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Metal-dependent anaerobic methane oxidation in marine sediment: Insights from marine settings and other
systems
Lewen Liang, Yinzhao Wang, Orit Sivan and Fengping Wang
Citation: SCIENCE CHINA Life Sciences ; doi: 10.1007/s11427-018-9554-5
View online: http://engine.scichina.com/doi/10.1007/s11427-018-9554-5
Published by the Science China Press
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Special Topic: Elemental cycling by microorganisms in the hydrosphere https://doi.org/10.1007/s11427-018-9554-5
•REVIEW•
Metal-dependent anaerobic methane oxidation in marine sediment:
Insights from marine settings and other systems
Lewen Liang1,2, Yinzhao Wang1,2, Orit Sivan3& Fengping Wang1,2*
1State Key Laboratory of Microbial Metabolism, School of Life Sciences and Biotechnology, Shanghai Jiao Tong University, Shanghai 200240,
China;
2Joint International Research Laboratory of Metabolic & Developmental Sciences, Shanghai Jiao Tong University, Shanghai 200240, China;
3Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel
Received March 5, 2019; accepted April 22, 2019; published online June 14, 2019
Anaerobic oxidation of methane (AOM) plays a crucial role in controlling global methane emission. This is a microbial process
that relies on the reduction of external electron acceptors such as sulfate, nitrate/nitrite, and transient metal ions. In marine
settings, the dominant electron acceptor for AOM is sulfate, while other known electron acceptors are transient metal ions such as
iron and manganese oxides. Despite the AOM process coupled with sulfate reduction being relatively well characterized,
researches on metal-dependent AOM process are few, and no microorganism has to date been identified as being responsible for
this reaction in natural marine environments. In this review, geochemical evidences of metal-dependent AOM from sediment
cores in various marine environments are summarized. Studies have showed that iron and manganese are reduced in accordance
with methane oxidation in seeps or diffusive profiles below the methanogenesis zone. The potential biochemical basis and
mechanisms for metal-dependent AOM processes are here presented and discussed. Future research will shed light on the
microbes involved in this process and also on the molecular basis of the electron transfer between these microbes and metals in
natural marine environments.
anaerobic methane oxidation, metal-AOM, marine sediment, archaea, electron transfer
Citation: Liang, L., Wang, Y., Sivan, O., and Wang, F. (2019). Metal-dependent anaerobic methane oxidation in marine sediment: Insights from marine settings
and other systems. Sci China Life Sci 62, https://doi.org/10.1007/s11427-018-9554-5
Introduction
Methane is one of the most potent greenhouse gases that is
mainly produced through the methanogenesis process by
methanogens (Ciais et al., 2014). Anaerobic oxidation of
methane (AOM) coupled with sulfate reduction is considered
to be the major methane sink below the ocean floor; this
process consumes up to 90% of the methane generated in
anoxic marine sediments (Knittel and Boetius, 2009;Re-
eburgh, 2007). AOM was first observed in vertical geo-
chemical profiles, particularly at the sulfate-methane
transition zones (SMTZ) in marine sediment cores, where
upward-diffusing methane is consumed coupled with sulfate
depletion (Reeburgh, 1976;Iversen and Jorgensen, 1985).
