān)。將微塑料(0.05 μm)設(shè)置梯度濃度(0.1、1、10、25 μg·mL–1)對長牡蠣的生殖細(xì)胞和受精卵進(jìn)行暴露實驗, 發(fā)現(xiàn)25 μg·mL–1的微塑料致使受精率、孵化率嚴(yán)重下降(19.6%、76.4%); 同研究表明在濃度為1、10、25 μg·mL–1時, 含氨基成分的微塑料(NH2-50 nm)對胚胎形成的抑制率為100%[105]。

此外, 微塑料可能通過干擾生物的能量收支, 對生物的繁殖造成損害[102]。微塑料使珍珠貝()能量攝入降低, 但是代謝率并未降低, 彌補能量收支平衡的方式為減少用以繁殖的能量支出[106]。

2.2.4 免疫系統(tǒng)

微塑料污染可以使海洋生物產(chǎn)生一系列的應(yīng)激反應(yīng), 干擾生物的免疫防御系統(tǒng)。由于更容易嵌入生物組織[82]、停留的時間更長, 纖維微塑料對斑馬魚()腸道造成的毒性更強, 包括黏膜損傷、通透性增加和炎癥等, 破壞其腸黏膜的免疫屏障[83]。高濃度(9.0×1010個·L–1)聚苯乙烯微塑料暴露能夠激活造礁石珊瑚()的應(yīng)激反應(yīng), 并通過c-Jun氨基末端激酶(JNK)和細(xì)胞外調(diào)節(jié)蛋白激酶(ERK)信號通路抑制其免疫系統(tǒng)[107]。海膽()體腔液中添加氨基聚苯乙烯(PS-NH2, 10～25 g·mL–1)顆粒, 可使得吞噬細(xì)胞的溶酶體膜的不穩(wěn)定性增加，引起細(xì)胞凋亡[108]。海洋生物接觸特定濃度的微塑料可能會在短時期內(nèi)產(chǎn)生免疫反應(yīng), 但一段時間后機體會產(chǎn)生適應(yīng)機制。用高密度聚乙烯(HDPE)微塑料混合糠蝦對海馬()進(jìn)行了為期45 d的喂養(yǎng)實驗, 僅在實驗早期(0～15 d)發(fā)現(xiàn)海馬體內(nèi)超氧化物歧化酶(SOD)和過氧化氫酶(CAT)活性的增加, 但很快又恢復(fù)了正常水平[109]。首次暴露于HDPE微塑料中, 貽貝()體內(nèi)的免疫和應(yīng)激反應(yīng)相關(guān)基因出現(xiàn)了差異性表達(dá), 第2次暴露實驗后消化腺中免疫和應(yīng)激相關(guān)基因的表達(dá)量降低, 推測貽貝產(chǎn)生了適應(yīng)機制[110]。

2.2.5 基因表達(dá)與遺傳

微塑料可通過影響相關(guān)基因的表達(dá)而干擾生物的內(nèi)環(huán)境穩(wěn)定。長牡蠣()濾食微塑料后, 胰島素信號通路相關(guān)基因表達(dá)下調(diào), 對生殖細(xì)胞增殖和成熟產(chǎn)生負(fù)面影響[102]。在海洋環(huán)境中放置3個月的微塑料(PE)暴露日本青鳉()2個月, 對雌激素受體(ERa)介導(dǎo)的基因表達(dá)產(chǎn)生影響, 使卵殼前體蛋白H的表達(dá)量顯著降低[111]。

生物體攝入和積累微塑料可能會產(chǎn)生遺傳毒性。滕瑤[112]研究發(fā)現(xiàn), 微塑料的存在可以使十溴代聯(lián)苯醚對櫛孔扇貝細(xì)胞造成的DNA損傷程度由輕度提高至中度, 對扇貝產(chǎn)生遺傳毒性。高濃度以及小粒徑的聚苯乙烯微塑料使日本虎斑猛水蚤()的F1代存活率顯著下降[113]。浮游動物大型蚤()在聚苯乙烯微塑料暴露21 d后, 其后代的體型變小, 且畸形率高達(dá)68%[91]。

