Concentrations of the secondary algal metabolite dimethylsulphoniopropionate (DMSP) and its breakdown product, the climate-active trace gas dimethylsulphide (DMS), are sensitive to changes in pCO2. Data on the response of marine macroalgae to future pCO2 levels are lacking. Here we report the first measurements of DMSP and DMS production in two species of chlorophyte macroalgae (Ulva lactuca and U. clathrata). Laboratory cultures were grown in pH–stated medium that received pulses of CO2 to create pCO2 conditions ranging from ambient (432 μatm) to future (1514 μatm). Intracellular concentration of DMSP remained unaffected in both species (101 ± 21 and 69 ± 20 mmol g−1 FW in U. lactuca and U. clathrata, respectively) but significant differences in extracellular production of DMSP and DMS were observed for U. lactuca: Whilst production of total DMSP (the sum of external DMSP and DMS) was different between replicated experiments, the percentage of total DMSP produced throughout each experiment increased significantly by up to 65% with increasing pCO2 to 1514 μatm. In contrast, DMS production decreased from 0.4 to 0.25 nmol g−1 FW h−1. This decrease was not a linear function of pCO2 but an almost 50% step-wise loss of DMS production was indicated between 635 and 884 μatm, a pCO2 predicted for the next 100 years. Since Ulva spp. form massive harmful blooms (‘green tides’) that can occur free-floating and faraway (>100 km) from the coast, they may provide a locally significant source of DMS to the remote marine boundary layer.