This was initially believed to be an abiotic process due to the
extremely low energy yield available to support life (re-
viewed in (Knittel and Boetius, 2009)). However, through
the isotope fractionation of lipid biomarker, and fluorescence
in situ hybridization (FISH) and FISH coupled-secondary
ion mass spectrometry (FISH-SIMS) analyses, it was con-
firmed that AOM is a microbial process, mainly conducted
by the consortia consisting of anaerobic methane-oxidizing
archaea (ANMEs) and sulfate-reducing bacteria (SRB)
(Boetius et al., 2000;Hinrichs et al., 1999;Orphan et al.,
2001). Furthermore, in addition to sulfate, the well-known
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019 life.scichina.com link.springer.com
SCIENCE CHINA
Life Sciences
* Corresponding author (email: fengpingw@sjtu.edu.cn)
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electron acceptor in SMTZ, a variety of other en-
vironmentally essential compounds such as nitrate, nitrite,
and transient metal oxides (iron, manganese, and chromium)
have also been found to be capable of coupling to the AOM
process (Timmers et al., 2017). Sulfate and metal (iron,
manganese)-dependent AOM were identified in marine set-
tings while nitrate/nitrite and metal (iron, chromium)-de-
pendent AOM were spotted in freshwater samples (á Norði
and Thamdrup, 2014;Ettwig et al., 2010;Haroon et al.,
2013;Raghoebarsing et al., 2006;Shen et al., 2016). In
marine settings, whereas the microbes mediating sulfate-
dependent AOM have been identified, those involved in
metal-dependent AOM remain to date largely unknown.
Although identified in marine seep sediment (Beal et al.,
2009), AOM process coupled with metal oxides reduction is
presumed to be a challenging task for microbes in diffusive
marine sediments. Under natural environmental settings in
marine sediments, metal oxides exist in the form of minerals
which are barely soluble and hard for microbes to use
(Canfield, 1989). Based on the microbial respiration cascade,
the primarily available electron acceptors such as oxygen,
nitrate, manganese, iron, and sulfate are sequentially used to
couple organic matter remineralization above the SMTZ in
the sediment (Emerson and Hedges, 2003) (Figure 1). In this
way, relatively easily used metal oxides are reduced above
the methane-rich zone, and the “survived” metal oxides are
in the inert form such as the iron minerals bound in sheet
silicates, which are assumed to be preserved even on time-
scales of up to thousands of years (Canfield et al., 1992).
However, the reduced form of metal ions such as Fe2+ and
Mn2+ are usually found increasing in deep methanogenesis
zones in marine sediment cores, which suggests the occur-
rence of reduction of metal oxides in this layer where other
electron acceptors are already depleted (D’Hondt et al.,
2004;Oni et al., 2015;Vigderovich et al., 2019).
Furthermore, studies on modeling have revealed that me-
tal-dependent AOM accounts for the decrease of methane
and reduction of metal oxides observed in the methanogenic
zone. A metal-dependent AOM zone in deep marine sedi-
ment (Figure 1) (Riedinger et al., 2014;Sivan et al., 2007) as
well as in brackish coastal sediments (Egger et al., 2017;
Egger et al., 2016;Egger et al., 2015;Rooze et al., 2016;
Segarra et al., 2013;Slomp et al., 2013), has been proposed.
However, the responsible microbes and the strategies they
employ for metal-dependent AOM in the marine environ-
ment are still unknown. In this mini-review, we summarize
the geochemical evidences from sediment cores in various
marine environments and discuss potential biochemical
processes involved in metal-dependent AOM. In addition,
we discuss the microbes which are possibly responsible for
Figure 1 (Color online) Sequential utilization of electron acceptors in marine sediment (modified from Emerson and Hedges, 2003;Riedinger et al., 2014).
2Liang, L., et al. Sci China Life Sci
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these processes and their metabolic properties and call for
future research attention in this regard.
Geochemical evidence of metal-dependent AOM in
marine sediment
The first documented evidence of potentially metal-driven
AOM could be dated back to the year 1980. The presence of
iron and manganese in microcosm incubation systems was
found to increase the ratio of methane oxidation to methane
formation as revealed by isotopic data (Zehnder and Brock,
1980), suggesting that these metal ions may stimulate the
AOM process. Although sulfate was the only known electron
acceptor for AOM in marine sediment at the time, accumu-
lating geochemical profiles revealed the decoupling of me-
thane oxidation from sulfate reduction in anoxic marine
sediment; suggesting the presence of other electron acceptors
such as iron and manganese (Hoehler et al., 1994). Besides,
high concentrations of reduced metal ions were also ob-
served below the SMTZ in the deep methanogenic zone,
which implies the existence of an unrecognized relationship
between metal reduction and methane cycling (D’Hondt et
al., 2004). Diffusion and reaction simulation of the pore
water profiles from West African Margin deep sea sediment
further indicates metal-dependent AOM as the possible
process for methane decrease in the methanogenic zone
(Sivan et al., 2007).
