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Geomicrobiology
Methanotrophs in moss
Peat bogs release large quantities of methane to the atmosphere. A global survey of peat mosses reveals a
ubiquitous symbiotic relationship with methane-oxidizing bacteria.
Yin Chen and J. Colin Murrell
P
eat bogs, prevalent in northern
latitudes, store around a third of the
Earth’s carbon1. Acidic, nutrientdepleted conditions in these bogs impede
decomposition of organic matter, facilitating
the formation of peat. Sphagnum mosses
represent one of the dominant forms of
vegetation in these systems. Anaerobic
breakdown of these mosses produces
large quantities of methane, a potent
greenhouse gas. There is growing concern
that global warming will stimulate the
anaerobic breakdown of peat mosses, and
hence methane production. Writing in
Nature Geoscience, Kip and colleagues2 show
that Sphagnum mosses form symbioses with
methane-oxidizing bacteria — which have
the potential to diminish methane release —
in peat bog systems around the globe.
Aerobic methane-oxidizing bacteria,
known as methanotrophs, function as a
terrestrial methane sink. Methanotrophs
not only thrive as free-living bacteria in
peatland soils, but also form symbioses
with submerged Sphagnum mosses.
Methanotrophs embedded in moss tissues
collected from a peat bog in the Netherlands
were found to convert methane to carbon
dioxide, which was subsequently taken
up by the plants3. However, the global
extent of this symbiosis, together with
its significance for attenuating methane
emissions from peatland ecosystems, has
remained uncertain.
Kip and colleagues2 collected Sphagnum
mosses from the pools, lawns and
hummocks of nine Sphagnum-dominated
wetlands across the globe. They observed
significant rates of methane oxidation in
all Sphagnum mosses; rates reached up to
80 μmol per day per gram dry weight in
mosses sampled from northern Siberia.
Oxidation was greatest in mosses collected
from waterlogged pools and was lower
in mosses collected from hummocks
and lawns. The authors attribute the
consumption of methane to a symbiotic
association with methanotrophs, because in
a laboratory experiment, methane oxidation
ceased following the addition of acetylene, a
Methane
Oxygen
Other plants
(e.g. Eriophorum)
Symbiotic
methanotrophs
Sphagnum
Alternative
carbon sources
CO2
Photosynthesis
Methane
Water
Peat
Methane
Mineralization of
organic matter
Rhizosphere
methanotrophs
Figure 1 | Methane oxidation by methanotrophs in a peat bog. Kip et al.2 reveal that Sphagnum mosses
form symbioses with methane-consuming bacteria in Sphagnum-dominated peat bogs across the globe.
Methane oxidation by the symbionts was greatest in submerged mosses. Carbon dioxide derived from
methane oxidation — and potentially from methanotrophic utilization of alternative carbon sources — is
taken up by the plants for photosynthesis. The symbiosis could help to explain why Sphagnum-dominated
peatlands emit less methane than other peatland types that are dominated by vascular plants which
channel methane from the soil to the atmosphere. Free-living methanotrophs that inhabit the rhizosphere
of peat-adapted plants also play an important role in methane oxidation.
compound that specifically inhibits methane
oxidation by methanotrophs. Stableisotope labelling experiments subsequently
confirmed that methane-derived carbon
was taken up by submerged mosses in the
form of carbon dioxide (Fig. 1). The authors
estimate that methane-derived carbon
accounted for up to 35% of the carbon
dioxide assimilated by Sphagnum mosses in
these experiments. DNA-based microarray
analyses revealed that the diversity of
methanotrophs in the moss samples
was surprisingly high for this specific
ecological niche. One would expect a higher
diversity in the surrounding peat soils4–5,
as the heterogeneous nature of soils creates
different ecological niches, with differing
concentrations of methane, oxygen and
other essential nutrients.
nature geoscience | VOL 6 | SEPTEMBER 2010 | www.nature.com/naturegeoscience
The symbiotic relationship seems
to be mutually beneficial. Submerged
Sphagnum is unable to access sufficient
carbon dioxide from the atmosphere
for photosynthesis owing to a lack of
stomata, and would therefore benefit from
a local, methanotrophic supply of carbon
dioxide. Concomitantly, methanotrophs
in submerged mosses would presumably
benefit from a plant-derived supply of
elevated oxygen concentrations, which
would normally be supplied by diffusion
from the atmosphere.
