Iron-dependent AOM

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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.

Dull as Ditchwater?

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19th April 2010.

The murky, smelly sediments at the bottom of a canal or ditch might not be the obvious place to find a novel form of bacterial life which prompts questions about the early evolution of metabolism, the nature of the biological methane sink, or the possible role of man-made pollution in creating a microbe which ingeniously generates its own fuel supply.  A study by researchers in Netherlands (Ettwig et al., 2010, Nature) has revealed a new type of oxygen-producing, methane-eating bacteria living in just such an unpromising environment, and the findings have expanded our understanding of the interaction between methane and the biosphere.

Earlier studies investigating an enrichment culture of nitrate-rich sediments of the Twente Canal revealed that these sediments contain bacteria which oxidise methane anaerobically via a process apparently fuelled by nitrite reduction (Raghoebarsing et al., 2006, Ettwig et al. 2008). Hitherto, anaerobic methane oxidation was thought to be the preserve of ANME (ANaerobic MEthane oxidising) archaea acting in consortium with sulfate-reducing bacteria in marine sediments.

The same research group, based at Radboud University, Nijmegen, has now demonstrated that the anaerobic methane oxidation is performed by a novel bacterium, provisionally namedMethylomirabilis oxyfera, which acts as its own DIY oxygen generator. M. oxyfera first reduces nitrite to nitric oxide (NO) which is then split by a putative nitric oxide dismutase enzyme into O2and N2. The self-produced oxygen is then used to fuel methane oxidation as in conventional oxygen-dependent methanotrophy. Genomic studies of M. oxyfera revealed the presence of genes encoding for the well-established pathway for aerobic methane oxidation. M. oxyfera’slack of genes encoding enzymes known to be crucial for anaerobic methane oxidation in ANME archaea highlights the novelty of the ‘crypto-aerobic’ metabolism revealed in this study.

This clear demonstration that microbial life can generate oxygen from nitrogen oxides provides fresh material for speculation about the evolution of metabolism in early Earth, prior to the Great Oxidation Event (2.45 Gyr ago). Alternatively, the possibility that man’s agricultural activities have provided a niche which microbial activity has evolved to exploit, is also suggested by the capabilities of the newly discovered Dutch ditch dweller.

References:

Ettwig et al. (2010). Nitrite-driven anaerobic methane oxidation by oxygenic bacteria. Nature,464, 543-548.

Ettwig et al. (2008). Denitrifying bacteria anaerobically oxidise methane in the absence ofArchaea. Environmental Microbiology, 10, 3164-3173.

Raghoebarsing et al. (2006). A microbial consortium couples anaerobic methane oxidation to denitrification. Nature, 440, 918-921.