Responses of coastal phytoplankton populations to nitrogen additions: dynamics of cell-associated dimethylsulfoniopropionate (DMSP), glycine betaine (GBT), and homarine

Abstract

Dimethylsulfoniopropionate (DMSP) and glycine betaine (GBT) are organic osmolytes abundant in many marine phytoplankton. Herein, we field tested for the first time the hypothesis that GBT production might be favored over DMSP in natural phytoplankton populations growing in high-N environments or when N is added to a system. Concentrations of particulate DMSP (DMSP_p; 15–45 nmol·L^–1) were equal to, or greater than, concentrations of particulate GBT (GBT_p; 0–15 nmol·L^–1) in the upper water column. Homarine, another N-containing osmolyte, was detected at lower levels in all samples. These are the first reported values of GBT_p, and homarine in seawater. During N enrichment experiments, no consistent pattern of response in the DMSP pool resulted. Under N stress, nitrate addition either caused DMSP to be released but without an equivalent increase in GBT or DMSP dynamics were not affected but GBT increased. In populations under less N stress, GBT levels were similar to those of DMSP_pthroughout the experiments. Homarine levels remained low at all times. We conclude that no simple switch between DMSP and GBT occurs as a function of N availability in natural populations. Variable responses to N supply probably resulted from differences in species composition and physiological state of the populations present.