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and altered precipitation, is less clear. This is a crucial factor,
as it determines the nature and extent of terrestrial ecosystem
feedback responses. However, understanding the responses of
microbial communities to climate change is complicated by the
vast and largely unexplored diversity of microbiota found in the
terrestrial environment, for which only a few examples of food
webs have been fully constructed (Morgan, 2002). Also, dif-
ferent terrestrial ecosystems comprise different microbial com-
munities, and this is further compounded by the effects of land
use, other disturbances (such as management practices) and dif-
ferent biogeographical patterns (distribution of microbial com-
munities over space and time).
18.2 Greenhouse gas emissions by microbial control
Understanding the physiology and dynamics of microbial com-
munities is essential if we are to increase our knowledge of
the control mechanisms involved in greenhouse gas fluxes
(Schimel and Gulledge, 1998; Allison et  al. 2010). This topic
has received little attention owing to the assumption that micro-
bial community structure has little relevance to large-scale eco-
system models (Schimel, 1995) and to the lack of theoretical
background and technologies to measure the vast diversity of
microbial communities in natural environments and to deter-
mine their link to ecosystem functioning. Nevertheless, recent
advances in molecular techniques and their application to the
characterisation of the so-called uncultivable microorganisms
have started to provide an improved understanding of microbial
control of greenhouse gas emissions.
Carbon dioxide
gas
In the global carbon cycle, annual emissions of CO 2 from the
burning of fossil fuels are dwarfed by the natural fluxes of CO 2 ,
to and from the land, oceans and atmosphere. Current levels of
atmospheric CO 2 depend largely on the balance between pho-
tosynthesis and respiration. In oceans, photosynthesis is pri-
marily carried out by phytoplankton, whereas autotrophic and
heterotrophic respiration return much of the carbon taken up
during photosynthesis to the dissolved inorganic carbon pool
(Del Giorgio and Duarte, 2002; Arrigo, 2005). For terrestrial
ecosystems, the uptake of CO 2 from the atmosphere by net
primary production is dominated by higher plants, but micro-
organisms contribute greatly to net carbon exchange through
the processes of decomposition and heterotrophic respiration,
as well as indirectly, through their role as plant symbionts or
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