Diversity of carbon cycle-linked phyllosphere microorganisms: A key driver of CO2 flux in macrophyte-dominated aquatic systems

Planktonic microorganisms have been recognized as important components in biogeochemical cycling in lakes. However, research into the impact of phyllosphere microorganisms, particularly those involved in carbon cycling, on CO₂ fluxes in macrophyte-dominated lakes within the context of global environmental changes remains scarce. Here, by employing high-throughput sequencing techniques, we experimentally tested how nutrient enrichment, top-down effects of fish and increases in dissolved organic carbon (DOC) affect CO₂ exchange flux at the water-air interface by altering the community structure and functioning of phyllosphere bacteria on macrophytes. We found that our mesocosm ecosystems exhibited a net absorption of CO₂, but nutrient enrichment significantly decreased the absorption ability. Mantel tests and multiple regression modeling also showed that eutrophication-associated parameters (total nitrogen, total phosphorus and ammonium nitrogen), pH, and extinction coefficient were the key drivers influencing abundance of phyllosphere functional microorganisms. In addition, these experimental treatments significantly altered the composition, diversity and co-occurrence networks of carbon cycling-associated phyllosphere microorganisms, which impacted the CO₂ flux. Structural equation models and linear regression further showed that the Shannon Index of phyllosphere functional microorganisms related to carbon cycling (rather than plant volume inhabited - PVI) had a significant positive impact on CO₂ fixation. This means that environmental changes—especially eutrophication—may hinder carbon sequestration by decreasing the diversity of phyllosphere microorganisms rather than reducing the abundance of submerged macrophytes. This study increases our understanding of carbon cycling processes in aquatic environments from a management perspective by emphasizing the importance of protecting the diversity of phyllosphere microorganisms in macrophyte-dominated lakes.