Natural marine microbial communities sourced from under fast ice at an Antarctic coastal site were incubated in tanks under differently attenuated natural sunlight for 2 wk in late spring (Expt 1) and early summer (Expt 2). In the 18 d period between the 2 sampling episodes, the ice edge retreated from 10 to within 1.5 km of the sampling site, and the fast ice began to break up. Expt 1 rapidly produced significant quantities of total DMSP (DMSPt) with concentrations increasing from 16.6 nmol l-1 to 192.7-204.5 nmol l-1 in 2 d. We believe this is the largest observed increase in DMSPt in a semi-natural community over this time frame. Abundances of Phaeocystis antarctica increased significantly during this initial period, while other phytoplankton species/groups remained stable. DMSPt concentrations then declined at rates averaging 39.2-50.0 nmol l-1 d-1 between Days 2 and 4. No major DMSPt production event occurred during Expt 2 despite strong community similarities. Sea ice breakout exposes phytoplankton to significant light-related oxidative stress, and these results suggest the rapid production of DMSPt during Expt 1 was due to the initiation of anti-oxidant mechanisms by a low-light-acclimated community in response to solar radiation stress. DMS concentrations remained comparatively low throughout Expt 1, suggesting oxidation of DMSP to products other than DMS. Rapid sea ice breakout in coastal regions of Antarctica may result in similar fast DMSP production events during spring. © Inter-Research 2013.