Methane flux at the water-gas interface is influenced by complex interactions among phytoplankton, phosphorus inputs and methane-functional bacteria: A microcosm systems study

Abstract

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.