Methane flux at the water-gas interface is influenced by complex interactions among phytoplankton, phosphorus inputs...

Phytoplankton affect carbon cycling and emissions in eutrophic reservoirs dramatically, but our knowledge about carbon emissions response to phytoplankton bloom and phosphorus enrichment is rather limited. Here we performed a microcosm experiment with five treatments to investigate how phytoplankton blooms and phosphorus addition will impact the carbon emissions and the methane-functional bacterial community. During the 43-day incubation, the CH₄ and CO₂ flux at the water-air interface in the five water columns fluctuated between 7.536 and 16.689 μmol and between 2788.501 and 4142.726 μmol, respectively. The flux of CH₄ and CO₂ during phytoplankton decay was 1.542 to 10.397 times and 4.203 to 8.622 times higher, respectively, compared to that during phytoplankton growth. Furthermore, exogenous phosphorus increases bloom biomass of phytoplankton and subsequent CH₄ production, even with low nitrogen concentrations. The addition of 1 mg KH₂PO₄ resulted in a conservative increase of 0.0715 μmol in CH₄ emission and 11.911 μmol in CO₂ emission in the water column, respectively, compared to the in-situ water column. High throughput sequencing determined that hydrogenotrophic Methanoregula dominated methanogens (MPB) and Methylocystaceae dominated methanotrophs (MOB) in the sediment. Phosphorus inhibited the relative abundance of Methanoregula after incubation, resulting in a significant decrease. Real-time quantitative polymerase chain reaction indicated that the absolute abundance of MPB and MOB (i.e., the mcrA gene and the pmoA gene) in the sediments ranged from 5.1354E+06 to 6.3176E+07 copies·g^-1 and 1.1656E+06 to 9.5056E+06 copies·g^-1, respectively. The mcrA gene showed a preference for sediments with high organic carbon content. The effect of eutrophication on CH₄ emissions is closely related to nutrient load and distinct niche of methane-functional bacteria.