In the year 2009, Beal et al. in their study conducted a
microcosm experiment by adding metallic minerals (bir-
nessite and ferrihydrite) to the marine seep sediment col-
lected from the Eel River Basin and revealed the potential for
metal (manganese and iron)-dependent AOM in a marine
environment (Beal et al., 2009). Sivan et al. (2014) in their
study showed that iron-dependent AOM in these seeps could
occur concurrent with sulfate-dependent AOM (Sivan et al.,
2014). Additionally, a study of the decoupling of AOM and
sulfate reduction in the incubated seep sediments from the
Santa Monica Basin revealed that ferric citrate could be used
as an alternative electron acceptor (Scheller et al., 2016).
The AOM process usually causes higher alkalinity and
increasing concentration of dissolved inorganic carbon in
pore waters and leads to precipitation of authigenic carbo-
nates as well as the formation of carbonate deposits (Chen et
al., 2014). By combining nanoscale secondary ion mass
spectrometry ion mapping and transmission mode dual en-
ergy analyses, Peng et al. in their study found that iron is
widely distributed in the veins of the carbonate pipe in the
northern Okinawa Trough (OT), which indicates the co-
precipitation of iron with carbonate during methane oxida-
tion. Because the organic matter content in this in situ en-
vironment was poor (~0.5 wt%), iron-dependent AOM has
recently been proposed as one of the driving forces in the
formation of the iron-rich carbonate pipe (Peng et al., 2017).
In addition, similar iron or other metals-rich carbonates have
also been found in the northern OT, Black Sea, South China
Sea, and Gulf of Cadiz cold-seep environments (Han et al.,
2013;Merinero et al., 2012;Reitner et al., 2005b;Sun et al.,
2015;Tong et al., 2013); which suggests extensive dis-
tribution of metal-dependent AOM in natural marine sedi-
ments.
However, metal-dependent AOM in the deep methano-
genic zone of diffusive marine sediments has mainly been
considered by modeling approaches. Additional to the early
modeling studies already mentioned (Sivan et al., 2007),
geochemical profiles of the Argentine Basin sediment
showed that microbial iron reduction occurs below the
SMTZ, and it has also been suggested that the low reactivity
of organic matters precludes the possibility of organo-clastic
dissimilatory iron reduction and thus iron-dependent AOM
in the methanogenic zone is likely to happen (Riedinger et
al., 2014). Meanwhile, measured manganese usually exhibits
similar behavior as iron; inferring that manganese-dependent
AOM is also likely to occur at the same layer (Riedinger et
al., 2014;Treude et al., 2014). A one-dimensional reactive
transport model was developed for the Baltic Sea sediment
sample to investigate the contribution of iron-dependent
AOM as a source of ferrous iron, which is abundant in pore
water (Egger et al., 2017). The model includes physical
transport as well as biogeochemical transformations, as-
suming that iron-dependent AOM is the dominant iron oxide
reduction process and that fits well with both the pore water
and solid phase profiles.
Biogeochemical consideration of metal-dependent
AOM
Thermodynamically, the sustainability of a living cell re-
quires energy to synthesize adenosine triphosphate (ATP) for
metabolism. Moreover, it has been considered that a favor-
able metabolic reaction must provide at least 15 kJ mol−1
energy per ATP generation (Caldwell et al., 2008). Due to its
low energy yield (16.6 kJ mol−1), sulfate-dependent AOM
has been considered for a long time to be a thermo-
dynamically unfavorable process in natural environments.
