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Box 2.1 Could microbial methane oxidation boost acidification and oxygen depletion in the ocean?

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).

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)).

FROZEN HEAT 38