38 / 78
FROZEN HEAT
38
Methane consumption by methane-eating microbes in sediments
and in the water column is an important global mechanism that
prevents methane from reaching the atmosphere. There are two
main processes by which methane is consumed: aerobic and
anaerobic methane oxidation. But do these processes generate
ecological issues of their own?
Aerobic oxidation of methane consumes both oxygen and methane
to produce carbon dioxide. Excessive oxygen consumption,
particularly in the deep ocean where it is not easily replenished,
can be detrimental to oxygen-breathing life forms. Carbon dioxide
dissolves in water to form carbonic acid, acidifying the water. In
theory, if methane vents rapidly into the water column, aerobic
oxidation of methane could cause significant local decreases in
oxygen levels and increased acidity (lower pH values, see Fig. TB2.1
for an example of potential acidification effects).
There are indications in the geologic record that massive methane
releases from gas hydrates might have driven ocean acidification
in the past (Zachos
et al.
2005; Pelejero
et al.
2010). Model
predictions for the future (Biastoch
et al.
2011) suggest that
methane consumption could lead to pH values dropping by up to
0.25 units within the next century in some deep areas of the Arctic
Ocean. In addition, microbial consumption of methane could
decrease local bottom-water oxygen concentrations by up to 25
per cent. Regional methane-induced sea-water acidification from
the sea floor would occur, in addition to ocean-wide acidification
caused by the uptake of anthropogenic carbon dioxide from the
atmosphere. The combined effect of the two processes would
accelerate acidification in parts of the Arctic Ocean, including in
deeper waters. Research has so far been based on the premise of
a projected pH decrease due to the anthropogenic carbon dioxide
uptake of about 0.3 units by the end of this century. Methane-
induced acidification could nearly double the pH decrease in parts
of the Arctic Ocean (Biastoch
et al.
2011).
The effects of anaerobic oxidation of methane (AOM) in sediments
are not as easily predicted. AOMconsumes no oxygen and produces
bicarbonate instead of carbon dioxide (Barnes and Goldberg 1976).
Box 2.1
Could microbial methane oxidation boost acidification and oxygen
depletion in the ocean?
However, sulphide, another end-product of AOM, might be re-
oxidized with oxygen by chemoautotrophic organisms (Jørgensen
and Nelson 2004) or simply through abiotic chemical reactions.
So although the microbial process itself does not directly consume
oxygen, consumption occurs during re-oxidation of sulphide at the
sediment-water interface.
Figure TB-2.1:
Potential effects of ocean acidification on marine
organisms. The planktonic coccolithophore Calcidiscus leptoporus
cultured under present-day carbon dioxide conditions (pCO
2
~380
µatm, left panel) and under conditions projected for the end of this
century, assuming business-as-usual carbon dioxide emissions
(pCO
2
~780 µatm, right panel). With increasing carbon-dioxide-
induced ocean acidification, the energetic costs of calcification go
up. While some organisms are able to compensate for this, others
find it increasingly difficult to produce their carbonate shells and
skeletons (courtesy Ulf Riebesell, GEOMAR, Kiel)).