However, it has been proposed that cells that adapt to energy-
deficient conditions may require even less energy than ex-
pected to sustain life (Knab et al., 2008). Table 1 shows a list
of the potential Gibbs free energy changes of the AOM
process coupled to different electron acceptors. However, the
identified electron acceptors of the AOM process in marine
settings are restricted to sulfate, iron, and manganese.
Compared to the prevalent sulfate-dependent AOM (ΔG0’=
−16.6 kJ mol−1), which serves as an example of how life
operates close to the thermodynamic limit, the metal-de-
3
Liang, L., et al. Sci China Life Sci
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pendent AOM is thermodynamically favorable (iron-de-
pendent AOM ΔG0’=−81.6 kJ mol−1; manganese-dependent
AOM ΔG0’=−494.0 kJ mol−1) (He et al., 2018).
However, in the in situ environment, the Gibbs free energy
(in situ ΔG in Table 1) is quite different from the ΔG0’ cal-
culated under standard conditions for biological processes.
Furthermore, the total energy yield should include the reac-
tion rate in the in situ environment. In the microcosm ex-
periment for metal-dependent AOM using the Eel River
Basin seep sediment, it was found that in the in situ condi-
tion, the birnessite-dependent AOM has the potential to gain
10 times more energy, while ferrihydrite-dependent AOM
gains twice as much energy compared to that of sulfate-
dependent AOM (Beal et al., 2009).
Additional to the direct metal-dependent AOM, metal
oxides can indirectly influence the AOM process coupled
with other electron acceptors. Moreover, it has been shown
that the addition of hematite to the Hydrate Ridge seep se-
diment results in iron reduction co-occurring with increasing
sulfate-dependent AOM rates (Sivan et al., 2014). Besides,
the redox reaction between sulfur and iron is assumed to be
similar to that in the “cryptic sulfur cycle” (Holmkvist et al.,
2011;Pellerin et al., 2018). It was speculated that either the
iron oxides reduction coupled with sulfur disproportionation
could stimulate sulfate recycling, thus increasing the rates of
sulfate-dependent AOM, or that the precipitation of pyrite
could remove the end product sulfide in the system, and
consequently accelerates the process.
Microbial processes in metal-dependent AOM
Known microbial processes in AOM
At present, the majority of the known microbes responsible
for AOM are ANMEs in the Archaea domain, this being with
the exception of NC10 bacteria which are capable of nitrite-
dependent AOM (Ettwig et al., 2010) and other methano-
gens, as well as aerobic bacteria, that are suggested con-
ducting iron-dependent AOM in lake sediments (Bar-Or et
al., 2017). Based on 16S ribosomal RNA (rRNA) gene
phylogeny, all ANMEs belong to the archaeal phylum Eur-
yarchaeota in the Archaea domain and cluster closely with
the cultivated methanogens in the class Methanomicrobia
(Knittel and Boetius, 2009). ANMEs that are responsible for
sulfate-dependent AOM can be divided into three groups:
ANME-1 (subgroups a and b); ANME-2 (subgroups a, b, and
c); and ANME-3, with 16S rRNA gene similarity between
75%–92% (Knittel and Boetius, 2009). Nitrate-dependent
AOM was carried out by ANME-2d lineage enriched from
freshwater systems (Haroon et al., 2013;Hu et al., 2009;
Raghoebarsing et al., 2006), and was later found to be cap-
able of catalyzing iron-dependent AOM (Ettwig et al., 2016).
Recently, a new microorganism in ANME-2d lineage, which
is also enriched from freshwater sediment, was found cap-
able of iron-dependent AOM (Cai et al., 2018).
In marine sediments, ecological niche separation of AN-
MEs usually occurs where ANME-2a/b dominates upper
layers and ANME-2c and/or ANME-1 take over in deeper
zones, which indicates the diversity/specificity of metabolic
capabilities of ANMEs (Niu et al., 2017). ANMEs that are
responsible for sulfate-dependent AOM tend to live in syn-
trophy with bacterial partners and form cell consortia cov-
ered by a thick siliceous envelope (Chen et al., 2014).
