| Summary: | In exploring the temperature and salinity tolerances of bacteria in sea ice, we hypothesized a seasonal synergy between bacteria and algae, based on bacterial uptake and metabolism of compatible solutes. In this model, algae release choline into the brine pockets of new sea ice, which can then be converted to compatible solutes by bacteria as a means to resist the osmotic stress of inhabiting brines that will become colder and more saline (> 210 ppt) in winter. In spring, as the ice warms and brine salinities (and osmotic stress) decrease, bacteria will metabolize these compatible solutes as nitrogen-rich carbon sources, potentially recycling ammonia to the algae. For initial testing of this hypothesis, samples of sea ice, brines and seawater were collected from Nuuk, Greenland, in spring 2013. Sample treatments included incubation at 1°C and at salinities higher than, similar to or lower than original conditions after amendment with 14C-choline. The proportions of respired choline (implying metabolism) versus retained choline (implying conversion to compatible solute for osmotic protection) were compared between treatments. Samples exposed to lower salinities respired a greater fraction of the total 14C-choline transported into cells (51103% converted to 14CO2) compared to samples exposed to higher salinities (328% respired), validating key aspects of the proposed model. Ongoing work involves similar experiments with model organisms and plans to study more natural samples obtained from coastal Greenland at different times in the lifecycle of sea ice.
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