3 微塑料的載體作用

作為聚合物、殘余單體和化學(xué)添加劑組成的復(fù)雜混合物, 微塑料具有比表面積大、疏水性高的特征, 可吸附重金屬或抗生素等化學(xué)污染物[114], 還可以成為細(xì)菌等微生物生長的載體[115]。此外, 為了提高使用性能, 在塑料制作過程中通常還加入一些添加劑, 例如烷基酚、雙酚A、多溴聯(lián)苯醚和鄰苯二甲酸酯[11], 它們也會隨著微塑料進(jìn)入海洋環(huán)境中。微塑料攜帶污染物隨海水運動進(jìn)入海洋的每個角落, 在生物體內(nèi)富集并沿著食物鏈進(jìn)一步傳遞, 對海洋生態(tài)系統(tǒng)的穩(wěn)定與健康產(chǎn)生影響[116]。

3.1 微塑料作為微生物的載體

作為微塑料污染相關(guān)的問題之一, 海洋中微塑料與微生物之間的生態(tài)相互作用逐漸引起人們的關(guān)注[117-118]。微生物通過生物膜的形式在微塑料表面進(jìn)行附著、增殖[119], 最終形成生物群落。微塑料的存在不僅減弱了環(huán)境因素改變對細(xì)菌群落的影響, 還可以作為細(xì)菌在海水與沉積物之間積累、遷移的載體[120], 使生物群落在海水運動作用下進(jìn)行長距離輸送或因改變微塑料的浮力而一起匯集到沉積物中[121-122]。

微塑料表層形成的生物膜中, 微生物的群落結(jié)構(gòu)和多樣性與環(huán)境中不同[121]。Wu等[120]對渤海灣沉積物、水體中以及微塑料附著的細(xì)菌群落進(jìn)行分析, 發(fā)現(xiàn)微塑料選擇性富集了某些細(xì)菌(例如鹽桿菌科和假交替單胞菌科), 其中潛在致病菌(假單胞菌和芽孢桿菌)的豐度明顯高于周圍環(huán)境。將微塑料置于環(huán)境中培養(yǎng)生物膜, 與天然基質(zhì)相比, 微塑料不僅可以改變微生物群落結(jié)構(gòu), 還可以影響微生物的生態(tài)功能[123]。Sun等[124]也發(fā)現(xiàn)微塑料可抑制海水嗜堿鹽單胞菌的生長, 對海洋氮循環(huán)有潛在影響。有研究表明弧菌和假單胞菌是微塑料表面上的優(yōu)勢菌[117], 對海洋生物均存在致病性, 前者是導(dǎo)致珊瑚白化的主要病原體[118]。

當(dāng)微塑料吸附病原微生物[125-126]或當(dāng)外來物種通過微塑料運輸而轉(zhuǎn)移到新環(huán)境中時[121], 可能會引起生態(tài)問題。Zhang等[127]調(diào)查了工廠化養(yǎng)殖水體中微塑料對抗生素耐藥菌(ARB)的富集作用, 微塑料中可培養(yǎng)ARB的多樣性和豐度均高于水樣, 微塑料表層多重抗生素耐藥細(xì)菌中檢測到更多的抗生素抗性基因(ARG)。研究人員對珊瑚礁生態(tài)區(qū)水體中微塑料表面生物膜進(jìn)行分析發(fā)現(xiàn), 附著的優(yōu)勢細(xì)菌中包括與珊瑚組織損傷密切相關(guān)的弧菌()、紅細(xì)菌()和黃桿菌()[118]。

生物膜的形成可以降低微塑料的疏水性, 增加微塑料表面親水基團(tuán)的豐度, 促進(jìn)金屬離子、抗生素等物質(zhì)在微塑料表面的吸附[128]。將聚乙烯(PE)置于海水中, 微塑料表面生物膜的親水性、C-O和C=O基團(tuán)的豐度隨著時間的推移而增加[129]。Wang等[130]研究表明, 生物膜可以通過改變微塑料的吸附性能促進(jìn)Cu2+和四環(huán)素(TC)的吸附和穩(wěn)定。與相對裸露的微塑料表面相比, 含生物膜的微塑料(HDPE和PP)表面硅、鋁含量更高, 并且可以為放射性核素提供環(huán)境“匯”[131]。

此外, 生物膜的形成對水中的微塑料分布具有重要意義。黏附在微塑料表面的微生物可以保護(hù)微塑料免受紫外線輻射而間接延長塑料顆粒的壽命[132]。McCormick等[133]的研究表明, 微塑料可以作為降解碳?xì)浠衔镂⑸锏母街? 從而提高微塑料的降解速率。生物膜還可以增加微塑料密度從而改變微塑料在海水中的分布, 導(dǎo)致其下沉, 最終到達(dá)沉積物中[134]。

