Abstract:

Shallow lakes produce and emit substantial amounts of methane (CH₄). Part of the CH₄ produced in lakes is consumed by methane-oxidizing bacteria (MOB) present in the sediment and water column, thus reducing the overall CH₄ emissions. However, the role of aquatic plants as habitat for CH₄ oxidation by MOB is poorly understood. In this study, we compared CH₄ oxidation rates and MOB abundance associated with different types of aquatic plants (periphyton, filamentous algae, and both above-ground macrophytes and their rhizosphere). The plants were collected from shallow lake mesocosms exposed to experimental nutrient enrichment and warming treatments for 17 years prior to this study. Incubations of all sampled plants showed CH₄ oxidation, with above-ground macrophyte tissue and filamentous algae having the highest rates of up to 0.25 µmol CH₄ h⁻¹ g⁻¹ dw. Oxidation rates associated with macrophytes were species dependent, with consumption rates on rhizospheres of Potamogeton crispus higher than those on Elodea canadensis. The increase in nutrients and dissolved CH₄ in the water tended to increase MOB abundance and activity for all plant types, while no effect of long term warming was detectable. Our results showed that MOB associated with periphyton, filamentous algae and macrophytes oxidize CH₄ in shallow lakes at different rates across species or plant types. We also found that high macrophyte biomass is associated with reduced CH₄ concentration in the water. This study shows that CH₄ oxidation occurs on many plant surfaces but that oxidation rates alone cannot explain the reduced CH₄ emissions at higher plant biomass.

https://doi.org/10.1016/j.aquabot.2022.103610