Biosynthesis of Dimethylsulfide and Dimethylpropiothetin by Hymenomonas carterae in Relation to Sulfur Source and Salinity Variations

The marine coccolithophorid phytoplankton species Hymenomonas carterae (class Prymnesiophyceae) produces both dimethylpropiothetin (DMPT) and dimethylsulfide (DMS) in axenic cultures. The rate of DMS production is closely regulated by the cell; it remains independent of environmental sulfate concentration down to levels of 2.5% of the seawater value. Below this sulfate level, DMS production decreases with decreasing sulfate concentration, but significant amounts of DMS are released even under conditions of sulfate-limited growth. Hymenomonas carterae can grow on sulfite, thiosulfate, and cysteine as sulfur sources, but not on methionine. The rate of DMS output is similar for the different sulfur sources. Both the intracellular concentration of DMPT and the rate of output of DMS by H. carterae increase with increasing salinity of the medium. This increase is observed when either salt or sucrose is used to control the osmolarity of the growth medium. Variations in DMPT levels and DMS output were observed within hours after transferring cells to a medium of different osmotic pressure. The intracellular DMPT concentration is of the order of 0.3 mole per liter and contributes significantly to the osmotic pressure in the cell. These results suggest that DMPT plays an important role in osmoregulation by H. carterae.