10th December 2014.
Nitrate/nitrite-dependent anaerobic oxidation of methane (n-damo) is a recently discovered process that is, to some extent, responsible for limiting emissions of methane from water bodies to the atmosphere. Despite its potential importance for carbon cycling, numerous questions remain unanswered about the process. A new paper published in PNAS addresses some of these questions.
The researchers investigated the microorganisms responsible for n-damo, which are bacteria belonging to the candidate phylum NC10 and related to Candidatus Methylomirabilis oxyfera. Through the analysis of sediments from Lake Constance in Germany, it was discovered that the abundance of M. oxyfera-like bacteria was greatest where methane and nitrate profiles intersected, lending support to the hypothesis that these bacteria carry out n-damo. Furthermore, results suggested that denitrifying methanotrophs were more numerous than aerobic methanotrophs at deep, undisturbed locations in the lake.
The authors conjectured that the need for a stable environment could drive the within-lake distribution of n-damo bacteria. For instance, numerous processes could introduce oxygen to sediments in shallow areas of the lake, such as waves, bioturbation, or the actions of algae and macrophytes. Because the bacteria grow relatively slowly, such disturbances would be difficult to recover from. Additionally, due to biological factors, nitrate concentrations are depleted in the upper layers of the lake, but remain higher in the lower layers, thus favouring n-damo in deeper waters.
The authors conclude by stating that n-damo is currently an overlooked process, and that it functioned as the dominant methane sink in Lake Constance. They also include the caveat that care must be taken when studying this pathway, as the close juxtaposition between oxygen and nitrate profiles could result in n-damo being misidentified as aerobic methane oxidation.
Reference: Deutzmann, J.S., Stief, P., Brandes, J., Schink, B. 2014. Anaerobic methane oxidation coupled to denitrification is the dominant methane sink in a deep lake. PNAS, 10.1073/pnas.1411617111.
Photo: Satellite image of Lake Constance, NASA.