Iron-dependent AOM

New research using sediments from a Dutch canal has yielded important insights into the anaerobic oxidation of methane (AOM).  A group of scientists from the Institute for Water and Wetland Research at Radboud University identified archaea of the order Methanosarcinales as being responsible for AOM coupled to iron and manganese reduction.

AOM proceeds as methane is oxidised with various terminal electron acceptors including sulphate, nitrite and nitrate, and the importance of oxidised metals in the process has been investigated, but the microorganisms involved have remained unknown.  To address this, a culture of methanotrophs was established that was enriched with canal sediment.  When this culture was supplied with nitrate as the only available electron acceptor, it became dominated by AOM-associated archaea (AAA).  This culture linked methane oxidation to nitrate reduction, with N₂ being the main end product, although approximately 10% of nitrate was converted to ammonium.

The enrichment culture with AAA was used for further experiments, where methane oxidation was observed following the addition of ferric citrate, nanoparticulate ferrihydrite (Fe³⁺) or birnessite (Mn⁴⁺).  Analysis of the AAA genome showed that it could couple methane oxidation to nitrate reduction or Fe³⁺ reduction, or that the reverse methanogenesis pathway could also operate.  The authors therefore suggest that AAA could act as a versatile methanotroph, switching electron acceptors depending on their availability.

The paper was jointly led by Katharina Ettwig and Baoli Zhu. One of the co-authors, Boran Kartaldescribed the possible application of their findings: “A bioreactor containing anaerobic methane and ammonium oxidizing microorganisms can be used to simultaneously convert ammonium, methane and oxidized nitrogen in wastewater into harmless nitrogen gas and carbon dioxide, which has much lower global warming potential.”  The authors conclude that their work “may also shed light on the long-standing discussion about Fe²⁺ -producing processes on early Earth, when AAA-related organisms may have thrived under the methane-rich atmosphere in the ferruginous Archean oceans.”

Ettwig, K.F., Zhu, B., Speth, D., Keltjens, J.T., Jetten, M.S.M., Kartal, B. 2016. Archaea catalyse iron-dependent anaerobic oxidation of methane. PNAS, doi: 10.1073/pnas.1609534113.

Image is from the paper and shows fluorescence in situ hybridization of biomass from the enrichment culture of AAA and M. oxyfera-like bacteria.

Anaerobic Methane Oxidation Coupled to Denitrification

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.

Anaerobic Oxidation of Methane in Peatlands

12th July 2013. A new study published in Environmental Science and Technology has shed light on anaerobic oxidation of methane (AOM) in peatlands.  The importance of AOM as a methane sink has been recognised in marine sediments for a number of years, but the process is only poorly understood in peatlands, and has received relatively little attention.

In the latest study, researchers collected soil samples from 15 different peatlands and carried out lab incubation experiments.  The addition of ¹³C-CH₄ isotope tracers to the anaerobic incubations resulted in the production of ¹³CO₂ for samples from all 15 peatlands, implying that AOM occurred at all sites.  Rates of AOM were sustained for longer in fens compared to bogs, and the authors hypothesised that this was due to groundwater inputs supplying the electron acceptors needed to maintain the process.  However, the second part of the experiment that attempted to determine the relevant electron acceptor found that additions of nitrate, sulphate and iron had no effect on AOM.  It therefore appears that the pathway of AOM in peatlands is fundamentally different to that in other systems.

As a final piece of work, the authors calculated the amount of methane consumed through AOM for northern peatlands using low, average, and high estimations.  The results from their average scenario suggested that AOM could act as a sink of 24 Tg of methane each year; a significant amount.  Methane fluxes in peatlands are traditionally modelled using variables such as temperature, vegetation and water table, with methane consumption and production being considered to be oxic and anoxic processes.  However, the growing body of research on peatland AOM may well have implications on the use of such simple models.

Reference: Gupta, V., Smemo, K.A., Yavitt, J., Fowle, D.A., Branfireun, B.A., Basiliko, N. 2013. Stable isotopes reveal widespread anaerobic methane oxidation across latitude and peatland type. Environmental Science and Technology, in press.  DOI: 10.1021/es400484t