3.2 微塑料作為其他污染物的載體

3.2.1 重金屬

目前, 有關(guān)微塑料與金屬相互作用的研究主要涉及微塑料對金屬離子的吸附機理以及影響微塑料對金屬離子吸附效應(yīng)的因素, 包括微塑料類型、微塑料濃度、pH、接觸時間、溫度和作用介質(zhì)等[135-137]; 僅有少數(shù)研究對環(huán)境中微塑料吸附的金屬含量與環(huán)境中、生物體內(nèi)金屬含量的相關(guān)性進(jìn)行了探討[138-139]。

金屬離子能夠直接吸附到微塑料表面[140]。相對于原生微塑料, 次級微塑料表面更易吸附金屬離子[141]。光照、溫度等因素也可以改變微塑料的比表面積以及表面的含氧基團(tuán)數(shù)量[142], 增強微塑料對金屬離子的吸附能力。Wang等[143]使用紫外線輻射聚對苯二甲酸乙二酯(PET)模擬環(huán)境中微塑料的光老化, 結(jié)果發(fā)現(xiàn)老化微塑料對重金屬(Cu2+和Zn2+)的吸附能力高于未經(jīng)紫外處理的微塑料, 且隨著輻射時間的延長而增強。芬頓試劑處理導(dǎo)致聚苯乙烯微塑料的比表面積、表面羰基和羧基數(shù)量增加, 對Cd2+的吸附能力增強[144]。除吸附在微塑料表面, 金屬還可以作為塑料制品的添加劑, 例如鎘(Cd)、鋅(Zn)[145], 以及用作聚氯乙烯(PVC)熱穩(wěn)定劑的鉛(Pb)[146]。

結(jié)合在微塑料表面或作為添加劑的金屬很容易解吸附或浸出[137-146], 因此, 微塑料不僅對重金屬具有載體功能, 還可能提高金屬污染物的生物利用度, 對生物健康產(chǎn)生影響。Martin和Turner[147]發(fā)現(xiàn)在模擬無脊椎動物體內(nèi)消化環(huán)境的化學(xué)條件下, 微塑料作為載體會增加Cd的生物利用度, Cd總量為1 000 μg·g–1的微塑料所含生物可利用的金屬比沉積物多3 200倍左右。體外模擬探究吸附在5種微塑料表面的重金屬在人體消化系統(tǒng)中的釋放過程, 結(jié)果表明聚乳酸(PLA)微塑料中Cr4+的生物利用度在胃中最高, 為19.9%, 原因可能為PLA在消化酶和胃酸中易降解, 從而使PLA表面的Cr4+的釋放速率提高[137]。單獨使用微塑料或汞暴露歐洲鱸魚(), 幼魚的游泳速度和運動耐受時間顯著降低, 而微塑料和汞混合暴露會加劇對幼魚游泳能力的影響[148]。Wang等[149]發(fā)現(xiàn), 相對于微塑料或重金屬(Cd、Zn、Pb)單一因子的作用, 微塑料和重金屬的混合物可以更顯著地改變海水青鳉()腸道微生物的多樣性。此外, Imran等[150]的研究表明, 當(dāng)攜帶金屬離子的微塑料沿食物鏈進(jìn)入人體后, 已經(jīng)存在于人體腸道中的致病菌會接觸其表面的金屬, 可能會進(jìn)一步引起金屬驅(qū)動的多重耐藥性人類病原體的產(chǎn)生。

3.2.2 添加劑及有機化學(xué)污染物

除金屬外, 調(diào)查研究發(fā)現(xiàn)環(huán)境中微塑料還可以吸附多環(huán)芳烴(PAHs)、有機氯農(nóng)藥(OCPs)、有機鹵化物、鄰苯二甲酸酯(PAEs)和有機磷酯類(OPEs)等化學(xué)污染物[114, 151-155]。海洋環(huán)境中微塑料攜帶的化學(xué)污染物可以分為兩類: 內(nèi)源化學(xué)添加劑和外源吸附的化學(xué)污染物[155]。Mato等[156]從日本4個海岸收集的聚丙烯(PP)樹脂球中檢測出多氯聯(lián)苯(PCBs)、二氯二苯二氯乙烯(DDE)和壬基酚(NP), 室內(nèi)實驗發(fā)現(xiàn)原生PP顆粒吸附的PCBs和DDE濃度有顯著、穩(wěn)定的增加且是海水中105～106倍; NP濃度卻發(fā)生顯著變化, 表明野外微塑料的PCBs和DDE來源于海水中的疏水性有機污染物, 而NP可能來源于添加劑。

