Benthic bacterial communities are shaped by browning in boreal headwater streams

Owing to the rapid progress of high‐throughput sequencing technologies, microbial assemblages have gained growing interest in environmental impact assessment. However, research on microbial community responses, particularly those of benthic biofilm, to browning (increased concentrations of dissolved organic carbon [DOC]), is scarce. We used data from 55 boreal streams to examine if biofilm bacterial communities exhibit changes in diversity and community composition along a gradient of browning (3.6–27 mg DOC L−1). Species richness increased slightly with increasing DOC, whereas community composition changed markedly across the gradient, especially in the active community. Pseudomonadota and Bacteroidota were overall dominant bacterial phyla. In the active community, Bacteroidota became relatively less abundant and Pseudomonadota more abundant with increasing DOC. Nitrate‐N (NO3‐N) and DOC were the most important predictors of bacterial community turnover. The greatest change in community composition occurred between 75 and 100 μg NO3‐N L−1. For DOC, the first change point was at the low‐end of the gradient, followed by a major change in strongly brownified waters (> 20 mg L−1). Bacterial communities became phylogenetically more similar than expected by chance as DOC increased. Concordance between bacterial and benthic invertebrate communities was very high, indicating that browning exerts a strong control over both taxonomic groups. Our results suggest that microbial communities, particularly the active portion of the community, may provide a sensitive and reliable tool for stream bioassessment. We defined a threshold‐type response in bacterial assemblages to water browning but more research is needed on microbial responses to multiple simultaneous stressors related to global warming and land‐use intensification.

Methods
Please see the detailed description of sample collection and data processing in the article "Benthic bacterial communities are shaped by browning in boreal headwater streams".

得益于高通量测序（high-throughput sequencing）技术的飞速发展，微生物群落（microbial assemblages）在环境影响评估中的关注度持续攀升。然而，针对微生物群落——尤其是底栖生物膜（benthic biofilm）——对褐变作用（即溶解性有机碳[DOC]浓度升高）的响应研究仍较为匮乏。我们利用55条北方溪流的相关数据，探究生物膜细菌群落是否会随着褐变梯度（DOC浓度范围为3.6~27 mg·L⁻¹）发生多样性与群落组成的变化。物种丰富度随DOC浓度升高呈现小幅上升趋势，而群落组成则随该梯度发生显著改变，在活性群落中这一特征尤为突出。假单胞菌门（Pseudomonadota）与拟杆菌门（Bacteroidota）是总体上占优势的细菌门类。在活性群落中，随着DOC浓度升高，拟杆菌门的相对丰度逐渐降低，而假单胞菌门的相对丰度则随之上升。硝态氮（NO₃-N）与溶解性有机碳是驱动细菌群落更替的最关键预测因子。群落组成的最大变化发生在硝态氮浓度介于75~100 μg·L⁻¹之间的区间。对于DOC而言，第一个变化拐点出现在浓度梯度的低端，随后在高褐变水体（>20 mg·L⁻¹）中发生显著的群落结构转变。随着DOC浓度升高，细菌群落的系统发育相似度较随机预期结果更高。细菌群落与底栖无脊椎动物群落之间的契合度极高，这表明褐变作用对这两类生物类群均具有极强的调控作用。本研究结果表明，微生物群落——尤其是群落中的活性组分——有望成为溪流生物评估的敏感且可靠的工具。我们明确了细菌群落对水体褐变的阈值型响应，但仍需开展更多研究，以探索微生物对全球变暖和土地利用集约化带来的多重复合胁迫因子的响应机制。

方法
详见论文《Benthic bacterial communities are shaped by browning in boreal headwater streams》中关于样品采集与数据处理的详细描述。