260
16.2.4.4 Methane Emissions to the Atmosphere
The CH 4 concentrations at the water surface being higher
than the concentrations in the atmosphere, CH 4 is emitted
from the lake. These emissions, estimated at ~ 22 mmol.m−2.
yr−1 (Guimbaud et al. 2011 ), are several orders of magnitude
below the average emissions of temperate lakes (see Sect.
16.2.3.1).
16.3 Biogenic Methane Production
“The details of microbial physiology can have large impacts
on global biogeochemical cycles and the planet’s climate
system” (Schimel 2004 ).
16.3.1 The Anaerobic Foodweb
Anaerobic microorganisms perform a variety of fermenta-
tion processes, and also have the ability to couple the oxida-
tion of organic substrates with the reduction of diverse
inorganic compounds other than O 2 (e.g., nitrate, manganese,
iron, sulfate, CO 2 ). However, their bioenergetic capacities
compel them to establish a tissue of interactions including
cooperation, syntrophy and competition.16.3.1.1 Carbon and Electron Fluxes
Between Metabolic Types
In anoxic environments, the degradation of organic matter is
performed in several steps (Fig. 16.2) involving different
metabolic groups of anaerobic Bacteria and Archaea.(1) The polymers (polysaccharides, proteins, nucleic acids,
lipids) are converted to oligo- and monomers (sugars,
amino acids, fatty acids, glycerol, purines, pyrimidines)
through extracellular hydrolytic enzymes produced by
the hydrolytic bacteria.
(2) The fermentative bacteria ferment these monomers in
volatile fatty acids with short carbon chains (acetate,
propionate, butyrate, valerate, isovalerate, etc..), alco-
hols, aromatic compounds, etc. The simplest metabolites
(acetate, H 2 , CO 2 , or other one-carbon compounds) areFig. 16.2 Sequential pattern
of microbial degradation of
complex organic matter in
anoxic environments.
Macromolecules are
hydrolysed by hydrolytic
bacteria. The monomers are
fermented by fermentative
bacteria into a range of
fermentation products. The
acetate is produced directly
by fermentation or indirectly
by acetogenesis. Fermentation
products are consumed by
methanogens or others
microorganisms performing
terminal electron accepting
processes such as sulfate
reduction, ferric-iron
reduction, denitrification
A.-C. Lehours et al.