During the 1980s, the importance of a constituent of DOM called dimethylsulphide (CH3)2S was realized. DMS is a gas found in solution near the sea surface.
The gas escapes from the ocean surface and it has been estimated that it accounts for as much as 25 per cent of the sulphur that enters the atmosphere. The exact process by which it is liberated remains unknown but it originates from dimethylsulphoniopropionate (DMSP) found in many marine algae, both seaweeds and phytoplankton, where it performs various roles in metabolism. DMSP released directly from plants into seawater is somehow rapidly converted by free-living bacteria into DMS. Digestion of plant material by animals such as copepods, also converts DMSP to DMS via bacteria living in the gut.
DMS released into the atmosphere is oxidized to sulphur dioxide, which may result in the acidification of rain. Airborne sulphate ions (SO4=) are also produced and these, often in combination with atmospheric ammonia, are a major consituent of the tiny particles known as cloud condensation nuclei. Water condenses on these particles and clouds are built up. Thus the metabolism of marine phytoplankton, by producing DMS in the ocean surface, is thought to be linked with cloud formation which in turn affects the radiative (heat) balance of the atmosphere. This simplified account of a complex series of interactions provides another example of intricate connections between the activities of marine organisms and world climate. This is discussed further in Section 10.2 in the context of global warming. Recent experiments also suggest that some seabirds such as petrels may be able to smell DMS given off by phytoplankton as this is eaten by the krill on which the birds feed.
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