Electrochemical study of sulfur species in seawater and marine phytoplankton cultures

Abstract

Abstract Using a direct voltammetric method in seawater samples from the northern Adriatic Sea we have observed a peak at −0.6 V which belongs to sulfur 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 phytoplankton cultures ( Phaeodactylum tricornutum, Emiliana huxleyi, Tetraselmis suecica, Isochrysis galbana, Prorocentrum micans, Thalassiosira weissflogii and a mixed culture from the Adriatic Sea) incubated with dimethylsulfoniopropionate (DMSP) suggested that this sulfur peak is, in addition to other possible sources, closely connected with degradation processes of DMSP which is a direct or indirect precursor of several marine sulfur species: dimethyl sulfide (DMS), methanethiol (MSH), 3-methiolpropionate (MMPA), 3-mercaptopropionate (MPA), carbonyl sulfide (COS), dimethyl disulfide (DMDS), H 2 S, S 0 . The observed voltammetric peak at −0.6 V was stable on acidification and purging procedure, followed by a readjustment of the solution pH to 8–10. The characterization of the electroactive sulfur species responsible for this peak was made by comparison with model substances. No inorganic or organic sulfur species alone revealed completely the voltammetric behaviour of natural samples. We concluded that in seawater and in phytoplankton cultures sulfur is very probably bound and stabilized with organic matter 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.