Extracellular polysaccharide substances (EPS) play critical roles in microbial ecology, including the colonization of extreme environments in the ocean, from sea ice to the deep sea. After first developing a sugar-free growth medium, we examined the relative effects of temperature, pressure, and salinity on EPS production (on a per cell basis) by the obligately marine and psychrophilic gamma-proteobacterium, Colwellia psychrerythraea strain 34H. Over growth-permissive temperatures of approximately 10 to -4 degrees C, EPS production did not change, but from -8 to -14 degrees C when samples froze, EPS production rose dramatically. Similarly, at growth-permissive hydrostatic pressures of 1-200 atm (1 atm = 101.325 kPa) (at -1 and 8 degrees C), EPS production was unchanged, but at higher pressures of 400 and 600 atm EPS production rose markedly. In salinity tests at 10-100 parts per million (and -1 and 5 degrees C), EPS production increased at the freshest salinity tested. Extreme environmental conditions thus appear to stimulate EPS production by this strain. Furthermore, strain 34H recovered best from deep-freezing to -80 degrees C (not found for Earthly environments) if first supplemented with a preparation of its own EPS, rather than other cryoprotectants like glycerol, suggesting EPS production as both a survival strategy and source of compounds with potentially novel properties for biotechnological and other applications.

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Cryopegs are sub-surface hypersaline brines at sub-zero temperatures within permafrost; their global extent and distribution are unknown. The permafrost barrier to surface and groundwater advection maintains these brines as semi-isolated systems over geological time. A cryopeg 7 m below ground near Barrow, Alaska, was sampled for geochemical and microbiological analysis. Sub-surface brines (in situtemperature of -6 °C, salinity of 115 ppt), and an associated sediment-infused ice wedge (melt salinity of 0.04 ppt) were sampled using sterile technique. Major ionic concentrations in the brine corresponded more closely to other (Siberian) cryopegs than to Standard seawater or the ice wedge. Ionic ratios and stable isotope analysis of water conformed to a marine or brackish origin with subsequent Rayleigh fractionation. The brine contained ∼1000× more bacteria than surrounding ice, relatively high viral numbers suggestive of infection and reproduction, and an unusually high ratio of particulate to dissolved extracellular polysaccharide substances. A viral metagenome indicated a high frequency of temperate viruses and limited viral diversity compared to surface environments, with closest similarity to low water activity environments. Interpretations of the results underscore the isolation of these underexplored microbial ecosystems from past and present oceans.

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 used 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. Working with the marine psychrophilic gammaproteobacterium, Colwellia psychrerythraea 34H, and with natural microbial assemblages present in sackhole brines drained from sea ice in Kanajorsuit Bay (2013) and Kobbefjord (2014), Greenland, we measured the utilization of 14 C-choline (precursor to glycine betaine, a common compatible solute) at −1°C upon salinity shifts to double and to half the starting salinity. In all cases and across a range of starting salinities, when salinity was increased, 14 C-solute (choline or derivatives) was preferentially retained as an intracellular osmolyte; when salinity was decreased, 14 C-choline was preferentially respired to 14 CO 2 . Additional experiments with cold-adapted bacteria in culture indicated that an abrupt downshift in salinity prompted rapid (subsecond) expulsion of retained 14 C-solute, but that uptake of 14 C-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.

Sea ice and its snow cover are critical for global processes including climate regulation and biogeochemical cycles. Despite an increase in studies focused on snow microorganisms, the ecology of snow inhabitants remains unclear. In this study, we investigated sources and selection of a snowpack-specific microbial community by comparing metagenomes from samples collected in a Greenlandic fjord within a vertical profile including atmosphere, snowpack with four distinct layers of snow, sea ice brine and seawater. Microbial communities in all snow layers derived from mixed sources, both marine and terrestrial, and were more similar to atmospheric communities than to sea ice or seawater communities. The surface snow metagenomes were characterized by the occurrence of genes involved in photochemical stress resistance, primary production and metabolism of diverse carbon sources. The basal saline snow layer that was in direct contact with the sea ice surface harbored a higher abundance of cells than the overlying snow layers, with a predominance of Alteromonadales and a higher relative abundance of marine representatives. However, the overall taxonomic structure of the saline layer was more similar to that of other snow layers and the atmosphere than to underlying sea ice and seawater. The expulsion of relatively nutrient-rich sea ice brine into basal snow might have stimulated the growth of copiotrophic psychro- and halotolerant snow members. Our study indicates that the size, composition and function of snowpack microbial communities over sea ice were influenced primarily by atmospheric deposition and inflow of sea ice brine and that they form a snow-specific assemblage reflecting the particular environmental conditions of the snowpack habitat.

Hypersaline aqueous environments at subzero temperatures are known to be inhabited by microorganisms, yet information on community structure in subzero brines is very limited. Near Utqiaġvik, Alaska, we sampled subzero brines (–6°C, 115–140 ppt) from cryopegs, i.e. unfrozen sediments within permafrost that contain relic (late Pleistocene) seawater brine, as well as nearby sea-ice brines to examine microbial community composition and diversity using 16S rRNA gene amplicon sequencing. We also quantified the communities microscopically and assessed environmental parameters as possible determinants of community structure. The cryopeg brines harbored surprisingly dense bacterial communities (up to 108 cells mL–1) and millimolar levels of dissolved and particulate organic matter, extracellular polysaccharides and ammonia. Community composition and diversity differed between the two brine environments by alpha- and beta-diversity indices, with cryopeg brine communities appearing less diverse and dominated by one strain of the genus Marinobacter, also detected in other cold, hypersaline environments, including sea ice. The higher density and trend toward lower diversity in the cryopeg communities suggest that long-term stability and other features of a subzero brine are more important selective forces than in situ temperature or salinity, even when the latter are extreme.