Distribution and composition of dissolved extracellular polymeric substances (EPS) in Antarctic sea ice

Concentrations and chemical composition of carbohydrates in dissolved organic carbon (DOC) and in extracellular polymeric substances (EPS) from brines, ice cores, gap layers in sea ice and associated surface waters were determined during 2004 and 2006 in the Weddell Sea, Antarctica. High levels of spatial heterogeneity were a common feature in these habitats, with DOC concentrations ranging from 45 to 669 μmol C kg–1. The highest concentrations of DOC and carbohydrate were measured in bulk sea-ice brines. Concentrations of dissolved carbohydrate (>8 kDa in size) varied between 31 and 255 μmol C kg–1 (glucose-carbon equivalent), and consisted of ca. 40% of the DOC in melted ice cores, and 10 to 20% of the DOC in ice brines. Dissolved carbohydrate and DOC concentrations were significantly correlated to chl a. Carbohydrate present as EPS (determined by selective alcohol precipitation) made up >68% of dissolved carbohydrates in all sea ice habitats. There were significant differences in concentration and relative importance of different EPS size fractions, and in the uronic acid content and monosaccharide composition (especially for glucose contribution), of different sea ice habitats, in relation to chl a concentrations, and between the 2 sampling cruises. High algal biomass was associated with greater relative abundance of glucose-rich EPS. Fractionation of EPS on the basis of solubility found that the least soluble EPS fraction contained substantial amounts of uronic acids and higher proportions of mannose, xylose, fucose and rhamnose than the more soluble EPS fractions. This relatively insoluble EPS fraction had characteristics that could contribute to a hydrophobic and structured microenvironment surrounding cells. EPS thus modify the local environment in sea ice brine channels, and form a chemically diverse source of DOC to the water column upon ice melt.