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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
43
methane seeps) imply that such sea-floor ecosystems have
been occurring for millions of years (Goedert and Benham
2003; Peckmann and Thiel 2004).
2.4.1
Overview of chemosynthetic
communities at methane seeps
Chemosynthetic communities found at methane seeps in-
clude both microorganisms and animals that depend on
The term chemosynthesis refers to metabolisms that obtain
energy to create biomass by using chemical energy through
reduction-oxidation reactions. If the carbon source used to
form the biomass is inorganic (carbon dioxide), the organisms
are called chemoautotrophs. Chemoautotrophs are different
from photoautotrophs, organisms that use light energy
through photosynthesis to obtain energy for growth from
carbon dioxide. Chemoautotrophs are bacteria, exclusively.
Animals that live symbiotically with chemoautotrophs are called
chemosymbiotic organisms. Animals (including humans) that
feed on algae, plants, or other animals are called heterotrophs.
Famous examples of chemosymbiotic animals are the giant
tubeworms, clams, and mussels thriving on chemical energy
in the permanent darkness of hydrothermal vent systems
and methane seeps (Corliss
et al.
1979; Felbeck 1981; Levin
2005). These ecosystems are often described as functioning
completely independent of sunlight, especially when located in
the deep sea. At first glance, this statement appears correct,
because primary producers at the basis of these ecosystems
gain energy and carbon from inorganic compounds, even in
the absence of light. However, the chemicals needed to oxidize
energy-rich molecules such as hydrogen sulphide, are oxygen
and nitrate. Both were rare in ancient oceans. Only through
photosynthesis did oxygen accumulate and react with reduced
nitrogen compounds to form nitrate.
Hence, modern complex chemosynthetic ecosystems, especially
those with higher organisms, are not completely independent
of sunlight. They depend on compounds produced directly or
indirectly through photosynthesis. This dependence is even
Box 2.3
What is chemosynthesis?
stronger at seeps, where methane, and sometimes petroleum,
form the basis for chemosynthetic food chains. Both are often
fossil-transformation products of photosynthetically produced
organic matter.
Figure TB-2.3:
Near the sea floor above active methane seeps,
gas hydrate can form mounds such as that pictured above in
the Gulf of Mexico. The gas hydrate mound is tinted orange
by small amounts of oil, and is partially covered by a thin
sediment drape (grey material). (Courtesy of Ian MacDonald).
hydrogen sulphide, which is a by-product of AOM (Paull
et
al.
1984; Sibuet and Olu 1998; Levin 2005), as well as ani-
mals that directly consume methane (Childress
et al.
1986;
Schmaljohann and Flugel 1987). Free-living chemosynthetic
bacteria include sulphur bacteria such as
Beggiatoa
,
Thiop-
loca
, or
Thiomargarita
(Jørgensen and Nelson 2004), as well
as aerobic methane-oxidizing bacteria. Filamentous sulphur
bacteria can sometimes form extensive white or orange mats
on the sediment surface (Fig. 2.4).