微塑料對(duì)海洋橈足類攝食的影響與微塑料的類型、尺寸以及橈足類的種類有關(guān)[1, 25]。微塑料對(duì)猛水蚤的攝食率和濾水率具有消極作用，且具有劑量-效應(yīng)關(guān)系，即隨著微塑料濃度的增大，攝食率和濾水率下降幅度越大。原因可能是由于微塑料對(duì)猛水蚤消化系統(tǒng)的機(jī)械阻塞，導(dǎo)致攝入微塑料后無(wú)法消化。微塑料會(huì)聚團(tuán)或堆積在動(dòng)物消化道內(nèi)，“飽腹感”使其攝食率降低，從而攝入營(yíng)養(yǎng)和能量不足，導(dǎo)致生殖能力等降低[13, 26]。另外，微塑料會(huì)附著在橈足類的觸角、附肢、背甲、尾叉等部位，影響其活動(dòng)能力，導(dǎo)致其有“不適感”，從而減少攝食率。除了被動(dòng)濾食，海洋橈足類能夠感知化學(xué)物質(zhì)，當(dāng)存在藻和微塑料時(shí)，橈足類表現(xiàn)為擇食性，橈足類會(huì)避開(kāi)攝食與微塑料大小、形狀相似的藻[27]。Cole等[28]研究了尼龍對(duì)飛馬哲水蚤攝食的影響，研究表明以不同藻作為餌料，尼龍纖維和尼龍微粒對(duì)飛馬哲水蚤的攝食影響不同，飛馬哲水蚤表現(xiàn)為選擇性攝食，當(dāng)餌料為圓海鏈藻、錐狀克里普藻時(shí)，尼龍纖維顯著抑制攝食率，而尼龍微粒對(duì)上述兩種藻的攝食率沒(méi)有明顯影響。我們的結(jié)果顯示尼龍6微粒對(duì)猛水蚤的攝食率有抑制作用，與Cole等[28]結(jié)果不同的原因主要是因?yàn)槲覀兯脛?dòng)物類型、微塑料類型及濃度不同，我們的微塑料濃度高于Cole等[28]所用的濃度。本實(shí)驗(yàn)所用的微塑料濃度12.5~50 mg·L-1大約為2.4×107~6.3×109 items·L-1，與餌料濃度1.5 ×108cells·L-1相近；微塑料濃度為100~200 mg·L-1大約為1.9×108~2.5×1010 items·L-1，高于所用的餌料濃度。隨著微塑料濃度的增大，盡管橈足類對(duì)微塑料有化感作用，但低選擇性濾食仍然導(dǎo)致橈足類對(duì)微塑料的攝食，并且微塑料濃度越大對(duì)攝食率和濾水率的有害作用越大。微塑料對(duì)海洋生物不利影響所表現(xiàn)出的劑量-效應(yīng)關(guān)系在Rehse等[22]中已有報(bào)道。本實(shí)驗(yàn)中的PA 6對(duì)猛水蚤的24 h·EC50與Frydkj?r等[29]的不規(guī)則微塑料聚乙烯(polyethylene，PE)對(duì)枝角類大型蚤Daphnia magna 48 h·EC50 (65 mg·L-1)比較相近。Rehse等[22]研究表明1 μm PE對(duì)大型蚤96 h·EC50為57.4 mg·L-1。目前已有一些微塑料對(duì)浮游動(dòng)物EC50的報(bào)道[22, 29]，但微塑料對(duì)浮游動(dòng)物的急性致死效應(yīng)尚未見(jiàn)報(bào)道，因此需要開(kāi)展微塑料長(zhǎng)期暴露對(duì)浮游動(dòng)物的亞致死實(shí)驗(yàn)。
