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微塑料對海洋橈足類攝食、排泄及生殖的影響（二）
發(fā)布日期：2020-03-02
3 討論3.1 微塑料對橈足類攝食的影響

微塑料對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猛水蚤消化系統(tǒng)的機械阻塞，導致攝入微塑料后無法消化。微塑料會聚團或堆積在動物消化道內(nèi)，“飽腹感”使其攝食率降低，從而攝入營養(yǎng)和能量不足，導致生殖能力等降低[13, 26]。另外，微塑料會附著在橈足類的觸角、附肢、背甲、尾叉等部位，影響其活動能力，導致其有“不適感”，從而減少攝食率。除了被動濾食，海洋橈足類能夠感知化學物質(zhì)，當存在藻和微塑料時，橈足類表現(xiàn)為擇食性，橈足類會避開攝食與微塑料大小、形狀相似的藻[27]。Cole等[28]研究了尼龍對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，當餌料為圓海鏈藻、錐狀克里普藻時，尼龍纖維顯著抑制攝食率，而尼龍微粒對上述兩種藻的攝食率沒有明顯影響。我們的結(jié)果顯示尼龍6微粒對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為我們所用動物類型、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。本實驗所用的微塑料濃度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，盡管橈足類對微塑料有化感作用，但低選擇性濾食仍然導致橈足類對微塑料的攝食，并且微塑料濃度越大對攝食率和濾水率的有害作用越大。微塑料對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]中已有報道。本實驗中的PA 6對猛水蚤的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對大型蚤96 h·EC50為57.4 mg·L-1。目前已有一些微塑料對浮游動物EC50的報道[22, 29]，但微塑料對浮游動物的急性致死效應(yīng)尚未見報道，因此需要開展微塑料長期暴露對浮游動物的亞致死實驗。

本文在攝食率實驗測定時發(fā)現(xiàn)，在不添加橈足類的對照組中，添加微塑料與無添加微塑料的對照相比，微塑料尼龍6的暴露使三角褐指藻的生長速率降低。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，但對藻的生長抑制效應(yīng)相近。微塑料對藻可能會產(chǎn)生抑制細胞生長、破壞細胞膜結(jié)構(gòu)、降低葉綠素含量和光合效率、引起細胞氧化損傷等毒性效應(yīng)[30-31]。Zhang等[30]研究了微塑料對中肋骨條藻的生態(tài)影響，結(jié)果表明微塑料聚氯乙烯(polyvinyl chloride，PVC)抑制藻的生長和光合作用，電鏡掃描結(jié)果表明，微塑料的吸附和聚團是造成抑制作用的原因。微塑料對微藻的致毒機理比較復雜，已提出的可能的致毒機理主要有機械損傷、遮蔽效應(yīng)、氧化損傷、吸附團聚，并且這些致毒效應(yīng)往往同時存在，致使藻細胞生長代謝異常甚至死亡[19]。微塑料影響藻細胞生長的同時，藻細胞也會產(chǎn)生應(yīng)激反應(yīng)以減小、修復微塑料對其造成的損傷[32]。微塑料對藻生理生化、營養(yǎng)的影響[32]推測也會對橈足類的攝食產(chǎn)生間接的影響，尚需進一步證實。

3.2 微塑料對橈足類排泄的影響

攝食率的降低直接導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對猛水蚤總排糞率的24 h·EC50(221.2 mg·L-1)高于對正常排糞率的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對猛水蚤總排糞率的24 h·EC50僅涉及微塑料對排糞顆粒數(shù)量的抑制，無法代表對顆粒大小、密度的影響，因此不能全面衡量微塑料對猛水蚤排泄的消極影響。同時因為微塑料的攝入，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]。本實驗發(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。同時因為猛水蚤糞便中微塑料含量增大，而微塑料黏度較差，導致其糞便顆粒更易散開，而導致異常糞便的出現(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 微塑料對橈足類生殖的影響

