The Effects of Salinity on Dimethylsulfoniopropionate Production in the Green Alga Ulva fenestrata Postels et Ruprecht (Chlorophyta)

Dimethylsulfoniopropionate (DMSP) in marine algae has been hypothesized to serve as a compatible solute which functions in osmotic acclimation or cryoprotection. However, many macroalgae that produce large quantities of DMSP live in habitats where they are unlikely to experience large fluctuations in salinities or freezing temperatures. We hypothesized that DMSP has other functions in these algae and that they should not show large changes in DMSP concentrations in response to salinity changes. We tested this hypothesis by placing 1.5 cm super(2) diameter disks of the chlorophyte Ulva fenestrata in artificial seawater (ASW) at salinities from 10% ASW to 300% ASW. Over the next 24 h, DMSP concentrations tended to be lower in the algae in the higher salinity media. After 4 weeks, the final DMSP concentrations tended to be highest in the algae grown at the highest salinities, but the mean DMSP concentrations were only 23% higher or 12% lower in algae grown in the high and low salinity media, respectively, relative to algae in 100% ASW. This suggests that osmotic acclimation is not a primary function of DMSP in U. fenestrata. Disks acclimated in 25% ASW, 100% ASW, or 200% ASW then transferred to a higher or lower salinity did not generate measurable amounts of dimethylsulfide (DMS), demonstrating that U. fenestrata is not using DMSP cleavage as a short-term mechanism for reducing internal DMSP stores, as occurs in some phytoplankton. Survival, as measured by the absence of bleaching, was highest in intermediate to high salinities. Growth was highest in low to intermediate salinities and reproduction only occurred in intermediate salinities. These results suggest that U. fenestrata can tolerate salinity changes, but uses metabolites other than DMSP for osmotic acclimation. Based on the results of this and previous studies, we propose that DMSP has other functions in U. fenestrata such as acting as an herbivore deterrent or antioxidant.