SIZE DISTRIBUTION OF DIMETHYLSULFONIOPROPIONATE (DMSP) IN AREAS OF THE TROPICAL NORTHEASTERN ATLANTIC-OCEAN AND THE MEDITERRANEAN-SEA

Size fractionation of dimethylsulfoniopropionate (DMSP) was carried ou t in 2 areas of the Mediterranean Sea and 2 areas of the tropical nort heastern Atlantic Ocean. In the Mediterranean Sea, particles in the si ze range 10-200 mum collected 15 m deep accounted for 16.0 +/- 4.6% an d 39% of total particulate DMSP off Villefranche/Mer and off Banyuls/M er, respectively. In the Atlantic Ocean, the study of the vertical siz e distribution of particulate DMSP revealed that DMSP containing parti cles in the size range 10-200 mum tend to accumulate at the pycnocline and accounted for 26.0 +/- 7.2% and 36.3 +/- 11.6% of total particula te DMSP at sites 18-degrees-30'N, 21-degrees-W and 21-degrees-N, 31-de grees-W, respectively. Although particulate DMSP in mixed layer waters of the tropical northeastern Atlantic Ocean was carried mainly by par ticles in the size range 0.7-10 mum, no significant correlation was fo und between concentration of dissolved compounds (dissolved DMSP+DMS i n samples filtered through GF/F filters) and this DMSP fraction. Disso lved DMSP + DMS levels were significantly correlated (r2 = 0.33, n = 2 4, P = 0.002) only with levels of particulate DMSP in the size range 1 0-200 mum. Since particulate DMSP in the size range 10-200 mum appeare d not to be associated with microphytoplanktonic populations (diatoms and dinoflagellates), it is suggested that some heterotrophic organism s (microzooplankton) and/or detrital microscopic material (aggregates, fecal pellets) could play a key role in controlling the concentration s of dissolved DMSP and DMS in these waters. Depth profiles of DMSP le vels in the size range 0.7-10 mum revealed that DMSP covaried with dia dinoxanthin and zeaxanthin, two nonphotosynthetic carotenoids with pho toprotective properties. This result provides the first in-situ indica tion of light-dependent DMSP accumulation in nannophytoplankton (most likely prymnesiophytes) and prochlorophytes.