Abundant microbial communities act as more sensitive bio-indicators for ecological evaluation of copper mine contamination than rare taxa in river sediments.

Bacterial and fungal communities have been widely applied as bio-indicators for ecological evaluation of copper (Cu) mine pollution in river sediments. However, the response pattern of their abundant and rare sub-communities is still unknown, limiting the further development of biological diagnostics. Here, the alpha-diversity, community composition, environmental contribution and co-occurrence network of total, abundant and rare taxa for bacteria and fungi in the Jiaopingdu Cu Mine wastewater treatment plant (WWTP) were investigated through high-throughput sequencing. The results revealed different responses of microbial alpha-diversity for abundant and rare sub-communities. The abundant taxa were ubiquitous in all sediments, while rare taxa exhibited increases of species richness in polluted areas because of heterochthonous inputs of WWTP drainage. Nevertheless, the variations of community composition were consistent for bacterial and fungal abundant and rare taxa, all of which showed significant dissimilarity between control and polluted areas. Distance-decay relationship and canonical correlation analysis indicated that abundant taxa assemblies (rbacteria = -0.924, rfungi = -0.684) were more strongly driven by environmental changes than rare ones (rbacteria = -0.626, rfungi = -0.349), because abundant microbes had higher proportions of significant variations in abundance. Co-occurrence networks revealed more keystone species with high node degree and centrality among abundant taxa compared with rare ones. Moreover, bacterial abundant and rare taxa were more sensitive to Cu mine pollution than relevant fungal taxa owing to different Cu tolerance. In conclusion, among all microbial sub-communities, abundant bacteria had the highest environmental sensitivity, suggesting their important application in biological diagnosis of Cu mine pollution. Accordingly, the abundant taxa could act as "key reservoir" for future selection of specific indicator species, for example Thiobacillus, while rare taxa no longer need excessive in-depth analysis, which would greatly improve microbial diagnosis efficiency.