However, the syntrophic pattern also varies among the
ANMEs. For example, ANME-1 and ANME-2 archaea are
usually associated with sulfate-reducing bacteria (SRB) of
the Desulfosarcina/Desulfococcus (DSS) clade within the
Deltaproteobacteria, while ANME-3 also formed consortia
with the SRB of branch Desulfobulbus in addition to the
syntrophy with DSS (Knittel and Boetius, 2009). Further,
ANME-2 was also identified to form aggregates with non-
SRB partners such as Limnobacter spp. of Betaproteo-
bacteria (Chen et al., 2016;Pernthaler et al., 2008). Besides,
ANME-1 in the Black Sea microbial mat was found to be
living without a bacterial partner (Reitner et al., 2005a).
ANMEs perform AOM through the reverse methanogen-
esis pathway. ANMEs and methanogens share the same sets
of enzymes for methane metabolism while working in dif-
Table 1 Gibbs free energy changes under standard conditions (ΔG0’) and the calculated in situ ΔG of different electron acceptors (based on He et al., 2018;
Lu et al., 2016;Beal et al., 2009)
Reaction ΔG0’ (kJ mol−1 CH4)In situ ΔG (kJ mol−1 CH4)
CH +S O HCO + HS + H O
4 4
2
3 2
−16.6 −14~−35
CH +8Fe(OH) +15H HCO + 8Fe + 21H O
43
+
3
2+
2
−81.6 −270.3
CH +4 MnO +7H HCO + 4Mn +5H O
4 2
+
3
2+
2
−494.0 −556
CH +4NO CO +4NO + 2H O
4 3 2 2 2
−519.8 N/A
CH +4 /3Cr O + 32 / 3H 8 / 3C r + C O +22/3H O
4 2 7
2 + 3+
2 2
−878.8 N/A
CH +8/3NO + 8/3H CO +8/3N +10/3H O
4 2
+
2 2 2
−928 N/A
4Liang, L., et al. Sci China Life Sci
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ferent directions (Hallam et al., 2004). This assertion has
been verified by the ANME-2a genome that contains all the
required genes that satisfy this hypothesis (Wang et al.,
2014). In this pathway, methane is activated by the methyl-
coenzyme M reductase (MCR), and then finally oxidized to
CO2(McGlynn, 2017). MCR is considered to be the key
enzyme in methane and short-chain alkanes (such as propane
and n-butane) metabolism for archaea (Scheller et al., 2010;
Laso-Pérez et al., 2016). However, genes that encode the
MCR complex are also found to exist in genomes of the
phyla Bathyarchaeota (formerly known as the Miscellaneous
Crenarchaeota Group (MCG) (Meng et al., 2014)), Ver-
straetearchaeota, and other archaeal phyla. Although the
functions of the MCR complex encoded in these archaeal
genomes have not been confirmed, it has opened the possi-
bilities of methane/short-chain alkane metabolism outside
the Euryarchaeota phylum, and indicating versatile methane/
short-chain alkane metabolic properties (Berghuis et al.,
2019;Borrel et al., 2019;Boyd et al., 2019;Colman et al.,
2019;Evans et al., 2015;McKay et al., 2019;Seitz et al.,
2019;Vanwonterghem et al., 2016;Wang et al., 2019a;Wang
et al., 2019b).
