Trophic uptake and transfer of DMSP in simple planktonic food chains

Recent field studies suggest that a large portion of phytoplankton-DMSP could be lost to grazing by protozoans, but the fate of the grazed DMSP remains uncertain. In the laboratory we studied trophic uptake and transfer of phytoplankton-DMSP through simple planktonic food chains using 2 experimental approaches: (1) A direct approach measured the ingestion and retention of phytoplankton-DMSP by the heterotrophic dinoflagellate Gyrodinium dominans. Overall, DMSP content of G. dominans estimated by the direct approach was highly variable, likely because of the low G. dominans biomass relative to phytoplankton in the samples. (2) An indirect approach, in which the omnivorous copepod Acartia tonsa was allowed to prey on a mixture of phytoplankton and G, dominans. Using this indirect approach, A. tonsa retained a high concentration of G. dominans biomass in its guts. Combined with other feeding parameters, the copepod gut contents were used to derive the DMSP content of G. dominans. When fed on Phaeocystis globosa, G. dominans retained 1.64 x 10(-4) nmol DMSP cell(-1), or 44% of the grazed DMSP. When fed on Isochrysis galbana, the protozoan retained 6.87 x 10(-5) nmol DMSP cell(-1), or 32% of the grazed DMSP. A. tonsa selectively preyed on G. dominans when offered a mixture of G. dominans and phytoplankton, deriving 63 to 84% of their dietary DMSP from the protozoan. Our study suggests that protozoans are an important trophic linkage to transfer phytoplankton-DMSP up food chains, and that the effectiveness of this linkage is dependent on the species composition of both the phytoplankton and the higher trophic levels.