Freshwater trophic status mediates microbial community assembly and interdomain network complexity

Freshwater microorganisms and their interactions are important drivers of nutrient cycling that are in turn affected by nutrient status, causing shifts in microbial community diversity, composition, and interactions. However, the impact of water trophic status on bacterial-archaeal interdomain interactions remains poorly understood. This study focused on the impact of trophic status, as characterized by trophic state index (TSI), on the interdomain interactions of freshwater microbial communities from 45 ponds in Hangzhou. Our results showed that the mesotrophic wetland bordering on lightly eutrophic (Hemu: TSI of 49; lightly eutrophic is defined as 50 ≤ TSI <60) harbored a much more complex bacterial-archaeal interdomain network, which showed significantly (P < 0.05) higher connectivity than the wetlands with lower (TSI of 38) or higher (TSI of 57) trophic levels. Notably, light eutrophication strengthened the network modules' negative associations with organic carbon through some network hubs, which could trigger carbon loss in wetlands. We also detected a non-linear response of interdomain network complexity to the increasing of nutrients with a turning point of approximately TSI 50. Quantitative estimates of community assembly processes and structural equation modelling analysis indicated that chlorophyll-a, total nitrogen, and total phosphorus could regulate interdomain network complexity (50% of the variation explanation rate) by driving microbial community assembly. This study demonstrates that microbial interdomain network complexity could be used as a bioindicator for ecological changes, which would helpful for improving ecological assessment of the freshwater eutrophication.