Origin and Importance of Picoplanktonic DMSP

The importance of picoplanktonic particulate DMSP (DMSPp GF/F-2 µm) was investigated using size fractionation during a time-series experiment in the Mediterranean Sea and in three areas of the subtropical Atlantic Ocean. Picoplanktonic DMSPp accounted for up to 25% of depth-integrated total DMSPp (GF/F-200 µm) in oligotrophic waters. In order to estimate the relative contribution of picophytoplankton to DMSPp in the GF/F-2 µm fraction, we measured the DMSPp content of representative strains of the three main groups of picophytoplankton, including Prochlorococcus (prochlorophytes), Synechococcus (cyanobacteria) and picoeukaryotes belonging to four classes (Prasinophyceae, Pelagophyceae, Chlorophyceae and Prymnesiophyceae). The average cellular DMSPp concentrations (mean ± 1SD) were 2.8.10-4 ± 1.8.10-4, 5.1.10-3 ± 7.8.10-3 and 46.5 ± 73.7 fg/cell for Prochlorococcus sp., Synechococcus sp. and the picoeukaryotes, respectively. The DMSP content was highly variable among taxonomic groups of picoeukaryotes even when normalized per unit biovolume. The prymnesiophytes produced the most DMSP (e.g. clone CCMP625, 196 mmol.liter cell volume-1) and the chlorophyte the least (0.74 mmol.liter cell volume-1). Using these average DMSP contents per cell and flow cytometric counts of natural populations of picophytoplankton, it is strongly suggested that the picoeukaryotes were the main DMSP producers in the GF/F-2 µm size class. The combined contribution of the two prokaryotes was negligible (less than 1%), even in oligotrophic waters.