The sulphur-containing osmolyte,dimethylsulphoniopropionate (DMSP),synthesisedby a variety of phytoplankton,has been attributed many roles.It is an effective compatible solute,it may act as a cryoprotectant,a methyl donor in transmethylation reactions,a precursor of thebiosynthesis of membrane phospholipdis,a by-product of the metabolic regulation of sulphurcontaining amino acids and a grazing deterrent.Hence a complex network of biological andphysiological processes dictate theproduction of DMSP,its transformation to DMS and the removal of DMS fromsurface waters.Recently,DMSP production,its lysis to DMS and oxidation toDMSO have been proposed to be mechanisms to relieve photo-oxidative stress due to UV radiation in phytoplankton (Sunda et al.2002),by scavenging potentially harmful reactive oxygen species (ROS).Since then,several studies have proposed links between DMSP levels in the naturalenvironment to a physiological response to relieve oxidative stress (Harada et al.,2004; Sunda et al.,2005).However,other than the in vitro ROS scavenging effect of DMSP,DMS and DMSO(Sunda et al.,2002),little experimental evidence has been presented to support this function.Our study was aimed at assessing the role of DMSP and the structurally analogous quaternaryammonium compound glycine betaine (GBT) vs other photoprotectant mechanisms (e.g.,non photochecmical quenching),in preventing,mitigating or repairing photosynthesis photoinhibition in response to acute exposures to high light.We performed series of culture experiments usingEmiliania huxleyi and Symbiodinium microadriaticum.The cultures were adapted to growth irradiance levels in sophisticated chemostat vessels and exposed to different light stressintensities.Fluorescence arising from photosystem II was used to assess photosynthetic electron transfer rates,inhibition and recovery of PSII function.Discrete samples for quantification of the intracellular concentrations of DMSP/DMSO,GBT and xanthophyll pigments were taken before and after thehighlight stress period and after a recovery period under growth irradiance.Studying the roles of DMSP,GBT and their breakdown products in relation to photo-oxidative stress may help toexplain their relative concentrations in phytoplankton assemblages,and the consequence this has for their production in surface waters.