Vertical sediment distribution of methanogenic pathways in two shallow Arctic Alaskan lakes

Abstract

Methanogenesis (MG) occurs in anaerobic lake sediments during the terminal step of organic matter degradation. Methane is typically produced via two primary catabolic pathways (acetoclastic or hydrogenotrophic) in which the primary substrates are acetate or H2/CO2, respectively. The acetoclastic pathway has been shown to dominate in a 2:1 ratio over the hydrogenotrophic pathway in freshwater sediments. Rates of methane production from each pathway are regulated primarily by the quantity and quality of organic matter. As acetate and H2 are produced through decomposition of organic matter, increased terrestrially derived organic matter loading can fuel sediment MG. Increased delivery of terrestrially derived organic matter to arctic lakes is expected under future climate change scenarios. Therefore, we compared unamended rates of MG in anaerobic sediment slurries to those amended with acetate or hydrogen. We also evaluated the vertical sediment distribution of MG pathways in 1-cm increments to a final depth of 5 cm using an inhibitor for the acetoclastic pathway, methyl fluoride. In both lakes, unamended rates of MG decreased with increasing sediment depth. Additions of acetate or hydrogen stimulated rates of MG at all depths in both lakes resulting in rates 1–3 orders of magnitude greater than MG rates in unamended slurries. The ratio of the acetoclastic to the hydrogenotrophic pathway decreased with increasing sediment depth in both lakes. Our findings suggest that increased delivery of terrestrial organic matter to shallow arctic lakes may increase sediment methane production.