Evaluating radioisotope-based approaches to measure anaerobic methane oxidation rates in lacustrine sediments

Abstract

The microbial anaerobic oxidation of methane (AOM) is the dominant sink for methane in anoxic sediments. AOM rate measurements are essential for assessing the efficacy of the benthic methane filter to mitigate the evasion of this potent greenhouse gas to the atmosphere. Incubation techniques with trace amounts of radiolabeled substrate (typically 14CH4) represent the most sensitive approach for methane oxidation rate measurements. Yet, radiotracer application can be performed in different ways, rendering the comparability of AOM rate measurements in field and laboratory investigations problematic. We compared four different 14CH4-based short-term incubation approaches to quantify methane turnover rates in lake sediments. Three of the applied methods yielded similar and reliable downcore rate profiles. They provided clear evidence for AOM with maximum rates of 15 nmol cm−3 d−1 at ~ 17 cm sediment depth. Using the short-term slurry incubation (SL) method, however, we were unable to detect the AOM activity maximum that we observed with the other approaches. We hypothesize that changes in the microbial structure and disruption of physical interactions due to mixing of sediments negatively affected the activity of AOM communities and longer incubation times are necessary to enhance the sensitivity of this approach. Minor variabilities in rate measurement that we found in the non-SL incubations may be related to small-scale sediment heterogeneity, differential partial methane loss during sample handling, and/or an uneven application of the radiotracer. Whole-core incubations interfere the least with the in situ conditions, but the ultimate choice of the AOM rate measurement method will depend on the individual sampling requirements.