ELECTROCHEMICAL STUDY OF SULFUR SPECIES IN SEAWATER AND MARINE-PHYTOPLANKTON CULTURES

Using a direct voltammetric method in seawater samples from the northe rn Adriatic Sea we have observed a peak at -0.6 V which belongs to sul fur species. The levels of sulfur (expressed as equivalent to sulfide concentrations) ranged between 10 and 50 nM, depending on the season. Maximum concentrations (appr. 500 nM) were found in the surface water during an intensive phytoplankton bloom.Experiments with different phy toplankton cultures (Phaeodactylum tricornutum, Emiliana huxleyi, Tetr aselmis suecica, Isochrysis galbana, Prorocentrum micans, Thalassiosir a weissflogii and a mixed culture from the Adriatic Sea) incubated wit h dimethylsulfoniopropionate (DMSP) suggested that this sulfur peak is , in addition to other possible sources, closely connected with degrad ation processes of DMSP which is a direct or indirect precursor of sev eral marine sulfur species: dimethyl sulfide (DMS), methanethiol (MSH) , 3-methiolpropionate (MMPA), 3-mercaptopropionate (MPA), carbonyl sul fide (COS), dimethyl disulfide (DMDS), H2S, S-0. The observed voltamme tric peak at -0.6 V was stable on acidification and purging procedure, followed by a readjustment of the solution pH to 8-10. The characteri zation of the electroactive sulfur species responsible for this peak w as made by comparison with model substances. No inorganic or organic s ulfur species alone revealed completely the voltammetric behaviour of natural samples. We concluded that in seawater and in phytoplankton cu ltures sulfur is very probably bound and stabilized with organic matte r assuming that some of the bound sulfurs are electrochemically active . Our results do not rule out the possibility that some metal sulfides stabilize sulfide in oxic waters too.