作物种植经常使用杀虫剂和除草剂，而养殖业使用大量抗生素。作为农业生态系统中水体的重要污染物，这三类农药可能分别影响不同的浮游微生物类群；复合污染可能比单一污染造成更大影响。本文作者推测，影响包括蓝细菌在内的原核生物的抗生素，与影响真核藻类的除草剂，都会降低水体初级生产力；二者复合污染可能对初级生产力造成更明显的影响。三种农药的复合污染可能造成浮游微生物群落多样性的明显下降。在污染停止一段时间后，群落的初级生产力可能会提升，来自健康水体的生物迁入有助于群落的恢复。

本研究以采集于北京西海湿地公园的淡水浮游微生物群落在实验室条件下建立微宇宙系统，研究了典型除草剂莠去津、杀虫剂吡虫啉和抗生素土霉素的单一和复合污染处理下水生浮游微生物群落的响应；并追踪了污染停止后的群落恢复动态，研究了有无来自对照组的迁入对群落恢复的影响。在实验中，以之前研究报告的地表水中检出浓度的较高数值作为三种污染物实验处理浓度。以叶绿素a含量作为初级生产力指标；基于扩增子测序、浮游微生物鉴定与计数来分析群落结构。研究结果表明：

1．莠去津单一污染使群落初级生产力显著降低。土霉素单一污染、吡虫啉和土霉素的复合污染使群落初级生产力提高。土霉素单一污染下的群落初级生产力显著高于莠去津单一污染下的群落初级生产力。

2．对真核浮游微生物群落而言，莠去津单一污染、土霉素单一污染、莠去津和吡虫啉、莠去津和土霉素的复合污染使生物多样性香农指数显著升高。不同污染处理间群落结构有明显差异。土霉素的单一污染、土霉素和吡虫啉复合污染、土霉素和莠去津复合污染群落结构更加相似，说明土霉素在复合污染中起主导作用。复合污染与单一污染对真核浮游微生物群落影响无明显差别。

3．对原核浮游微生物群落而言，土霉素单一污染、吡虫啉单一污染与三种农药复合污染使生物多样性香农指数显著降低。不同污染处理间群落结构有明显差异。土霉素、吡虫啉单一污染对群落结构的影响显著，而莠去津单一污染对群落结构的影响不显著。任何两种农药的混合处理对群落结构都有着极显著影响。

4．经历污染的微宇宙系统，在污染停止一段时间后，大部分处理组初级生产力均上升，其中土霉素处理组的初级生产力显著上升，只有吡虫啉和土霉素复合污染处理组的初级生产力下降。莠去津和吡虫啉复合污染处理组的真核生物多样性显著降低。复合污染处理组的原核生物多样性均下降，其中莠去津和吡虫啉、吡虫啉和土霉素的复合污染处理组的原核生物多样性均显著降低，而单一污染的原核生物多样性不变或略有上升。

5．在污染停止后群落恢复阶段有无生物迁入对初级生产力和真核生物多样性均无显著影响。吡虫啉单一污染处理组的有迁入恢复末期比胁迫阶段末期的原核生物多样性显著增加，莠去津和土霉素、吡虫啉和土霉素复合污染组的有迁入恢复末期比胁迫阶段末期的原核生物多样性均显著降低。大多数的处理组无论有无迁入均未能恢复到与未经历污染的对照组一样。

本研究通过构建微宇宙系统开展农药胁迫和恢复实验，研究典型除草剂莠去津、杀虫剂吡虫啉和抗生素土霉素的单一和复合污染处理下水生浮游微生物群落的响应，弥补了农药复合污染对群落影响研究的不足。特别是说明了农药污染可以通过间接作用对非目标微生物类群产生重要影响，也说明了复合污染可能产生复杂的生态后果。另外，也暗示着原核微生物比真核微生物对多种农药污染更敏感。研究结果对于农牧区建设规划、水生生态风险评价和地表水污染治理具有一定的借鉴意义。

Pesticides and herbicides are commonly employed in agricultural practices, while antibiotics are extensively used in livestock farming. Those drugs are common pollutants in waterbodies of agroecosystems, which may individually or collectively affect various planktonic microbial taxa.  I hypothesize that antibiotics that target prokaryotes including cyanobacteria, and herbicides that impact eukaryotic algae, may reduce primary productivity. Synergistic effects between these pollutants may exist.

Here I report an experiment to study the responses of freshwater planktonic microbial communities to single and combination of the following pollutants, the herbicide atrazine, the insecticide imidacloprid, and the antibiotic oxytetracycline.  I also examine how microbial communities would recover after pollutants being terminated.  Samples of planktonic microbes were collected the West Lake Park, Beijing, based on which experimental microcosms were established.  This study has the following major findings. 

1. The herbicide Atrazine alone notably diminished primary productivity, while the antibiotic oxytetracycline alone, or in combination with the insecticide imidacloprid, showed an enhancement effect. The primary productivity of community polluted by oxytetracycline was significantly higher than that polluted by atrazine.

2. The shannon diversity index of the eukaryotic microbial  communities were higher under single pollution of atrazine, single pollution of oxytetracycline, combined pollution of atrazine and imidacloprid, and combined pollution of atrazine and oxytetracycline. Eukaryotic species composition differed among pollution treatments. However, similar community structure was found among the following treatments: single pollution of oxytetracycline, combined pollution of oxytetracycline and imidacloprid, and the combined pollution of oxytetracycline and atrazine, implying that oxytetracycline has played a predominant role in affecting species composition. On average, combined pollution treatments were not different from single pollution treatments in biodiversity index or community composition of eukaryotic planktonic taxa.

3. The shannon diversity index of prokaryotic planktonic microbes was lower under the treatments of single pollution of oxytetracycline, single pollution of imidacloprid, and combined pollution of three drugs . There were significant differences in community structure among different pollution treatments. Single pollution of oxytetracycline and imidacloprid had significant effects on community structure, but single pollution of atrazine had no significant effects on community structure. Combined pollution of any two drugs had a significant effect on the community structure.

4. Subsequent to the termination of the pollution, primary productivity of the oxytetracycline treatment group increased significantly, while that of the imidacloprid and oxytetracycline combined pollution treatment group decreased. Biodiversity index shannon of eukaryotic microbes decreased in microcosms that had been exposed to combined treatment of atrazine and imidacloprid. The shannon diversity index of prokaryotic microbes decreased in microcosms that had been exposed to combined pollution of atrazine and imidacloprid, imidacloprid and oxytetracycline, while that of the single pollution group remained unchanged or slightly increased.

5. After pollutant termination, immigration of organisms to microcosms that had been exposed to pollutant treatments did not cause significant difference in  total primary productivity and shannon diversity index of eukaryotic microbes , and showed various impacts on prokaryotic communities, depending on the history of pollution.  Immigration increased  prokaryotic biodiversity index shannon  in microcosms with historical treatment of  imidacloprid, and decreased the shannon diversity index in microcosms with historical treatment of combined pollution of atrazine and oxytetracycline. Most of the treatment groups, with or without immigration , did not recover as well as the control group that had not experienced pollution.

 In summary, the present study fills a gap in the literature by investigating comparing single and combined pollutant of three major types of drugs commonly used in agriculture.  The results suggest that a drug pollutant may very likely affect non-targe microbial tax through indirect effects, and combination of different types of pollutant can cause complex ecological consequences.  It is also suggested that  prokaryotic microbes are more susceptible to drug pollution than eukaryotic microbes. The results have implications for aquatic ecological risk assessment and surface water pollution control in agricultural and pastoral ecosystems.