A dominant feature of polar coastal ecosystems is the presence of seasonal sea ice. Depending on the season, sea ice covers 4 to 20 10⁶ km² of the Southern Ocean. Within sea ice, complex and highly heterogeneous ecosystems develop. Variations in ice type determine the composition of the microbial community.

Sea ice micro-algae start to grow in early spring, when incident light becomes sufficient. The extremely high accumulation of biomass (>1000 µg chlorophyll L^-1) can be a significant source of the climate-active gas dimethyl sulphide (DMS). There are some data in the literature describing extremely high amounts of dimethyl sulphoniopropionate (DMSP), the algal precursor of DMS, in sea ice. There is however, no data on the occurrence of DMS in ice, let alone on the mechanisms that control the production and conversion of DMS and DMSP.

There are several factors that make living in ice stressful: high salinity (up to 150 psu), light inhibition and UV-radiation in the top layers of the ice, at the same time light limitation in the bottom layers and a steep temperature gradient of about 15 to 20 °C within the ice column. In addition, sea-ice assemblages are densely packed in narrow brine channels. Here, a depletion of inorganic carbon almost to exhaustion and an increase in pH to 11, can be found as a consequence of high photosynthetic activity. Given the high biomasses of viable algae in the sea ice, some species have been able to adapt to this multiple stress environment, but little is known about their strategies. In this project it is hypothesised that the production and conversion of DMSP can play a crucial role. Specific foci of the project are:

• How heterogeneous is the spatial and temporal distribution of DMS(P) in Antarctic sea ice? What is the species dependency in the DMS(P) production?
• What controls the vertical distribution of DMS(P) in the ice cover and how does it relate to other biological and chemical indicators?
• What processes govern production and conversion of DMSP and related compounds in sea ice algae?