Sulfur assimilation by Oxyrrhis marina feeding on a 35S-DMSP-labelled prey

A laboratory grazing experiment was conducted with the aim of quantifying the sulfur assimilation by a herbivore protist feeding on a dimethylsulfoniopropionate (DMSP)-containing phytoplankter. When supplied with dissolved (35)S-DMSP, cultures of an axenic strain of the diatom Thalassiosira pseudonana took up 60-95% of the added radioisotope and accumulated it untransformed in the cytoplasm. Radiolabelled diatom cells were offered as prey to the heterotrophic dinoflagellate Oxyrrhis marina. After 32 h in the dark, all the prey had been grazed and digested, leaving only radiolabelled O. marina in the grazing bottles and thus providing an estimate of the percentage of DMSP-sulfur retained by the predator. Subsequent precipitation with cold trichloroacetic acid (TCA) provided the fraction of retained DMSP-S that had been assimilated into the micrograzer macromolecules. In parallel incubations with predator and dissolved (35)S-DMSP only (no prey), O. marina (and their closely associated bacteria) took up the radiolabelled substrate osmotrophically to an activity of 0.04 dpm cell(-1) and assimilated it all into macromolecules. By correcting grazing (35)S-DMSP assimilation for osmotrophic (35)S-DMSP assimilation, and comparing it with the ingested radioisotope, the percentage of ingested DMSP-sulfur retained and assimilated by the predator was determined to be 32 +/- 4%. This is the first study that provides direct evidence that ingestion of a DMSP-containing prey supplies structural sulfur to a herbivore protist and that quantifies this assimilative supply at one-third of ingested DMSP.