Microorganisms participate in metal-dependent AOM
To the best of our knowledge, evidence of microbes that are
potentially responsible for metal-dependent AOM in marine
samples is very little. One of these evidences is based on the
seep sediment collected from the Eel River Basin; where
after 10 months enrichment using manganese (birnessite) as
the electron acceptor, a shift of the microbial community was
observed compared to the original sample and control
groups. Based on the community shift during the enrichment,
the authors of this study suggested that either ANME-1 or
methanococcoides/ANME-3 with a bacterial partner might
be responsible for the metal-dependent AOM (Beal et al.,
2009). Furthermore, another study on the methane seep se-
diment of the Santa Monica Basin revealed that the enriched
samples which contained high abundance of ANME-2a and
ANME-2c and relatively low abundance of ANME-1 could
decouple the AOM process from SRB activities when ferric
iron compounds (ferric citrate and ferric-EDTA) were added
(Scheller et al., 2016). Research on the North Sea sediment
showed that members of the candidate division JS1, metha-
nogens and Methanohalobium/ANME-3-related archaea, are
closely linked to the profile of the dissolved iron in the
methanogenic sediments, as revealed by the result of the
correlation analysis between the microbial populations and
geochemical profiles in sediment cores. This indicates their
potential involvement in iron-dependent AOM in marine
environments (Oni et al., 2015).
In freshwater systems, ANME-2d has been found to be
capable of performing metal-dependent AOM. An enrich-
ment culture of denitrifying anaerobic methane oxidation,
containing ANME-2d and the bacterial partner, are shown to
separately support iron and manganese-dependent AOM
(Ettwig et al., 2016) and chromium-dependent AOM (Lu et
al., 2016). Also, long-term enrichment fed with ferrihydrite
and methane revealed that most of the archaea belong to
ANME-2d; which further indicates iron-dependent AOM
(Cai et al., 2018). Besides, a much more complex interplay
between methanogens and methanotrophic bacteria has also
been suggested to be responsible for the iron-dependent
AOM in lake sediment (Bar-Or et al., 2017). The authors in
this research set up a long-term incubation, adding 13CH4and
iron minerals to the systems; where the incorporation of 13C
into the fatty acids and the pmoA gene increase indicated the
involvement of methanotrophic bacteria (probably not NC-
10), while inhibition of activity of methanogens (probably
not ANMEs) completely stopped iron-dependent AOM in
the system.
Potential metabolic pathways for metal-dependent
AOM
Since there is no representative of metal-dependent AOM
microbes from the marine environment, it is still unclear
which pathway is involved. Nevertheless, genome re-
construction of ANME-2d from freshwater sediment en-
richment that is capable of iron-dependent AOM possesses
all the genes for the “reverse methanogenesis” pathway,
which are highly expressed, as revealed by metatran-
scriptomic data (Figure 2A) (Cai et al., 2018). Besides, a
recent biochemical study has revealed that Methanosarcina
acetivorans, a cultured methanogen, could reduce iron dur-
ing the oxidation of methane using the reverse methano-
genesis pathway as ANME-2a (Figure 2B) (Yan et al., 2018).
Nevertheless, how the electron is transferred to ferric iron
remains to date unknown. In most ANMEs, multi-heme c-
type cytochromes (MHCs) serve as a possible electron
shuttling. This has already been observed in iron-reducer
Geobacter sulfurreducens that use MHCs to transfer elec-
trons during Fe (III) and Mn (IV) oxide reduction (Mehta et
al., 2005), and might also work for metal-dependent AOM.
