Environment-driven geographical distribution of bacterial communities and identification of indicator taxa in Songhua River

Bacterial communities play crucial roles in biogeochemical cycles in riverine ecosystems. However, the use of bacteria as bio-indicators to assess the pollution of water environments is not as common as macroorganisms, especially in large rivers disturbed by anthropogenic activities. In this study, we explored the relationship between geographical distribution of bacterial communities and associated environmental variables, and further selected effective indicators in Songhua River, a crucial river highly disturbed by various anthropogenic activities in Northeastern China. Our results revealed significant differences of environmental variables between the highly-disturbed area (HDA) and the lightly-disturbed area (LDA), with HDA encountering a higher level of nutrient pollution than LDA. The bacterial community composition was significantly different between the two areas (p < 0.05), with nitrate (NO3-N) being selected as the most important factor for this dissimilarity. By assigning bacteria into HDA- and LDA-associated groups whose abundance was significantly higher (p < 0.05) in a specific group, we further examined the variation of bacterial communities in different areas. Furthermore, to indicate the degradation status of river ecosystems, we selected Cyanobacteria Family I, Planctomycetaceae, FukuN18 freshwater group, and Rhizobiales Incertae Sedis at the family level, and Pirellula, Synechococcus, Alsobacter, and Prochlorococcus at the genus level as bio-indicators. To indicate the remediation of river ecosystems, we recommended Comamonadaceae and Xanthomonadaceae at the family level, and Limnohabitans, Flavobacterium, Limnobacter, Rhodoferax, Zavarzinia, Pseudarcicella, and Pseudorhodobacter at the genus level. Some indicator taxa were specific to Songhua River, suggesting the importance of river-specific indicators based on unique ecological features of a river ecosystem; while some were common among rivers, supporting the generalization of these indicator taxa in river ecosystems.