Carbon - Freshwater Ecology: Concepts and Environmental Applications - page 239

The methanotrophs are generally found living in close proximity to anoxic
habitats from which there is a constant diffusive flux of methane.

Methanogenesis
Methane can be produced by anoxic Ar-
chaea in a process called methanogenesis in
extremely low redox conditions. CO 2 actu-
ally has more potential energy than the re-
duced methane at low redox (Figs. 11.6 and
12.4). Some of the reactions that produce
methane are presented in Table 12.1.
Microbe-mediated reactions similar to
methanogenesis include acetogenesis (mak-
ing acetate). Samples of these reactions are
also presented in Table 12.1. The rates of
methane and CO 2 formation occasionally
can be so great that gas bubbles containing
methane are released directly from anoxic
sediments. Biogas production as an alterna-
tive energy source is driven by methanogen-
esis. Methane is a greenhouse gas, and cur-
rent increases of methane concentration in
the earth's atmosphere related to human ac-
tivities are evident. Anoxic habitat is com-
mon in wetlands, and freshwater wetlands
contribute significantly to the global
methane budget (Sidebar 12.3).

Sidebar 12.2.
Acid Bogs in Archeology,
Paleolimnology, and Palynology

Bogs lead to preservation of organic materials
for thousands of years. The most spectacular
finds of archeological interest are preserved
human bodies. Several circumstances charac-
teristic of carbon cycling in bogs are required
for preservation of bodies: (i) The water must
be deep enough to prevent carrion-eating ani-
mals from consuming the body when it is first
deposited; (ii) the water must remain anoxic to
inhibit microbial growth and oxidation (pre-
served materials will decay within days when
reexposed to O 2 ); (iii) sufficient concentrations
of tannic acids must be present to cause tan-
ning of the skin (in nonacidic waters, only the
bones are preserved); and (iv) water must be
cold, generally below 4C, to inhibit microbial
growth. Above this temperature, the flesh rots
and the acids attack and degrade the bones
(Coles and Coles, 1989). Bodies 2000 years old
have been found so well preserved that the
color of the hair and eyes could be determined,
as well as the last meal eaten. Many human
artifacts preserved in wetlands have been un-
covered as well.
Scientists also use the preservation of or-
ganic materials in anoxic bogs to indicate
changes in plant communities over time. The
sediments can be dated, and preserved pollen,
pigments, or other plant parts can be used to
indicate the local plant community. Such infor-
mation may be useful in detecting environ-
mental trends over centuries or longer time
periods (Taylor and Taylor, 1993). For example,
isotopic composition of sedges and mosses
preserved in peat bogs can be used to esti-
mate atmospheric CO 2 content with a temporal
resolution of about a decade (White et al.,
1994). These data are useful to climate model-
ers who are interested in rapid climate change
events that have occurred during the past
12,000 years.

A GENERAL INTRODUCTION TO
NUTRIENT CYCLING AND THE
CARBON CYCLE

In this section, I diagram a complete nu-
trient cycle, rather than considering individ-
ual fluxes. Such cycling is a key feature of
all compounds used by organisms. All the
fluxes that occur in an environment make
up the cycle . An account of the relative mag-
nitude of the fluxes is called a budget . If any
chemical form in a cycle is not recycled, it
builds up in the environment and eventually
becomes unavailable to all organisms. Evo-
lution has led to organisms that can metab-
olize most naturally occurring organic car-
bon compounds as well as novel compounds
synthesized by humans. Such adaptations
can occur in days or weeks in the case of
novel organic compounds.