No abstract

SUMMARYThe principal volatile sulphur species found in seawater are dimethyl sulphide (DMS), carbonyl sulphide (COS) and carbon disulphide (CS # ). Of these, DMS is the most abundant and widespread in its distribution. The predominant oceanic source of DMS is dimethylsulphoniopropionate (DMSP), a compatible solute synthesized by phytoplankton for osmoregulation and\or cryoprotection. Not all species have the same ability to form DMSP ; for example, diatoms generally produce little, whereas prymnesiophytes and some dinoflagellates make significantly larger amounts. Much of the release of DMSP and DMS to the water occurs on death or through predation of the plankton. Our recent field data strongly suggest that oxidation of DMS to dimethyl sulphoxide (DMSO) is an important process in the water column, and it is clear that considerable internal cycling in the DMSP\DMS\DMSO system occurs in the euphotic zone. A fraction of the DMS crosses the sea surface and enters the atmosphere where it is oxidized by radicals such as OH and NO $ to form products such as methanesulphonate (MSA), DMSO and non-sea salt sulphate (NSSS) particles. These particles are the main source of cloud condensation nuclei (CCN) over oceanic areas remote from land.

Dirnethyl sulphoxide (DMSO) represents a major pool of dissolved dimethylated sulphur in seawater However, the origin and fate of this compound in the marine environment, and its role in the biogeochermcal cycle of dimethyl sulphide (DMS), remain unclear. The only established route for the formation of DMSO in oxygenated seawater is photochemical oxidation of DMS. It is not known whether significant biotic production pathways exist. In a study of methylated sulphur speciation in coastal North Sea waters and cultures of marinc unicellular algae, we measured pools of particulate DMSO (DMSO,) at nanomolar and micromolar concentrations, respectively. Analyses of size-fractionated seawater particulates and incubation experiments provded evidence that DiviSO, wds dssociaiea with mlcroplanktonic organisms. Log-phase cultures of Amphidinium carterae and Emilianja huxleyi exhibited intracellular dimethylsulphoniopropionate (DMSP) to DMSO molar ratios of 25 and 8, respectively. Our results strongly suggest the existence of biological production and release of DMSO in eukaryotic microplankton.

Dimethylsulphoniopropionate (DMSP), an algal compatible solute, has for many years been considered to play a key role in dimethylsulphide (DMS) production, influencing the concentrations of DMS in sea water available to be transferred to the atmosphere. However, in recent years it has been shown that dimethylsulphoxide (DMSO) can also be produced directly within the cells of marine phytoplankton. The exact role DMSO plays in cells is still subject to debate, but it is thought that it may act as an antioxidant or cryoprotectant. Whatever the reason, it has been suggested that release through algal mortality and permeative loss of DMSO from cells may contribute to dissolved DMSO concentrations and as such this pathway must also be considered an important component of DMS biogeochemistry. Experiments were conducted to investigate the intracellular concentrations of DMSO and the ratio of DMSP:DMSO in a range of phytoplankton species and in natural samples. Results indicate that prymnesiophytes and dinoflagellates are the main producers, generating relatively higher concentrations of particulate DMSO than diatoms. Results from both laboratory and field experiments show that there is a strong relationship between DMSOp and DMSPp, with DMSO generally representing between 10 and 20 % of the intracellular sulphur pool. Field data also indicates that dissolved DMSO concentrations in surface waters were not significantly correlated with those for particulate DMSO, but were significantly correlated with DMS concentrations.