本文在攝食率實(shí)驗(yàn)測(cè)定時(shí)發(fā)現(xiàn)，在不添加橈足類的對(duì)照組中，添加微塑料與無(wú)添加微塑料的對(duì)照相比，微塑料尼龍6的暴露使三角褐指藻的生長(zhǎng)速率降低。采用的尼龍6的濃度與Zhang等[30]的濃度相近，雖然所用的微塑料和藻的類型不同，但對(duì)藻的生長(zhǎng)抑制效應(yīng)相近。微塑料對(duì)藻可能會(huì)產(chǎn)生抑制細(xì)胞生長(zhǎng)、破壞細(xì)胞膜結(jié)構(gòu)、降低葉綠素含量和光合效率、引起細(xì)胞氧化損傷等毒性效應(yīng)[30-31]。Zhang等[30]研究了微塑料對(duì)中肋骨條藻的生態(tài)影響，結(jié)果表明微塑料聚氯乙烯(polyvinyl chloride，PVC)抑制藻的生長(zhǎng)和光合作用，電鏡掃描結(jié)果表明，微塑料的吸附和聚團(tuán)是造成抑制作用的原因。微塑料對(duì)微藻的致毒機(jī)理比較復(fù)雜，已提出的可能的致毒機(jī)理主要有機(jī)械損傷、遮蔽效應(yīng)、氧化損傷、吸附團(tuán)聚，并且這些致毒效應(yīng)往往同時(shí)存在，致使藻細(xì)胞生長(zhǎng)代謝異常甚至死亡[19]。微塑料影響藻細(xì)胞生長(zhǎng)的同時(shí)，藻細(xì)胞也會(huì)產(chǎn)生應(yīng)激反應(yīng)以減小、修復(fù)微塑料對(duì)其造成的損傷[32]。微塑料對(duì)藻生理生化、營(yíng)養(yǎng)的影響[32]推測(cè)也會(huì)對(duì)橈足類的攝食產(chǎn)生間接的影響，尚需進(jìn)一步證實(shí)。
3.2 微塑料對(duì)橈足類排泄的影響攝食率的降低直接導(dǎo)致猛水蚤的排糞率隨著微塑料濃度的升高而降低。微塑料尼龍6對(duì)猛水蚤總排糞率的24 h·EC50(221.2 mg·L-1)高于對(duì)正常排糞率的24 h·EC50(84.1 mg·L-1)，這與總糞便顆粒中含有異常糞便顆粒有關(guān)。由于尼龍6對(duì)猛水蚤總排糞率的24 h·EC50僅涉及微塑料對(duì)排糞顆粒數(shù)量的抑制，無(wú)法代表對(duì)顆粒大小、密度的影響，因此不能全面衡量微塑料對(duì)猛水蚤排泄的消極影響。同時(shí)因?yàn)槲⑺芰系臄z入，猛水蚤排出體積小、短小橢球狀的糞便顆粒，且隨著微塑料濃度的升高，異常糞便在總糞便顆粒里所占百分比不斷升高。微塑料的攝入使糞便顆粒的組成和密度發(fā)生改變，導(dǎo)致密度低于正常糞便的密度，而不同密度的微塑料也表現(xiàn)為不同的沉降速率[27]。本實(shí)驗(yàn)發(fā)現(xiàn)暴露微塑料的糞便不容易沉降，與正常糞便相比，其沉降速率顯著降低。同時(shí)因?yàn)槊退榧S便中微塑料含量增大，而微塑料黏度較差，導(dǎo)致其糞便顆粒更易散開(kāi)，而導(dǎo)致異常糞便的出現(xiàn)[24]。Coppock等[27]指出糞便顆粒的體積與微塑料的形狀有關(guān)，而且糞便沉降速率與微塑料的類型有關(guān)。
3.3 微塑料對(duì)橈足類生殖的影響微塑料會(huì)降低海洋橈足類的生殖能力和存活率[25]。本實(shí)驗(yàn)中，微塑料延遲猛水蚤雌體的首次抱卵，微塑料濃度越高，抱卵率越低。這主要是由于微塑料的攝入導(dǎo)致橈足類攝入的能量和營(yíng)養(yǎng)不足[33-34]，所以延遲抱卵，抱卵率降低。Sussarellu等[26]研究表明微塑料聚苯乙烯對(duì)太平洋牡蠣的生殖功能具有傷害作用，卵母細(xì)胞的數(shù)量、直徑顯著下降。本實(shí)驗(yàn)所用的微塑料濃度暴露未導(dǎo)致橈足類死亡，但推測(cè)可能已經(jīng)對(duì)猛水蚤的卵母細(xì)胞結(jié)構(gòu)產(chǎn)生影響，并影響其孵化率。本實(shí)驗(yàn)數(shù)據(jù)未能得到微塑料尼龍6對(duì)猛水蚤抱卵率的24 h·EC50，隨時(shí)間變化，48~144 h的EC50各不相同，因此，微塑料對(duì)猛水蚤生殖影響的作用機(jī)理及時(shí)間-效應(yīng)還需進(jìn)一步研究。