MHCs have been inferred to be involved in direct inter-
species electron transfer for exchange reducing equivalents
between the syntrophic AOM consortium (McGlynn et al.,
2015;Wegener et al., 2015). High numbers of MHCs are
present and are highly expressed in ANME-2d from fresh-
water iron-dependent AOM enrichment, including those lo-
cated adjacent to the menaquinone gene and the one that
contains an S-layer domain, indicating its capability to per-
form electron transfer from cytoplasmic membrane to iron
oxides (Cai et al., 2018). Biochemical data of Methano-
sarcina acetivorans revealed that methanogen enables iron-
5
Liang, L., et al. Sci China Life Sci
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Figure 2 (Color online) A proposed metabolic pathway of iron-dependent AOM inferred from the genomes of ANME-2d. A, ANME-2a and Methano-
sarcina acetivorans; (B, Abbreviations for major enzymes and co-factors. Mcr, Methyl-coenzyme M reductase; Mtr, Tetrahydromethanopterin S-methyl-
transferase; Mer, Coenzyme F420-dependent methylene-H4MPT reductase; Mtd, F420-dependent methylene-H4MPT dehydrogenase; Mch, methenyl-H4MPT
cyclohydrolase; Ftr, formylmethanofuran: H4MPT formyltransferase; Fmd, Formylmethanofuran dehydrogenase; CODH/ACS, CO dehydrogenase/acetyl-
CoA synthase; Pta, phosphotransacetylase; Ack, acetate kinase; Fd, ferredoxin; Hdr, heterodisulfide reductase; Cytb, b-type cytochrome; NrfD, polysulfide
reductase subunit D; FeS, ferredoxin iron-sulfur protein; Fpo, F420H2dehydrogenase; Mrp, multi-subunit type Na+/H+antiporter; Atp, ATP synthase. MK,
menaquinone; MP, methanophenazine. Readers may refer to the paper text and references for details (Cai et al., 2018;Wang et al., 2014;Yan et al., 2018).
6Liang, L., et al. Sci China Life Sci
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dependent AOM, and also showed that MHCs along with the
low-molecular-mass humic acids- tested by analog anthra-
quinone-2, 6-disulfonate (AQDS)- are the potential direct
electron donor to iron oxides as shown in Figure 2B.
Outlook
Large amounts of transient metals are provided to the ocean
annually (Poulton and Raiswell, 2000); for example, the
global input of iron to the ocean is 703.5~1217.5 Tg per year
(Jickells et al., 2005). Since transient metal elements may go
through recycling hundreds of times before they are finally
buried in the seabed, metal-dependent AOM potentially has a
great impact on methane removal in marine sediment (Beal
et al., 2009). Besides, during the Archean Eon of earth’s
history, anaerobic metabolic processes related to iron cycling
were thought to be one of the driving forces shaping the
biosphere (Canfield et al., 2006). As of then, supporting
evidence has insinuated the involvement of metal-dependent
AOM as the main methane sink (Beal et al., 2009;Crowe et
al., 2011;Riedinger et al., 2014). However, metal-dependent
AOM in marine settings has not been studied extensively.
Despite accumulating geochemical evidence and modeling
studies indicating the presence of metal-dependent AOM in
marine seep environments, the geological prevalence and
distribution of this process in the ocean is yet to be de-
termined. Besides, as shown in Figure 1, metal-dependent
AOM is likely to occur in the deep methanogenic zone,
which is usually overlooked on either methane oxidation or
metal reduction. The biochemical mechanism on metal-de-
pendent AOM as well as its contribution to global carbon and
metal elements cycling has not yet been studied. Due to the
ecological niche separation for different clades of ANMEs as
suggested earlier, microbes for metal-dependent AOM in the
methanogenic zone may have distinct metabolic properties
either to survive or carry out this process. As in the early age
of the study of sulfate-dependent AOM, microbial studies
such as enrichment and metabolic analysis are crucial in
order to fully understand the underlying process. The im-
portance and contribution of metal-dependent AOM to me-
thane sink as well as deep-biosphere element cycling needs
further investigation.
Compliance and ethics The author(s) declare that they have no conflict
of interest.
Acknowledgements The authors would like to thank Wenyue Liang,
Minyang Niu, Zeyu Jia and Yunru Chen from Shanghai Jiao Tong University
for providing help either on figure visualization or suggestions. This work
was supported by the National Natural Science Foundation of China
(91751205, 41525011), the National Key R&D project of China
(2018YFC0310800), China Postdoctoral Science Foundation Grant
(2018T110390), and the joint Israel Science Foundation-National Natural
Science Foundation of China (ISF-NSFC) (31661143022 (FW), 2561/16
(OS)).
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