目前實地調(diào)查環(huán)境中微塑料攜帶化學(xué)污染物特性的研究較少[154], 調(diào)查方法的局限性導(dǎo)致難以區(qū)分內(nèi)源添加劑和外源吸附化學(xué)污染物[157]。塑料加工過程中, 通常添加增塑劑、阻燃劑、穩(wěn)定劑、抗氧化劑和色素等添加劑，以此來改善成型性能和制品的使用性能[158]。添加劑通常不與聚合物形成共價鍵, 因此, 它們?nèi)菀讖乃芰现薪霾⑦M(jìn)入周圍環(huán)境中, 例如通過氫鍵或范德華力與聚合物結(jié)合的鄰苯二甲酸酯(PAEs)[159]。微塑料釋放的添加劑可對海洋生物產(chǎn)生影響[160]。Browne等[161]利用添加劑(三氯生)對微塑料進(jìn)行預(yù)處理, 將海蚯蚓()暴露在含有微塑料的沙子中, 結(jié)果發(fā)現(xiàn)海蚯蚓對沉積物的擾動能力降低, 死亡率超過55%。常作為增塑劑的鄰苯二甲酸二丁酯(DBP)和微塑料暴露能夠影響微藻細(xì)胞大小和葉綠素?zé)晒鈴姸? 導(dǎo)致藻細(xì)胞產(chǎn)生質(zhì)壁分離等結(jié)構(gòu)異常[162]。因此, 在探究微塑料對添加劑吸附和釋放的動力學(xué)基礎(chǔ)上, 需深入了解微塑料結(jié)合添加劑對海洋生物的影響。

外源吸附有機化學(xué)污染物主要來源于工業(yè)、農(nóng)業(yè)和生活, 例如工業(yè)生產(chǎn)使用的多溴聯(lián)苯醚[163]、抗生素以及防曬霜中的氧芐酮[164-165]。目前有關(guān)微塑料對不同有機化學(xué)污染物吸附動力學(xué)的研究較多[136, 166]。影響有機化學(xué)污染物吸附和釋放的因素較多, 包括微塑料類型、性質(zhì)、粒徑和海洋環(huán)境條件等[167-170]。例如在海水中, 聚乙烯(PE)、聚苯乙烯(PS)和聚氯乙烯(PVC)三種材料的微塑料中, 密度最小的聚乙烯(PE)對四環(huán)素的吸附容量最大[171]。Ma等[172]研究表明微塑料的表面特征和結(jié)晶程度影響污染物在不同密度微塑料中的擴散系數(shù), 從而影響其對污染物的吸附容量。但Li等[173]對比微塑料在淡水與海水中的吸附行為發(fā)現(xiàn), 相對于低密度的聚乙烯(PE), 密度較高的聚酰胺(PA)在淡水生態(tài)系統(tǒng)中對抗生素的吸附能力反而最強。因此, 密度是影響微塑料吸附有機污染物的重要因素, 但也與反應(yīng)條件有關(guān)。

微塑料攜帶的化學(xué)污染物具有在生物體內(nèi)解吸附的能力[151]。模擬海鳥消化系統(tǒng)的化學(xué)條件下, 在胃和肝中檢測出微塑料浸出的大量多溴聯(lián)苯醚[174]。海洋青鳉()的胚胎和幼體接觸含3種污染物的微塑料后, 含全氟辛基磺酸的微塑料(MP-PFOS)會降低胚胎存活率并阻止孵化; 含苯并芘(MP-BaP)或苯并酮(MP-BP3)的微塑料使幼體生長發(fā)育和行為異常[175]。持久性有機污染物(POPs)可以通過食物鏈從微塑料進(jìn)入鹵蟲無節(jié)幼體, 進(jìn)而轉(zhuǎn)移到斑馬魚體內(nèi)[176]。因此, 了解有機化學(xué)污染物與微塑料相互作用對生物的影響是評估其對海洋生態(tài)系統(tǒng)潛在風(fēng)險的基礎(chǔ)。