Further findings from Kip et al. suggest
that methanotrophic symbionts could help
to counteract increased methane emissions
from peat bogs in a warmer world. First,
rates of methane oxidation almost doubled
when the temperature of the incubations
1
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was raised from 10 °C to 20 °C. Second,
methane emissions increased almost
four-fold following removal of Sphagnum
mosses from intact soil cores, suggesting
that Sphagnum-associated methanotrophs —
rather than free-living methanotrophs — are
largely responsible for attenuating methane
release from these soils.
However, it remains unclear why
Sphagnum mosses that grow above the
water table exhibit lower rates of methane
oxidation than mosses growing beneath
the water table, as the methanotrophic
communities proved to be similar.
Methanotrophic activity may have been
diminished in hummock- and lawnderived mosses owing to a lack of methane,
or methanotrophs may start feeding on
alternative carbon sources, such as acetate.
Indeed, some methanotrophs isolated from
acidic peatlands can use acetate6–8, one of the
key intermediates in the breakdown of peat
mosses. If the symbiotic methanotrophs are
capable of feeding on alternative substrates,
the authors may have overestimated the
contribution of methane-derived carbon to
2
Sphagnum. Their estimation was based on
incubations carried out in closed systems;
in vivo, carbon dioxide may also be generated
during the respiration of alternative carbon
sources by methanotrophs.
Furthermore, the relative contribution of
symbiotic versus free-living methanotrophs
to methane oxidation in peatlands is
uncertain. Submerged Sphagnum pools
occupy a relatively small proportion of
the peatland landscape, although their
prevalence is likely to expand owing to
permafrost thawing in a warmer world.
At present it is difficult to assess the global
significance of Sphagnum symbionts in
attenuating peatland methane emissions. It
has been well documented that peatlands
covered with vascular plants, another
principal peatland landscape, release more
methane owing to the presence of vascular
tissue, which channels methane directly to
the atmosphere, bypassing the oxic zone of
the soil that methanotrophs inhabit 9,10.
Nevertheless, the study by Kip et al.2
highlights the ubiquitous nature of the
Sphagnum–methanotroph symbiosis in
submerged peatland mosses and could
help to explain why Sphagnum-dominated
peatlands generally emit less methane
than other peatland types10–11. Further
investigation is now needed to determine the
extent to which this symbiotic relationship
could help to reduce methane emissions
from peatlands in a warmer world.
❐
Yin Chen and J. Colin Murrell are in the
Department of Biological Sciences, University of
Warwick, Coventry CV4 7AL, UK.
e-mail: Cheny98@gmail.com
References
1.
2.
3.
4.
5.
Smith, L. C. et al. Science 303, 353–356 (2004).
Kip, N. et al. Nature Geosci. 3, xxx–xxx (2010).
Raghoebarsing, A. A. Nature 436, 1153–1156 (2005).
Chen, Y. et al. Environ. Microbiol. 10, 446–459 (2008).
Chen, Y. et al. Environ. Microbiol. 10, 2609–2622 (2008).
6. Dedysh, S. N. et al. J. Bacteriol. 187, 4665–4670 (2005).
7. Dunfield, P. F. et al. Int. J. Syst. Evol. Microbiol. doi:ijs.0.020149-0
(2010).
8. Belova, S. E. et al. Environ. Microbiol. Rep. doi:10.1111/j.17582229.2010.00180.x (2010).
9. Frenzel, P. et al. Biogeochemistry 51, 91–112 (2000).
10.Joabsson, A. et al. Trends Ecol. Evol. 14, 385–388 (1999).
11.Whiting, G. J. et al. Nature 364, 794–795 (1993).
nature geoscience | VOL 6 | SEPTEMBER 2010 | www.nature.com/naturegeoscience