微塑料會降低海洋橈足類的生殖能力和存活率[25]。本實驗中，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致橈足類攝入的能量和營養(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太平洋牡蠣的生殖功能具有傷害作用，卵母細胞的數(shù)量、直徑顯著下降。本實驗所用的微塑料濃度暴露未導致橈足類死亡，但推測可能已經(jīng)對猛水蚤的卵母細胞結(jié)構(gòu)產(chǎn)生影響，并影響其孵化率。本實驗數(shù)據(jù)未能得到微塑料尼龍6對猛水蚤抱卵率的24 h·EC50，隨時間變化，48~144 h的EC50各不相同，因此，微塑料對猛水蚤生殖影響的作用機理及時間-效應(yīng)還需進一步研究。

3.4 其他影響

除了攝食、排糞、生殖，微塑料還會對橈足類其他方面產(chǎn)生影響，如代謝以及行為等。Deng等[35]研究表明，5和7 μm的微塑料微球均會引起魚類肝部的局部感染及脂質(zhì)積累，代謝組學分析顯示，塑料微球會引起魚類肝部代謝產(chǎn)物的變化并擾亂肝臟部分機制和能量的代謝。Cole等[13]研究表明微塑料聚苯乙烯(Polystyrene，PS)對橈足類Calanus helgolandicus的耗氧率沒有明顯影響。微塑料對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，微塑料對動物的行為還會有影響。叢藝等[36]研究10 μm聚苯乙烯微粒(PS)對沙蠶掘穴行為影響和細胞超微結(jié)構(gòu)的改變，結(jié)果表明PS造成沙蠶掘穴行為延長、體壁表皮細胞凋亡和肌肉細胞線粒體水腫。目前，針對PS對海洋生物的研究較多，PS主要用于光學玻璃及儀器、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具有無毒、質(zhì)輕、優(yōu)良的機械強度、耐磨性及較好的耐腐蝕性，廣泛應(yīng)用于代替銅的機械、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Ciwalengke河的微塑料進行了調(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/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對海洋橈足類的毒理效應(yīng)對于研究微塑料污染對海洋食物網(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)時間的抱卵率降低?？赡苁俏⑺芰系拇嬖趯е旅退閿z食率的降低，進而營養(yǎng)和能量攝入不足，進一步導致對生殖活動的影響。

本實驗所用的微塑料濃度高于現(xiàn)場環(huán)境，因此用本結(jié)果推到現(xiàn)實海洋環(huán)境中時，還需考慮現(xiàn)場微塑料種類、濃度、作用時間及其不同的生態(tài)效應(yīng)。

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Effect of Microplastics on Feeding, Excretion and Reproduction of Marine Copepod
YU Juan1,2 , 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     
Abstract: In recent years, there has been a growing concern for microplastics. Marine copepod Harpacticus sp., commonly lived in coastal waters of Qingdao, was exposed to high concentrations of microplastics polyamide 6 (PA 6) in laboratory, and the effects of PA 6 on the feeding, excretion and reproduction of the copepod were investigated. The results showed that PA 6 exposure had adverse effects on the ingestion rate, clearance rate, pellet production rate, and proportions of gravid female, and the effects were dose-dependent. 24 h·EC50(median effect concentration of 24 h) for microplastics PA 6 on ingestion rate, clearance rate, and pellet production rate were 67.7, 62.2, and 84.1 mg·L-1, respectively, and 144 h·EC50 for PA 6 on proportions of gravid female was 30.3 mg· L-1. Decreased ingestion rate caused by "satiety" led to the limited energy and nutrition intake, which might explain the decreased proportions of gravid female. Ingestion in the microplastic led to decreased size and volume of fecal pellets, and the shape changed from long ellipsoid to short ellipsoid, which might be due to the changed viscosity and physical structure of fecal pellets. Volume and sinking rate of fecal pellets for exposure to PA 6 significantly decreased when compared with the controls (without PA 6). Our results can supply the data to the study of the effects of microplastics on the ecological toxic effects in marine copepod and filter-feeding zooplankton.

來源：中國海洋大學