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Red coralline algae produce significant quantities of dimethylsulphoniopropionate (DMSP), whose breakdown products include the important climate gas dimethylsulphide (DMS) but little is known about how environmental factors influence this DMS(P) production. The effect of photosynthetically active radiation (PAR) on intracellular DMS(P) concentrations in the red coralline algae Lithothamnion glaciale was investigated using short (30 min) and longer-term (up to 507 h) acclimatory responses and control and high-PAR light regimes. Longer-term acclimatory intracellular DMS(P) concentrations were significantly reduced following exposure to high-PAR (220–250 μmol m −2 s −1 ). No short-term acclimatory effects were observed. We conclude that while DMS(P) content in L. glaciale does respond to changes in irradiance, the effect takes place over hours – days rather than minutes, suggesting a continued turnover of DMS(P) to combat oxidative stress induced by prolonged high-PAR exposure. Immediate short-term acclimatory responses do not appear to occur.