Reverse Methanogenesis and Respiration in Methanotrophic Archaea.

Peer H A Timmers,Jasper J Koehorst,Alfons J M Stams,Mike S M Jetten,Caroline M Plugge,Cornelia U Welte

doi:10.1155/2017/1654237

Abstract

Anaerobic oxidation of methane (AOM) is catalyzed by anaerobic methane-oxidizing archaea (ANME) via a reverse and modified methanogenesis pathway. Methanogens can also reverse the methanogenesis pathway to oxidize methane, but only during net methane production (i.e., “trace methane oxidation”). In turn, ANME can produce methane, but only during net methane oxidation (i.e., enzymatic back flux). Net AOM is exergonic when coupled to an external electron acceptor such as sulfate (ANME-1, ANME-2abc, and ANME-3), nitrate (ANME-2d), or metal (oxides). In this review, the reversibility of the methanogenesis pathway and essential differences between ANME and methanogens are described by combining published information with domain based (meta)genome comparison of archaeal methanotrophs and selected archaea. These differences include abundances and special structure of methyl coenzyme M reductase and of multiheme cytochromes and the presence of menaquinones or methanophenazines. ANME-2a and ANME-2d can use electron acceptors other than sulfate or nitrate for AOM, respectively. Environmental studies suggest that ANME-2d are also involved in sulfate-dependent AOM. ANME-1 seem to use a different mechanism for disposal of electrons and possibly are less versatile in electron acceptors use than ANME-2. Future research will shed light on the molecular basis of reversal of the methanogenic pathway and electron transfer in different ANME types.

Highlights

ANME were first discovered in marine sediments where anaerobic oxidation of methane (AOM) was coupled to sulfate reduction (SR) (Table 1, reaction (1))
ANME formed metabolically interdependent consortia with sulfate-reducing bacteria (SRB) that belong to the Deltaproteobacteria [1,2,3]
We describe the reversibility of the central methanogenic pathway, including the key enzyme in methanogenesis and anaerobic methanotrophy

Summary

Introduction

Methanoperedens nitroreducens,” was recently discovered to perform nitrate-dependent AOM (N-AOM) [28] (Table 1, reaction (2)). This cluster was named “ANME-2d” [29] but later renamed to “GOM Arc I” [30] and “AOM-associated archaea (AAA)” [6]. More ANME phylotypes in different environments and possibly new archaeal clades involved in AOM may yet have to be discovered. Methyl coenzyme M reductase A genes (mcrA) from Bathyarchaeota (formerly known as Miscellaneous Crenarchaeota Group) and from the new archaeal phylum Verstraetearchaeota were recently found, indicating their involvement in methane metabolism [37, 38].

ANME versus Methanogens

Reversal of the Methanogenesis Pathway

Respiration during Anaerobic

Findings

Future Challenges