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

Elements of the proposed seasonal synergy between microorganisms encased in sea ice were examined by a combination of field and laboratory approaches. Sea-ice brines were drained from sea ice in Kanajorsuit Bay (2013) and Kobbefjord (2014), Greenland during late winter/early spring. The natural communities present in these brines were tested for choline uptake, conversion and retention as compatible solute, and remineralization, using radiolabeled choline-based incubations at -1°C upon salinity shifts to double and to half the starting salinity. Measurements of in situ temperature, salinity, pH, bacterial and viral abundance, chlorophyll pigments, and extracellular polysaccharide substances were made to provide environmental and biological context. Pure cultures of available sea ice-adapted bacteria were also examined after exposure to salinity shifts to clarify the key processes. In all cases and across a range of starting salinities, when salinity was increased, 14C-solute (choline or derivatives) was preferentially retained as an intracellular osmolyte; when salinity was decreased, 14C-choline was preferentially respired to 14CO2. Additional experiments with cold-adapted bacteria in culture indicated that an abrupt downshift in salinity prompted rapid (subsecond) expulsion of retained 14C-solute, but that uptake of 14C-choline and solute retention resumed when salinity was returned to starting value. Overall, the results indicate that bacteria in sea-ice brines use compatible solutes for osmoprotection, transporting, storing and cycling these molecules as needed to withstand naturally occurring salinity shifts and persist through the seasons. Because choline and many commonly used compatible solutes contain nitrogen, we suggest that when brines freshen and bacteria respire such compatible solutes, the corresponding regeneration of ammonium may enhance specific biogeochemical processes in the ice, possibly algal productivity but particularly nitrification. Measurements of potential nitrification rates in parallel sea-ice samples are consistent with a link between use of the compatible solute strategy and nitrification. Detecting this link may have been possible because the sea ice sampled was not supporting significant algal productivity; the link to nitrification may be obscured or overwhelmed by algal uptake of ammonia in more biologically productive sea ice.