Bacterial use of choline to tolerate salinity shifts in sea ice brines, Kanajorsuit Bay and Kobbefjord (Greenland)

The purpose of this study was to examine the potential microbial use of compatible solutes to tolerate the salinity shifts that occur naturally in the sea ice environment. Microorganisms (and particularly bacteria) within the brine network of sea ice experience temperature-driven fluctuations in salinity on both short and long temporal scales, yet their means of osmoprotection against such fluctuations is poorly understood. One mechanism to withstand the ion fluxes caused by salinity shifts, well-known in mesophilic bacteria, is the import and export of low molecular weight organic solutes that are compatible with intracellular functions. Studies of sea-ice bacteria and algae, including genomic evaluations, have revealed potential links between compounds called compatible solutes (CS), commonly used by microbes to protect against osmotic shock (including potential death by cell explosion) in high-salinity surroundings, and the availability of regenerated nitrogen within the ice. This research was designed to test a seasonal-synergy hypothesis whereby CS precursor compounds (specifically N-containing choline) that are released by ice algae and other microorganisms are taken up by bacteria in the ice, converted to CS for survival in the high-salinity brines of winter sea ice, and then metabolized for carbon, nitrogen and energy as spring temperatures warm, salinities freshen, and the need to retain CS as osmoprotectants diminishes. An expected end product of this bacterial conversion of CS is ammonia, potentially increasing the within-ice availability of nitrogen to ice algae on the verge of blooming as photosynthetically active radiation penetrates the ice, or to other microorganisms in need of ammonia (e.g., nitrifiers).