DMSP synthesis and exudation in phytoplankton: a modeling approach

In the marine environment, phytoplankton are the fundamental producers of d imethylsulfoniopropionate (DMSP), the precursor of the climatically active gas dimethylsulfide (DMS). DMSP is released by exudation, cell autolysis, a nd zooplankton grazing during phytoplankton blooms. In this study, we devel oped a model of phytoplankton DMSP and DMS production allowing quantificati on of the exudation rates of these compounds during different growth phases . The model was tested on published data from axenic cultures of Prorocentr um minimum and Phaeocystis sp.; DMSP exudation rates vary considerably betw een the 2 species. Model results show that P. minimum exudes around 1% d(-1 ) of its DMSP quota during the latent, exponential and senescent phases. Th is is comparable to the average exudation rate estimated from previous labo ratory experiments. However, Phaeocystis sp. exudes from 3 to 11% d(-1) of its DMSP quota. For this species, DMSP exudation rates apparently show an i nverse relationship with the population growth rate. The maximum DMSP exuda tion rate in Phaeocystis sp. is 10 times higher than previously reported DM SP or DMS exudation rates. Our results suggest that exudation may be as imp ortant as cell autolysis in the release of DMSP during Phaeocystis sp. bloo ms. We conclude that exudation should be incorporated in models of DMS cycl ing in the marine environment. Moreover, our results for Phaeocystis sp. su ggest that a low and constant exudation rate, as sometimes used in models, is not suitable for all conditions.