3.4 其他影響除了攝食、排糞、生殖，微塑料還會(huì)對(duì)橈足類其他方面產(chǎn)生影響，如代謝以及行為等。Deng等[35]研究表明，5和7 μm的微塑料微球均會(huì)引起魚(yú)類肝部的局部感染及脂質(zhì)積累，代謝組學(xué)分析顯示，塑料微球會(huì)引起魚(yú)類肝部代謝產(chǎn)物的變化并擾亂肝臟部分機(jī)制和能量的代謝。Cole等[13]研究表明微塑料聚苯乙烯(Polystyrene，PS)對(duì)橈足類Calanus helgolandicus的耗氧率沒(méi)有明顯影響。微塑料對(duì)橈足類代謝的影響與所用微塑料類型不同以及微塑料濃度有關(guān)。除了上述影響，微塑料對(duì)動(dòng)物的行為還會(huì)有影響。叢藝等[36]研究10 μm聚苯乙烯微粒(PS)對(duì)沙蠶掘穴行為影響和細(xì)胞超微結(jié)構(gòu)的改變，結(jié)果表明PS造成沙蠶掘穴行為延長(zhǎng)、體壁表皮細(xì)胞凋亡和肌肉細(xì)胞線粒體水腫。目前，針對(duì)PS對(duì)海洋生物的研究較多，PS主要用于光學(xué)玻璃及儀器、燈罩、包裝材料，其發(fā)泡塑料可作絕熱材料、快餐盒等。與PS不同，尼龍6具有無(wú)毒、質(zhì)輕、優(yōu)良的機(jī)械強(qiáng)度、耐磨性及較好的耐腐蝕性，廣泛應(yīng)用于代替銅的機(jī)械、化工等工業(yè)中制造軸承、齒輪、泵葉，另外還用于頭盔、輸液管、醫(yī)用縫線、漁網(wǎng)等。尼龍的分布也非常廣泛，Alam等[37]對(duì)Ciwalengke河的微塑料進(jìn)行了調(diào)查研究，結(jié)果表明表層海水平均微塑料顆粒為(5.85±3.28)個(gè)/L，主要微塑料類型為聚酯纖維(Polyester)和尼龍纖維。因此研究尼龍6對(duì)海洋橈足類的毒理效應(yīng)對(duì)于研究微塑料污染對(duì)海洋食物網(wǎng)的影響具有重要意義。
4 結(jié)論(1) 微塑料尼龍6的短期暴露使猛水蚤攝食率、濾水率和排糞率降低，而且降低幅度與微塑料濃度有關(guān)，濃度越大，降低幅度越大。微塑料尼龍6對(duì)猛水蚤攝食率、濾水率、排糞率24 h·EC50分別為67.7、62.2、84.1 mg·L-1。另外，尼龍6使猛水蚤的糞便顆粒小型化，由長(zhǎng)橢球體變?yōu)槎绦E球體，可能與其粘度或物理結(jié)構(gòu)的改變有關(guān)。微塑料尼龍6暴露導(dǎo)致糞便顆粒的沉降速率顯著降低。
(2) 微塑料尼龍6暴露使猛水蚤抱卵延遲，相同培養(yǎng)時(shí)間的抱卵率降低?？赡苁俏⑺芰系拇嬖趯?dǎo)致猛水蚤攝食率的降低，進(jìn)而營(yíng)養(yǎng)和能量攝入不足，進(jìn)一步導(dǎo)致對(duì)生殖活動(dòng)的影響。
本實(shí)驗(yàn)所用的微塑料濃度高于現(xiàn)場(chǎng)環(huán)境，因此用本結(jié)果推到現(xiàn)實(shí)海洋環(huán)境中時(shí)，還需考慮現(xiàn)場(chǎng)微塑料種類、濃度、作用時(shí)間及其不同的生態(tài)效應(yīng)。
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, XU Rui1 , WEI Yu-Jie1 , TIAN Ji-Yuan3 
, WANG Xue-Dan1 , LAI Jing-Guang1 , CHEN Rong1, YANG Gui-Peng1,2 , ZHANG Zheng-Yu1 , SHAO Yun-Cui1
來(lái)源：中國(guó)海洋大學(xué)