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 ³⁵S‐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 ³⁵S‐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 ³⁵S‐DMSP assimilation for osmotrophic ³⁵S‐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.