Geoscience Reference
In-Depth Information
ammonia (NH
3
)-oxidising bacteria, belonging to the class beta-
proteobacteria (Teske, 1994). However, recent studies suggest
that some archaea also have an important role in nitrification
(Leininger, 2006), although their relative contribution to this
process is still debated.
By contrast, denitrification is a multistep process in which
each step is mediated by a specific group of microorganisms
that have the enzymes necessary to catalyse that particular
step. The production of N
2
O is typically the result of incom-
plete denitrification. Denitrifying activity is distributed among
phylogenetically diverse bacterial populations, although each
denitrifying enzyme catalysing a specific step in the process
(e.g. nitrate reductase) is highly conserved genetically (Ye
et al., 1994). A recent study provided direct evidence of a strong
link between denitrifying bacterial communities and the rate of
N
2
O emission from soils (Salles et al., 2009).
Methane gas
Global emissions of CH
4
are arguably even more directly con-
trolled by microorganisms than emissions of CO
2
. Natural
emissions (~250 million tonnes of CH
4
per year) are dominated
by microbial methanogenesis, a process that is carried out by
a group of anaerobic archaea in wetlands, oceans, rumens and
termite guts. However, these natural sources are exceeded by
emissions from human activities (mainly rice cultivation, land-
fill, fossil fuel extraction and livestock farming) (~320 million
tonnes of CH
4
per year), which aside from some emissions
from fossil fuel extraction are also predominately driven by
microorganisms Mclain and Ahmann (2008). Methanotrophic
bacteria serve as a crucial buffer to the huge amounts of CH
4
produced in some of these environments. The so-called low-
affinity methanotrophs (active only at a CH
4
concentration of
>40 parts per million; also called type I methanotrophs), which
mainly belong to the class Gammaproteobacteria, can often
consume a large proportion of the CH
4
produced in soils before
it escapes to the atmosphere. For CH
4
already in the atmo-
sphere, methanotrophic bacteria may also act as a net CH
4
sink.
The so-called 'high-affinity' methanotrophs (active at a CH
4
concentration of <12 parts per million), which mainly belong to
the class Alphaproteobacteria (also known as type II methano-
trophs), remove approximately 30 million tonnes of CH
4
from
the atmosphere each year (Reay and Grace, 2007).
Reducing CO
2
emissions by
microbial
communities
Currently, soils contain about 2000 Pg of organic carbon, which
is twice the amount of carbon in the atmosphere and thrice the
quantity found in vegetation (Smith, 2004). The capacity of
