Relief of Phosphate Limitation Stimulates Methane Oxidation

Abstract

Aquatic ecosystems such as shallow lakes and wetlands are important emitters of the greenhouse gas methane (CH 4 ). Increased phosphorus (P) loading is expected to increase CH 4 production in these ecosystems. This increased CH 4 production can potentially be mitigated by increased CH 4 oxidation, but how P availability affects methane-oxidizing bacterial (MOB) community composition and potential CH 4 oxidation remains to be tested. Here, we incubated MOB from sediments of four subtropical lakes of different trophic states for 7 days at different phosphate (PO 4 3- ) concentrations to determine the effects of P on MOB community composition and potential CH 4 oxidation. We measured CH 4 consumption daily and compared CH 4 oxidation during the exponential growth phase. Furthermore, we determined MOB community composition at the end of the incubations using qPCR of the pmoA gene. To test for differences in N and P uptake, we determined bacterial biomass N and P content. We found that increases in PO 4 3- concentrations until 10 µM significantly increased CH 4 oxidation. PO 4 3- also increased bacterial biomass P content, while N content was not affected. MOB community composition was not affected by PO 4 3- but more strongly correlated to lake of origin, likely due to the short duration of the incubations. Our results show that PO 4 3- can not only stimulate CH 4 oxidation indirectly through increased CH 4 production, but also directly by increasing MOB growth. Importantly, these effects only occur at low PO 4 3- concentrations, indicating that at high nutrient loads the increased CH 4 oxidation will likely not mitigate the increased CH 4 production.