Flourescence in situ hybridization micrograph of the nitrate-reducing bioreactor community
Figure 1: Flourescence in situ hybridization micrograph of the nitrate-reducing bioreactor community. The yellow cells show the dominant ANME-2d population flanked by other bacteria (blue).

Methane is the second most abundant greenhouse gas in our atmosphere; it is roughly 30 times more efficient at trapping heat than carbon dioxide and has an average atmospheric lifespan of 12 years.  Methane is released into the atmosphere from both natural and human activities. The primary source of natural emissions is from microorganisms that decompose organic material in wetlands and aquatic sediments. Human-derived emissions include natural gas and petroleum systems, livestock production, and decomposing waste in landfills and wastewater treatment plants. Globally, over 60% of methane emissions are from human activity.

Natural metabolic processes in the soil help reduce the amount of methane that reaches the atmosphere. In aquatic sediments, anaerobic oxidation of methane (AOM) is responsible for consuming 90% of methane before it reaches the atmosphere. Geochemical studies have found that AOM is coupled to other metabolic pathways, including sulfate, nitrate, nitrite, iron, and manganese reduction, where these compounds are used as terminal electron acceptors.  For example, methane is oxidized with sulfate to form bicarbonate, bisulfide, and water.

The microorganisms responsible for AOM play a vital role in regulating methane emissions to our atmosphere hence it is important to understand who they are and how they work.  Thus far, the microbes mediating AOM coupled to sulfate reduction have been identified as a partnership between an anaerobic methanotrophic Archaea, called ANME, and sulfate-reducing Bacteria.  Recently, our lab also discovered a new ANME species, Candidatus Methanoperedens nitroreducens, capable of mediating AOM coupled to nitrate reduction independently (Figure 1). This microorganism contains genes for the methane oxidation pathway as well as nitrate reduction.  The nitrate reduction genes were acquired through a lateral gene transfer from a bacterial donor (Haroon et al. 2013).

Bioreactor

Figure 2: A bioreactor mediating AOM coupled to iron reduction. Bioreactor was amended with methane as the sole carbon and energy source, and ferrihydrite (Fe3+) as the sole electron acceptors.

The microbes responsible for AOM coupled to metal reduction remain elusive.  The aims for this project include:                                                 

Identify the microbial groups responsible for AOM coupled to metal reduction
Increase our understanding of the ANME lineage across multiple scales from community level, to interspecies, to the individual cell.

In order to identify the microorganisms responsible for AOM coupled to metal reduction, bioreactors were set up and fed with ferrihydrite (an iron compound) and methane (Figure 2).   The bioreactors showed strong enrichment for a single ANME population performing AOM coupled to iron reduction. Different methods such as bioreactor performance, isotope labelling, metagenomics analysis, and fluorescence in situ hybridization (FISH) indicate this ANME population is capable of AOM coupled to iron reduction, independent of a bacterial partner.  It is likely this ANME population has a crucial role in linking the global carbon and iron cycles. 

Staff: 
Principal investigator: Prof. Gene Tyson
PhD student: Mr. Andy Leu

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