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Blinded by the light: Increased chlorophyll fluorescence of herbicide-exposed periphyton masks unfavorable structural responses during exposure and recovery
Aquatic Toxicology ( IF 4.5 ) Pub Date : 2018-08-19 , DOI: 10.1016/j.aquatox.2018.08.015
Alexander Feckler , Jelena Rakovic , Maria Kahlert , Rikard Tröger , Mirco Bundschuh

In surface waters within agricultural catchments, periphyton – i.e., biofilms containing algae, heterotrophs, and associated detritus – is subjected to multiple stressors including herbicides. Although herbicide effects on periphyton are frequently studied, the focus has been on photosynthesis-inhibiting herbicides while other modes of toxic action have received little attention. Against this background, a 21-days-lasting bioassay was conducted, during which mature periphytic communities were exposed to the carotenoid-biosynthesis-inhibiting herbicide diflufenican for 12 days (up to 10 μg/L; n = 4), followed by a 9-days-lasting recovery phase in herbicide-free medium. Variables related to periphytic functioning (photosynthetic efficiency and non-photochemical quenching) and structure (pigment concentrations, biomass, and algal community structure) were quantified every third day during both experimental phases. Exposure to ≥ 0.2 μg diflufenican/L resulted in 20–25% and 25–30% lowered carotenoid and chlorophyll a concentrations, respectively, likely explained by a reduced algal biovolume as well as diflufenican’s mode of toxic action and thus a shift towards a higher heterotrophy of the communities. Despite these adverse effects on the photosynthetic apparatus, the photosynthetic efficiency increased by up to ∼15% under diflufenican exposure judged on higher chlorophyll fluorescence. This may be explained by an up to ∼60% reduced non-photochemical quenching as well as binding of diflufenican to the pigment-protein membrane complex of the photosystem II, two processes causing higher chlorophyll fluorescence. Additionally, phototrophs may have actively increased energy assimilation to cope with higher energy demands under chemical stress. Although periphyton showed some recovery potential following the exposure phase, observed as increasing chlorophyll a concentrations and non-photochemical quenching, periphyton may not be able to quickly recover from stress given the persistent increase in the photosynthetic efficiency. While the processes underlying the observed effects yet remain speculative, the results suggest a shift towards a higher degree of heterotrophy in periphytic communities ultimately increasing the importance of heterotrophic ecosystem functions at impacted sites over the long term.



中文翻译:

被光遮住:暴露于除草剂的周生植物中增加的叶绿素荧光掩盖了暴露和恢复过程中不利的结构响应

在农业流域内的地表水中,附生植物(即包含藻类,异养生物和相关碎屑的生物膜)受到多种除草剂的胁迫。尽管经常研究除草剂对植物附生植物的影响,但重点一直放在抑制光合作用的除草剂上,而其他毒性作用方式却很少受到关注。在这种背景下,进行了为期21天的生物测定,在此期间,将成熟的周围生植物群落暴露于抑制类胡萝卜素生物合成的除草剂二氟苯醚中达12天(最高10μg/ L;n = 4),然后在无除草剂的培养基中进行为期9天的恢复阶段。在两个实验阶段中,每三天对与周生植物功能(光合作用效率和非光化学猝灭)和结构(色素浓度,生物量和藻类群落结构)相关的变量进行定量。暴露于≥0.2μg的二氟苯甲醚/ L导致类胡萝卜素和叶绿素a降低20–25%和25–30%藻类生物量的减少以及双氟芬尼的毒性作用模式,以及向群落更高异养性的转变,可以分别解释浓度的升高。尽管对光合作用装置有这些不利影响,但根据较高的叶绿素荧光判断,在二氟苯醚暴露下,光合作用效率提高了约15%。这可以通过最多减少约60%的非光化学猝灭以及双氟苯甲醚与光系统II的色素-蛋白质膜复合物的结合来解释,这两个过程导致较高的叶绿素荧光。另外,光养生物可能积极地增加了能量吸收,以应对化学胁迫下更高的能量需求。尽管附生植物在接触阶段后显示出一定的恢复潜力,但观察到叶绿素增加由于浓度和非光化学猝灭作用,鉴于光合作用效率的持续提高,周生植物可能无法从压力中快速恢复。虽然所观察到的影响的过程仍然是推测性的,但结果表明,在周围植物群落中向更高程度的异养性转变,最终从长远来看最终增加了异养生态系统功能在受影响地点的重要性。

更新日期:2018-08-19
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