Influence of CO2, nitrate, phosphate, and silicate limitation on intracellular dimethylsulfoniopropionate in batch cultures of the coastal diatom Thalassiosira pseudonana

We measured intracellular dimethyl sulfoniopropionate (DMSP) concentrations in nutrient-limited batch cultures of the coastal diatom Thalassiosira pseudonana. Under N, P, CO2, and Si growth limitation, intracellular DMSP concentrations decreased to low values during the exponential phase of growth and increased under nutrient limitation. The intracellular DMSP concentration increased exponentially with decreasing growth rate and cellular chlorophyll a, in response to the type and degree of nutrient limitation. For a given growth rate, N-limited cells showed the greatest increase in cellular DMSP concentrations, CO2 and Si limitation had an intermediate effect, and P limitation caused the smallest increase. The results demonstrate the importance of nutrient limitation in enhancing DMSP concentrations in marine algae. This enhancement may be linked in part to the role of DMSP as a cellular antioxidant and to increased oxidative stress within cells during nutrient limitation. The replacement of DMSP, a sulfur-containing osmolyte, for N-containing osmolytes such as proline may also explain at least some of the DMSP increase under N limitation. Our results also point to the possible importance of diatom blooms in global or regional sulfur cycles. In 1987, the CLAW hypothesis (after its authors, Charlson, Lovelock, Andreae, and Warren) postulated that planktonic marine emissions of the volatile organic sulfur compound dimethylsulfide (DMS) are involved in the biological regulation of climate (Charlson et al. 1987). The oxidation products of marine atmospheric DMS, mainly sulfuric acid, form aerosols that act as cloud condensation nuclei (CNN), thereby influencing the planetary albedo and climate (Bates et al. 1987; Charlson et al. 1987). This hypothesis is supported by the results of studies by Ayers et al. (1991) and Ayers and Gillett (2000), which showed direct correlations among DMS, aerosol non–sea salt sulphate, and CNN in the remote marine atmosphere. Significant questions remain regarding the physiological roles of dimethyl sulfoniopropionate (DMSP) and DMS in marine algae and the environmental factors that regulate their production. DMSP is mainly produced by Dinophyceae (dinoflagellates) and Prymnesiophyceae (including the coccolithophorids) (Keller et al. 1989). Given its high intracellular concentration (up to 400 mmol L21), it serves in these species as an important osmolyte. It is cleaved into DMS and acrylate by DMSP lyase, an enzyme that is present in both marine phytoplankton (Stefels and van Boekel 1993) and bacteria (Kiene 1990). Through its cleavage to acrylate, a putative antigrazing compound, DMSP may act as a chemical defense against zooplankton (Wolfe et al. 1997). Also, a role in cry1 Current address: LEMAR, UMR CNRS 6539, Institut Universitaire Européen de la Mer, Place Nicolas Copernic, 29280 Plouzané, France ([email protected]).