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

Abstract

The nitrogenous organic osmolyte glycine betaine (GBT) and its sulfur analog dimethylsulfoniopropionate (DMSP) are quantitatively significant solutes in many marine algae. Although an inverse relationship has been suggested between these two compounds in marine phytoplankton that may be regulated by nitrogen availability, our results in Part I of this work (same issue) showed no such relationship in batch cultures of six species. In experiments conducted in August 1994, we reexamined this relationship in three axenic strains of phytoplankton [Thalassiosira pseudonana (Hustedt) Hasle et Heimdal, Emiliania huxleyi Hay et Mohler, and Amphidinium carterae Hulbert] in chemostat cultures at three N-limited growth rates. Levels of DMSP, GBT, and homarine, another nitrogenous osmolyte, were inversely related in T. pseudonana, with GBT and homarine preferentially produced at the highest growth rate when cells were N-replete. DMSP concentrations did not change in E. huxleyi, although GBT and homarine levels did increase at the highest growth rate. In A. carterae, neither DMSP nor GBT varied in any systematic way. In all three algae, additions of nitrogen to N-limited cultures resulted in short-term increases in GBT production. Levels of cellular DMSP remained fairly constant in E. huxleyi and A. carterae and were much greater than levels of comparable nitrogenous osmolytes like GBT and homarine. The dominance of DMSP makes a reciprocal relationship with the nitrogenous osmolytes unlikely in most species. Phytoplankton appear to be capable of directly assimilating extracellular GBT, although it is not known if they eliminate equivalent amounts of DMSP in the process.