In the past 200 years,the oceans have absorbed half of the CO2 emitted by human activities such as fossil fuel burning.This uptake has lead to a reduction of the pH of the surface water.The effects of ocean acidification on marine ecosystems are mostly unknown.To understand how marine DMS fluxes will respond to ocean acidification we have conducted measurements of DMS,DMSP and the enzyme DMSP-lyase during a mesocosm experiment (PeECE III) in spring 2005.A natural phytoplankton bloom was studied for 24 days under present,double and triple pCO2.Our results indicate very similar DMSP and DMSP-lyase patterns for all treatments.In contrast,DMS concentration showed different temporal development in the low versus the high pCO2 treatments,presumably a reaction of the ecosystem to simulated ocean acidification.The treatment with present CO2 had increased DMS concentrations for the first 10 days but,integrated over the whole study period,concentrations were similar to those of the double and triple pCO2 treatments.However,the DMS production in the present CO2 scenario occurred during a shorter period with a more pronounced peak.Thus,under bloom conditions where the maximal gradients in DMS between the sea-air interface are crucial for the amount of DMS transferred to the atmosphere,the observed pattern differences could lead to altered sea-air fluxes in a future pCO2 world.