Selective retention in saline ice of extracellular polysaccharides produced by the cold-adapted marine bacterium Colwellia psychrerythraea strain 34H Marcela EWERT, Jody W. DEMING School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195-7940, USA E-mail: mewerts@u.washington.edu ABSTRACT. The retention of salts in laboratory-grown ice was compared to the retention of extracellular polysaccharide substances (EPS) produced by the cold-adapted marine gammaproteo- bacterium, Colwellia psychrerythraea strain 34H. Saline ice was formed, by means of a cold-finger apparatus, from artificial sea-water solutions containing either native dissolved EPS from strain 34H, the same EPS but heat-treated, or dissolved EPS from the uninoculated growth medium. Results indicated that only the native (unheated) EPS of strain 34H was retained preferentially in the ice. Temperature and volumetric measurements of the ice further suggested a link between the heat-labile fraction of this EPS of marine bacterial origin and potential habitat alteration. Bacterial EPS may join algal EPS in our understanding of how extracellular polymers help to establish and sustain the microbial community that inhabits sea ice. INTRODUCTION Extracellular polysaccharide substances (EPS) are complex polymers commonly produced by microbes (Decho, 2000) and composed mainly of neutral sugars with variable fractions of uronic acids, sulfates, amino sugars and proteins (Nichols and others, 2005). Some of these polymers are tightly bound to the microbial cell, whereas others are loosely attached, forming a more dispersed ‘slime’ (Decho, 1990). Among many roles, EPS act as ligands for metal cations, participate in the formation of marine aggregates and contribute to biogeochemical cycles (Passow, 2002; Nichols and others, 2005). Regarding sea ice, EPS produced by algae and bacteria have been considered to provide cryoprotection within the ice matrix (Krembs and others, 2002; Collins and others, 2008; Marx and others, 2009), as well as a mechanism for organisms, particularly EPS-coated algae, to entrain selectively into the ice (Meiners and others, 2003; Riedel and others, 2007) and remain anchored within it (Krembs and Deming, 2008). Here we consider, via controlled laboratory tests, the entrainment of bacterial EPS into growing sea ice. During the freezing process, sea ice retains a fraction of the microbial organisms, particles, salts and other solutes present in the source water; they are retained within liquid inclusions in the ice, brine-filled pores and channels (Petrich and Eicken, 2010) which constitute the habitable portion of the ice (Junge and others, 2001). A larger fraction of solutes and particles, though, is expelled back into the water column. The proportion by which solutes are retained in sea ice can be described by ‘effective segregation coefficients’, the ratio between the concentration of solutes in the ice and the concentration of solutes in the source liquid from which the ice forms (Eicken, 2003). Effective segregation coeffi- cients, k eff , were initially defined for salts in sea water: k effs ¼ S ice S source , where S ice is the salinity of the melted ice (bulk salinity) and S source is the salinity of sea water away from the ice–water interface. Extending this definition to other solutes allows, for example, calculation of the segregation coefficient of dissolved exopolymers: k effe ¼ ½EPS ice ½EPS source , where [EPS] ice is the concentration of exopolymers in the melted ice and [EPS] source is the concentration of exopoly- mers in the source solution. Solutes in sea water will segregate proportionally to dissolved salts unless they interact with the ice crystals, in which case the segregation of the interacting solute will diverge from the segregation of the salts. To assess this divergence, segregation coefficients of different solutes can be normalized to generate an enrichment index (Gradinger and Ika ¨valko, 1998), defined as: I s ¼ k effx k effs , where k effx is the segregation coefficient of the solute x. If I s = 1, the solute and the salts are expelled in the same proportion. If I s > 1, the solute is selectively retained (enriched) in the ice. In ocean environments that experience seasonal freezing, enrichment indices have been calculated from measurements of the concentrations of particles, cells and EPS in the ice and in underlying sea water (Garrison and others, 1989; Meiners and others, 2003; Riedel and others, 2007). The enrichment mechanism for organic ‘impurities’ (including microbes) present in sea water has usually been ascribed to a non-selective physical process, whereby suspended ice crystals, known as frazil ice, rise through the water column to consolidate an ice layer at the surface and collect particulate matter indiscriminately as they ascend (Garrison and others, 1983, 1989). Evidence for a selective mechanism in the enrichment of particles and microorganisms, affected by cell size and possibly EPS coatings, comes from enrichment indices calculated for a variety of organisms in new Arctic sea ice (Gradinger and Annals of Glaciology 52(57) 2011 111