Does hydrogenotrophic methanogenesis occur in seagrass beds? Insights from whole-ecosystem stable isotope labelling

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Abstract Seagrass beds are blue carbon ecosystems; however, they release methane (CH 4 ), a potent greenhouse gas. Although likely important, the influence of primary production on methanogenic pathways remains understudied. We labelled intact seagrass-sediment cores and in situ seagrass beds with 13 C-dissolved inorganic carbon ( 13 C-DIC) to trace the transfer to 13 C-CH 4 over a short (<24 h) and long (35 d) time frame. Labelled carbon was transferred from DIC to CH 4 in the absence of primary production, indicating hydrogenotrophic methanogenesis or mixotrophy. Longer-term persistent in situ 13 C-CH 4 release suggests that CH 4 was also produced from 13 C-labelled organic substrates. The transformation rate of 13 C-DIC to 13 C-CH 4 was reduced by 50% in the light, indicating interactions with primary production via CH 4 oxidation, oxidative stress, or substrate competition. Additionally, dark CH 4 fluxes (5.1 ± 0.7 μ mol m −2 d −1 ) were 16% higher than light CH 4 fluxes (4.3 ± 0.4 μ mol m −2 d −1 ) in seagrass beds under in situ conditions. These findings highlight the role of plant-sediment interactions in governing methanogenic pathways, with implications for seagrass beds as carbon-sequestering hotspots and CH 4 sources.

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