淡水硅藻的砷甲基化和砷氧化代谢机制

Metabolic mechanisms of arsenic methylation and oxidation in freshwater diatoms

硅藻砷甲基化和氧化过程在淡水生态系统砷生物地球化学循环中发挥重要作用,并且硅藻的重金属抗性与营养元素硅酸盐有效性紧密相关。然而,不同硅藻对无机砷的转化能力和砷抗性差异,硅酸盐对硅藻砷转化的影响机制尚不清楚。选取2种淡水硅藻,即针状菱形藻(Nitzschia acicularis)和谷皮菱形藻(Nitzschia palea),作为研究对象,探究其砷甲基化和氧化作用,以及硅酸盐影响下砷转化和此过程中的相关基因转录活性。结果表明,硅藻暴露于As(Ⅲ)的10 d培养期内主要产生二甲基砷,针状菱形藻的二甲基砷转化率为5.54%,高于谷皮菱形藻的转化率(0.80%)。谷皮菱形藻的As(Ⅲ)氧化作用比针状菱形藻强,前者和后者的氧化率分别为90.1%和3.2%。2种硅藻砷甲基化和氧化能力的差异表明不同硅藻As(Ⅲ)胁迫下的主要砷抗性策略不同。针状菱形藻在As(Ⅲ)胁迫下显著上调砷甲基转移功能基因(arsM)表达,并显著下调硅酸盐转运基因(sit)表达。这表明硅藻驱动胞内砷甲基化反应和减少硅/砷转运活性,从而有助于胞内砷解毒和减少As(Ⅲ)吸收。硅酸盐添加对针状菱形藻的arsM基因表达没有明显影响,但上调了线粒体核糖体RNA 12S基因表达,同时促进As(Ⅲ)向As(Ⅴ)转化,这表明硅酸盐可促进硅藻砷氧化和呼吸产能活性,有助于增加硅藻对砷的抗性。本研究证实了硅藻间砷甲基化和氧化能力差异,并且硅酸盐是影响其砷转化的重要因素,可为硅藻应用于砷污染水体修复提供参考。

Arsenic(As)methylation and oxidation processes in freshwater diatoms play an important role in the biogeochemical cycle of As in freshwater ecosystems,and the heavy metal resistance of diatoms is closely related to the availability of nutrient silicates.However,the transformation ability and As resistance of different diatoms to inorganic arsenic vary,and the mechanism of the silicate effect on the As transformation in diatoms are still unclear.In this study,two freshwater diatoms(Nitzschia acicularis and Nitzschia palea)were selected to explore their As methylation and oxidation,as well as the influence of silicate on the As transformation and the transcriptional activity of related genes during this process.The results showed that diatoms exposed to As(Ⅲ)mainly produced dimethylarsenic(DMAs)during the 10-day culture period mainly produced dimethyl-arsenic,and the conversion rate of DMAs in the Nitzschia acicularis was 5.54%,higher than that in the Nitzschia palea(0.8%).The oxidation of As(Ⅲ)was stronger in Nitzschia acicularis than in Nitzschia palea with the oxidation rate of 90.1%and 3.2%,respectively.The differences of As methylation and oxidation capacities bwtween the two diatoms indicated that the main arsenic resistance strategies under As(Ⅲ)stress were different.With exposing to As(Ⅲ),the expression of arsenic methyl transfer functional gene(arsM)was significantly up-regulated and the expression of silicate transport gene(sit)was significantly down-regulated.This suggested that diatoms drived intracellular arsenic methylation and reduced Si/As transport activity,thereby contributing to intracellular arsenic detoxification and reducing As(Ⅲ)absorption.The addition of silicate had insignificant effect on transcriptional activity of arsM in Nitzschia acicularis,but up-regulated the express of mitochondrial ribosomal RNA 12S gene,and promoted the conversion of As(Ⅲ)to As(V).These results indicated that silicate could promote As oxidation and respiratory activity of diatoms,and contribute to the increase of arsenic resistance of diatoms.This study confirmed the differences in As methylation and oxidation capacity among diatoms,and that silicate was an important factor affecting As transformation,wihch can provide theoretical support for the application of diatoms in the remediation of As-contaminated water.