Marine phytoplankton produce a variety of organic osmolytes including the nitrogenous compound glycine betaine (GBT) and its sulfur analog, dimethylsulfoniopropionate (DMSP). It appears that GBT and DMSP make large and hitherto unrecognized contributions to the cellular nitrogen and sulfur pools in many species of marine phytoplankton and to particulate organic nitrogen (PON) and particulate organic sulfur (POS) pools in surface sea water. Very little is known about the production, distribution and cycling of these labile biogenic compounds. Information from the literature and from preliminary studies suggests that many species of phytoplankton produce both GBT and DMSP simultaneously and that nitrogen availability may affect the relative proportions of GBT and DMSP. This project will conduct laboratory and field studies to test the following hypotheses: 1) GBT makes up a significant (5-20%) fraction of cellular nitrogen in certain species of marine phytoplankton and DMSP is a major constituent of cellular sulfur; 2) There is a relationship between taxonomy and GBT and DMSP production; 3) The levels of GBT and GMSP in phytoplankton are a function of nitrogen availability, with DMSP favored under N-limitation; 4) Phytoplankton take up GBT and DMSP from their surroundings and release them under certain conditions; 5) Production of GBT over DMSP, or selection of species which produce GBT, will be favored in high nitrogen environments. This study will provide new and important data on the quantitative role of methylated nitrogen and sulfur osmolytes to the cellular nitrogen and sulfur budgets of phytoplankton and to PON and POS in the sea. This information will lead to a better understanding of the biological cycling of nitrogen and sulfur in the ocean.
