Differential responses of dominant and rare epiphytic bacteria from a submerged macrophyte to elevated CO₂

Epiphytic bacteria develop complex interactions with their host macrophytes and play an important role in the ecological processes in freshwater habitats. However, how dominant and rare taxa respond to elevated atmospheric CO₂ remains unclear. A manipulated experiment was carried out to explore the effects of elevated CO₂ on the diversity or functional characteristics of leaf epiphytic dominant and rare bacteria from a submerged macrophyte. Three levels (high, medium, normal) of dissolved inorganic carbon (DIC) were applied to the overlying water. The physicochemical properties of the overlying water were measured. Elevated atmospheric CO₂ significantly decreased the pH and dissolved oxygen (DO) of overlying water. Proteobacteria, Cyanobacteria, Bacteroidetes, Planctomycetes, and Actinobacteria are the dominant phyla of leaf epiphytic bacteria from Myriophyllum spicatum, occupying over 90% of the accumulated relative abundances. The aquatic DIC level and further pH significantly drove the epiphytic community composition differences among the three DIC levels. For dominant epiphytic bacteria, the functional potential of nutrient processes and mutualistic relationships were strongly affected by a high DIC level, while responses of rare epiphytic bacteria were more related to trace element processes, pathogens, and defense strategies under a high DIC level. Our results showed the responses of epiphytic bacteria to elevated CO₂ varied across dominant and rare taxa.