Dimethyl sulfide production: what is the
contribution of the coccolithophores?
Gill MALIN and Michael STEINKE
School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ. U.K.
G.Malin@uea.ac.uk, M.Steinke@uea.ac.uk
Summary
It is hard to find a research paper or book on coccolithophores that does not include
a few sentences on the role of this fascinating and enigmatic marine phytoplankton
group in the production of dimethyl sulfide «CH3)zS; DMS). Our aim
here is to provide some general background information on DMS for nonspecialists,
but also to highlight current knowledge and what we believe to be significant
gaps, for those with a specific interest in coccolithophores, other haptophytes
and DMS.
Introduction
Lovelock et al. (1972) first discovered that DMS was ubiquitous in the sea and
suggested that sea-to-air emission of this compound was a key pathway in the
global sulfur cycle. Marine DMS emissions are thought to account for about 23%
of the sulfur entering the global atmosphere but, because anthropogenic sulfur
tends to deposit rapidly and close to source, the biogenic flux contributes 42% of
the atmospheric column burden (Chin and Jacob 1996). Hence DMS inputs to the
atmosphere are of greatest significance in areas that are distant from anthropogenic
sulfur sources (Bates et al. 1987; Savoie and Prospero 1989; Twomey 1991).
Once in the air DMS oxidizes quite rapidly mainly via hydroxyl radicals during
the day, nitrate radicals at night (the latter being more significant in polluted areas),
and reactions with halogen oxides may also be significant (Plane 1989; Ravishankara
et al. 1997; Ballesteros et al. 2002). DMS oxidation leads to the formation
of a number of sulfur compounds including sulfur dioxide, sulfate,
methanesulfonic acid (CH3S03H; MSA), dimethylsulfoxide «CH3)2S0; DMSO)
and dimethylsulfone «CH3)2S02; DMS02).
In the 1980's and early 1990's research largely focused on the contribution of
natural DMS emissions to the global sulfur budget, and the wet and dry deposition
H. R. Thierstein et al. (eds.), Coccolithophores
© Springer-Verlag Berlin Heidelberg 2004
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of the acidic atmospheric oxidation products of DMS (natural 'acid rain'). However,
there was a substantial shift in emphasis after Charlson et al. (1987) picked
up on the earlier suggestion of Shaw (1983) and put forward the hypothesis that
sulfate aerosols derived from marine DMS emissions influenced cloud albedo and
global climate. A simple depiction is shown in Fig. 1. Sulfur aerosol influences
climate in two ways: directly since aerosol particles scatter incoming radiation reflecting
some of it back into space, and indirectly because the aerosols
Fig. 1. Research on DMS has been stimulated by the "plankton-climate connection" hypothesis
of Charlson et al. (J 987). DMS derived from phytoplankton is emitted to the air
and is the primary source of the sulfur particles that form in the atmosphere over the remote
oceans. These acidic particles stimulate cloud formation and increase the Earth's reflectivity
(albedo). In turn these processes affect climate and may also influence upper ocean
ecology.