Production of glycine betaine and dimethylsulfoniopropionate in marine phytoplankton. II. N-limited chemostat cultures

The nitrogenous organic osmolyte glycine betaine (GBT) and its sulfur analo g dimethylsulfoniopropionate (DMSP) are quantitatively significant solutes in many marine algae. Although an inverse relationship has been suggested b etween these two compounds in marine phytoplankton that may be regulated by nitrogen availability, our results in Part I of this work (same issue) sho wed no such relationship in batch cultures of six species. In experiments c onducted in August 1994, we reexamined this relationship in three axenic st rains of phytoplankton [Thalassiosira pseudonana (Hustedt) Hasle et Heimdal , Emiliania huxleyi Hay et Mohler, and Amphidinium carterae Hulbert] in che mostat cultures at three N-limited growth rates. Levels of DMSP, GET, and h omarine, another nitrogenous osmolyte, were inversely related in T. pseudon ama, with GET and homarine preferentially produced at the highest growth ra te when cells were N-replete. DMSP concentrations did not change in E. huxl eyi, although GET and homarine levels did increase at the highest growth ra te. In A. carterae, neither DMSP nor GET varied in any systematic way. In a ll three algae, additions of nitrogen to N-limited cultures resulted in sho rt-term increases in GET production. Levels of cellular DMSP remained fairl y constant in E. huxleyi and A. car-tel ae and were much greater than level s of comparable nitrogenous osmolytes like GET and homarine. The dominance of DMSP makes a reciprocal relationship with the nitrogenous osmolytes unli kely in most species. Phytoplankton appear to be capable of directly assimi lating extracellular GET, although it is not known if they eliminate equiva lent amounts of DMSP in the process.