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砷(As)是一种广泛分布在自然环境中的有毒类金属元素,在美国有毒物质和疾病登记署列出的物质优先名录(ATSDR’s substance priority list)(2019年)中居于首位[1]。水体中的砷主要以砷酸盐(As(Ⅴ))形式存在,其次是亚砷酸盐(As(Ⅲ))和多种有机砷(如一甲基化砷(MMA)、二甲基化砷(DMA))[2-4]。微藻在水环境中广泛存在,可以通过不同的途径来降低砷的毒性,包括细胞表面结合、砷酸盐还原、亚砷酸盐氧化、甲基化、As(Ⅲ)与谷胱甘肽和植物螯合素的螯合以及砷的外排[5]。
微藻在生长过程中释放到细胞外的大分子物质形成胞外聚合物(EPS),可为自身提供碳源和能量,在藻际环境中发挥重要的生态学功能[6],如改变细胞的电荷、亲疏水及絮凝特性。在重金属胁迫下(如Cd2+、Pb2+等),微藻多糖合成基因(如capD、csaB、tagH和epsL)会大量表达,促使EPS分泌量增加[7-8]。自上世纪90年代以来,有关EPS去除重金属的研究不断增加[9-15]。EPS中含有多种官能团,通过表面吸附减少微藻细胞对砷的吸收[16-17];同时EPS的结合位点也有助于砷的形态变化,从而降低砷对藻细胞的胁迫[13]。
硅藻是海洋中的初级生产者,生物多样性丰富,生物量巨大,约占海洋初级产量的50% 和全球初级产量的25%,对水生环境具有重要的生态学意义[10]。大部分硅藻可以通过硅质壳的裂缝(壳缝)或顶端开孔处分泌多聚物[18]。三角褐指藻是模式硅藻,生长速度快,易于培养,是一种常用的实验材料。丁腾达等[19]研究发现三角褐指藻(Phaeodactylum sp.)在20 mg·L−1 As(Ⅲ)下正常生长,对砷有较高的耐性。近年来,有关砷和硅藻的研究关注了砷对硅藻的生长抑制、氧化损伤等方面,而对砷在硅藻细胞内外的分配及赋存形态的变化规律研究较少,需要进一步探讨EPS对硅藻砷的毒性、富集和转化的影响。
因此本研究以三角褐指藻为材料,探讨在不同砷酸盐处理下,硅藻生长和EPS分泌情况及特性,以及在去除和未去除EPS的条件下,三角褐指藻对As(Ⅴ)的富集和转化的规律,从而明确三角褐指藻EPS与砷毒性、富集和形态转化之间的关系,为利用硅藻胞外大分子有机物进行水体砷污染的控制和修复提供依据。
胞外聚合物对三角褐指藻砷酸盐富集和形态转化的影响
Effects of extracellular polymeric substances on the accumulation and transformation of As(Ⅴ) by Phaeodactylum tricornutum
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摘要: 微生物分泌的胞外聚合物(EPS)对重金属吸附有重要作用,然而EPS对硅藻生长及砷(As)的富集和转化有何影响尚不清楚。为此本文选取三角褐指藻,研究了不同浓度砷酸盐(As(Ⅴ))处理下EPS各组分含量、藻细胞生长以及细胞形貌的变化,并比较了EPS提取前后藻细胞对砷的富集和形态转化的差异。结果表明,三角褐指藻对As(Ⅴ)有一定的耐性,72 h 的EC10值为1.75 mg·L−1。随着As(Ⅴ)浓度的升高,EPS的分泌量呈增加的趋势,最大可达到对照组的1.47倍。藻细胞吸收As(V)后,转化为亚砷酸盐(As(Ⅲ))、二甲基砷(DMA)和一甲基砷(MMA)并外排至培养液,EPS能促进藻细胞对As(V)的富集、还原和甲基化。衰减全反射傅里叶变换红外光谱(ATR-IR)数据表明,EPS中存在—OH、—NH、C—O—C等官能团,增加了藻细胞对砷的吸附,从而限制了As的跨膜运输。总之,EPS增强了三角褐指藻对As(Ⅴ)的耐性、富集和形态转化。研究结果可为利用硅藻EPS治理水体As污染提供理论依据。Abstract: Extracellular polymeric substances (EPS) secreted by microorganisms play an important role in the adsorption of heavy metals. However, effects of EPS on the accumulation and transformation of arsenic (As) in diatoms are still unclear. As such, Phaeodactylum tricornutum was selected to study the variations in the composition of EPS, growth and morphology of algal cells under different treatments of arsenate (As(Ⅴ)). Besides, the accumulation and transformation of As by algae cells before and after EPS extraction were compared. The results showed that P. tricornutum had certain tolerance to As(Ⅴ), and the EC10 value at 72 h was 1.75 mg·L−1. With the increase of As(Ⅴ) concentration, the secretion of EPS showed an increasing trend, which maximally reached 1.47 times higher than those of the control group. Arsenate (As(Ⅴ)) absorbed into the cells was converted into arsenite (As(Ⅲ)), dimethyl arsenic (DMA) and monomethyl arsenic (MMA), which were then released into the culture medium. The EPS also promoted the As(Ⅴ) accumulation, reduction and methylation by the algae cells. At the same time, ATR-IR data confirmed the presence of —OH, —NH and C—O—C functional groups, which increased the extracellular adsorption of As by algal cells and thereby limited the As transport across membrane. In summary, EPS promoted the As(Ⅴ) tolerance, accumulation and transformation by P. tricornutum to As(Ⅴ). The results of this study may provide a theoretical basis for remediation of As pollution in water bodies using diatom EPS.
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Key words:
- EPS /
- Phaeodactylum tricornutum /
- arsenic /
- accumulation /
- transformation
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图 6 不同浓度As(V)处理下有EPS(a)和去除EPS(b)三角褐指藻的富集、吸附和吸收砷含量及比例(柱子和线条分别代表砷含量和吸附/吸收率;不同字母表示不同浓度As(V)之间差异显著,P<0.05)
Figure 6. The concentration and ratio of arsenic in the accumulation, adsorption and absorption of P. tricornutum with EPS (a) and without EPS (b) under different As(V) concentrations (Columns and lines represent Ascontent and adsorption/ absorption rate respectively; Different letters indicate significant differences among various As(V) concentrations, P<0.05)
图 7 不同浓度As(V)处理下有EPS(a、c)和去除EPS(b、d)的三角褐指藻培养液中(a、b)和胞内(c、d)砷形态及含量(不同字母表示不同浓度As(V)之间差异显著,P<0.05)
Figure 7. Speciation and contents of As in the culture solution (a, b) and cells (c, d) of P. tricornutum with EPS (a, c) and without EPS (b, d) under different As(V) concentrations (Different letters indicate (Different letters indicate significant differences among various As(V) concentrations , P<0.05)
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