總而言之, 漂浮在表層海水中密度較低的微塑料能夠吸附化學(xué)污染物或形成生物膜, 可以進(jìn)入浮游生物體內(nèi)[177]; 聚苯乙烯(PS)等微塑料具有比海水更高的密度, 可以攜帶化學(xué)污染物或微生物沉入水底, 進(jìn)入底棲生物體內(nèi)。此外, 微塑料作為載體可使化學(xué)污染物或微生物沿著食物鏈轉(zhuǎn)移, 并可能會影響人類健康。因此, 需要在了解微塑料在海洋環(huán)境中分布特征的基礎(chǔ)上, 探究微塑料攜帶的化學(xué)污染物的濃度和來源, 評估微塑料與化學(xué)污染物、微生物相互作用對海洋生物的影響, 維持海洋生態(tài)系統(tǒng)的健康與穩(wěn)定。

4 展望

微塑料廣泛分布在海洋生物棲息的每個角落。迄今為止, 微塑料的研究主要集中在環(huán)境中的微塑料污染特征以及生物攝入的有害影響, 其影響程度與微塑料的類型、濃度以及生物的大小有關(guān); 也有部分學(xué)者探究了微塑料和其他典型污染物的吸附機理和聯(lián)合毒性作用。大多數(shù)微塑料毒性效應(yīng)的研究都是在室內(nèi)條件下進(jìn)行的, 可能與現(xiàn)實環(huán)境的相關(guān)性較小。此外, 關(guān)于作為載體的微塑料與海洋初級生產(chǎn)者、微生物相互作用的研究，以及食用體內(nèi)積聚微塑料的水產(chǎn)品對人類健康影響的研究目前都較少。因此, 可進(jìn)一步探究微塑料對海洋初級生產(chǎn)者的影響及其影響機制, 通過同位素示蹤、分子生物學(xué)等技術(shù)全面的評估微塑料在食物鏈中的傳遞效應(yīng)以及對高營養(yǎng)級消費者的生態(tài)毒理作用, 還可利用體外模擬實驗更詳細(xì)地了解到微塑料對人體的潛在影響, 以及分析已有海洋水體、生物、沉積物中微塑料豐度的數(shù)據(jù), 對微塑料豐度的未來變化進(jìn)行預(yù)測。

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Distribution, ecological effects, and carrier function of microplastics in the marine environment

CHEN Meng-ling1, GAO Fei1, 2, WANG Xin-yuan1, WEI Yi-fan1, XU Qiang1, 2, LIU Chun-sheng1, 2

(1. Ocean College, Hainan University, Haikou 570228, China; 2. State Key Laboratory of Marine Resources Utilization in South China Sea, Hainan University, Haikou 570228, China)

In recent years, microplastics (defined as particles less than 5 mm in diameter) have become a ubiquitous plastic polymer present in marine environments. Due to their small size, microplastics might be ingested by a variety of marine organisms and can be migrated and transferred along the marine food chain, threatening the marine ecosystem’s health and stability. Microplastics in the marine environment also adsorb pollutants (such as heavy metals and organic pollutants) and carry microbes, which may cause more serious toxicological effects. This review mainly summarizes thecharacteristics and distribution of microplastics in the marine environment and the impact on the behavior and physiology of marine biota, especially the food chain. Moreover, the interactions and combined ecological effects of microplastics with other pollutants are analyzed. The main focus of the review is to provide an outlook for future studies on the effects of microplastics on the marine environment and biology.

microplastics; distribution; characteristic; ecological effects; carrier function

Jun. 11, 2020

S917.4

A

1000-3096(2021)12-0125-17

10.11759/hykx20200611001

2020-06-11;

2020-10-22

國家自然科學(xué)基金地區(qū)基金項目(41766005, 31760757); 國家重點研發(fā)計劃“藍(lán)色糧倉科技創(chuàng)新”重點專項(2019YFD0901304)

[National Natural Science Foundation of China, Regional Fund Projects, Nos. 41766005, 31760757; The National Key Research and Development Program of China, No. 2019YFD0901304]

陳孟玲(1994—), 女, 山東省菏澤人, 碩士研究生, 主要從事海水養(yǎng)殖生態(tài)學(xué)研究, 電話: 17330934375, E-mail: yolozzq@163.com; 高菲(1981—),通信作者, 副教授, 主要從事水產(chǎn)養(yǎng)殖生態(tài)學(xué)研究, E-mail: gaofeicas@126.com

(本文編輯: 楊 悅)