Zinc and laboratory emerge date impact honey bee gut microbiota and survival

In this study, we tested the effects of zinc on honey bee survivorship and gut microbiota. To evaluate survivorship, we exposed bees to seven concentrations of zinc (0, 50, 100, 250, 500, or 1000 mg/L) and assessed survival daily for 10 days. To evaluate effects of zinc on gut microbiota, we exposed bees to 5 or 100 mg/L zinc. Bees were sampled before (day 0), and after zinc exposure (days 3, 6, and 9). Abdominal contents underwent DNA extraction and 16S rRNA sequencing (V3-V4) on an Illumina MiSeq. Sequences were filtered and processed through QIIME2 and DADA2. Zinc treatment had minimal effects on bacterial DNA concentrations and absolute cell counts while emerge date (the date a bee emerged from the brood comb) had a significant effect with decreased bacterial concentrations and cell counts observed at later emerge dates. In this group, several beneficial taxa (Lactobacillus, Rhizobiaceae, Gilliamella) were found at reduced abundances, while Paenibacillus, a potentially pathogenic taxa, was found at increased abundances. This suggests that zinc exposure, even at relatively low levels, may negatively impact honey bee health, even if survivorship is not dramatically impacted. Notably, emerge date effects were also observed in microbial composition. These results highlight a clear need to control for emerge day in laboratory-based honey bee experiments. These results also demonstrate the need to include assessments of honeybee gut microbiota in addition to other metrics of honey bee health and survivorship when evaluating the potential effects of agrochemicals on honey bees.

本研究考察了锌对蜜蜂存活率与肠道菌群（gut microbiota）的影响。为评估蜜蜂存活率，我们将蜜蜂暴露于7种梯度浓度的锌溶液中（浓度分别为0、50、100、250、500、1000 mg/L），并连续10天每日监测其存活情况。为探究锌对肠道菌群的影响，我们将蜜蜂分别暴露于5 mg/L与100 mg/L的锌溶液中。分别在暴露前（第0天）及暴露后第3、6、9天对蜜蜂进行采样。采集蜜蜂腹部内容物进行DNA提取，并依托Illumina MiSeq测序平台完成16S核糖体RNA（16S rRNA）（V3-V4可变区）测序。测序序列经QIIME2与DADA2工具进行过滤与分析处理。锌处理对细菌DNA浓度与绝对细胞计数的影响甚微；而出房日期（蜜蜂从育幼巢脾羽化的日期）则对其存在显著影响，较晚羽化的蜜蜂其细菌浓度与细胞计数均有所降低。在该组样本中，若干有益菌群类群包括乳杆菌属（Lactobacillus）、根瘤菌科（Rhizobiaceae）、吉亚姆菌属（Gilliamella）的丰度均出现下降，而潜在致病类群的类芽孢杆菌属（Paenibacillus）丰度则有所升高。这表明，即便处于相对较低的浓度水平，锌暴露仍可能对蜜蜂健康产生负面影响，即使其存活率未出现显著变化。值得注意的是，出房日期对菌群组成同样存在显著影响。本研究结果明确指出，在基于实验室的蜜蜂实验中，需对出房日期进行严格控制。此外，本研究结果还表明，在评估农用化学品对蜜蜂的潜在影响时，除考察蜜蜂健康与存活率相关指标外，还需纳入肠道菌群的评估环节。