Response of dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO) cell quotas to salinity and temperature shifts in the sea-ice diatom Fragilariopsis cylindrus

Sea ice is an extreme environment known to host microbial communities which produce high concentrations of the metabolites dimethylsulfoniopropionate (DMSP) and dimethylsulfoxide (DMSO). These two compounds are involved in the cycling of the climate-cooling gas dimethylsulfide. Despite decades of research, the drivers of these large concentrations in sea ice remain largely unknown. In this study, we use a cell culture approach to quantify for the first time the DMSP and DMSO cell quotas for the diatom Fragilariopsis cylindrus under combined shifts of temperature and salinity typically encountered in the sea-ice brine habitat. In doing so, we investigate two suggested potential metabolic functions for DMSP and DMSO: osmoregulation and cryoprotection. We observed an increase of both DMSP:Chl-a and DMSO:Chl-a in multiple experiments with salinities of 75 and 100 at constant temperature, which suggest osmoregulation for both compounds in the diatom cell. Stronger salinity shifts to 150 induced lethal osmotic shock resulting in massive cell death. Interestingly, combining salinity shifts with temperature shifts (as low as − 7.4 °C) did not modify drastically the DMSP and DMSO cell quotas, which may indicate that the cryoprotectant function of DMSP and DMSO in our diatom cultures was not-relevant. Also, decreasing the salinity to 20 at constant temperature suggested no cellular adaptation in terms of DMSP and DMSO cell quotas.