Production and Characterization of the Intra‐ and Extracellular Carbohydrates and Polymeric Substances (EPS) of Three Sea‐Ice Diatom Species, and Evidence for a Cryoprotective Role for EPS

Abstract

Diatoms and their associated extracellular polymeric substances (EPS) are major constituents of the microalgal assemblages present within sea ice. Yields and chemical composition of soluble and cell-associated polysaccharides produced by three sea-ice diatoms, Synedropsis sp., Fragilariopsis curta, and F. cylindrus, were compared. Colloidal carbohydrates (CC) contained heteropolysaccharides rich in mannose, xylose, galactose, and glucose. Synedropsis sp. CC consisted mainly of carbohydrates <8 kDa size, with relatively soluble EPS, compared to high proportions of less-soluble EPS produced by both Fragilariopsis spp. F. curta colloidal EPS contained high concentrations of amino sugars (AS). Both Fragilariopsis species had high yields of hot bicarbonate (HB) soluble EPS, rich in xylose, mannose, galactose, and fucose (and AS in F. cylindrus). All species had frustule-associated EPS rich in glucose-mannose. Nutrient limitation resulted in declines in EPS yields and in glucose content of all EPS fractions. Significant similarities between EPS fractions from cultures and different components of natural EPS from Antarctic sea ice were found. Increased salinity (52) reduced growth, but increased yields of EPS in Fragilariopsis cylindrus. Ice formation was inhibited byF. cylindrus, EPS, and by enhanced EPS content (additional xanthan gum) down to -12°C, with growth rate reduced in the presence of xanthan. Differences in the production and composition of EPS between Synedropsis sp. and Fragilariopsis spp., and the association between EPS, freezing and cell survival, supports the hypothesis that EPS production is a strategy to assist polar ice diatoms to survive the cold and saline conditions present in sea ice.

Broad-scale predictability of carbohydrates and exopolymers in Antarctic and Arctic sea ice

“…Bacteria also contribute to the EPS pool in ice (15,20), although bacterial biomass and production is not necessarily closely coupled to algal autotrophic activity in sea ice (36). Ice diatoms and ice bacteria produce EPS with a substantial dUA content (14)(15)(16), but the relationship between dUA and dCHO was strongest in the algal-rich Resolute datasets.…”

“…Ice formed at the seawater interface and supporting active microbial assemblages becomes progressively cut off from underlying water as it is exposed increasingly to lower atmospheric temperatures. These changes decrease biological activity, decrease dCHO concentrations, and increase the relative importance of dEPS complex , bacterial cells, and particulate EPS (9,14,20). The carbohydrate and EPS present in interior and upper ice horizons will have been exposed to a longer period of biological (e.g., bacterial EPS derived from heterotrophic utilization of algal DOC) and physical transformation [salinity and temperature effects (23)].…”

“…However, there is increasing evidence that the processes of seawater freezing can be biologically mediated by ice-binding proteins and EPS secreted by bacteria and algae. These compounds can alter ice structure (14,15,(21)(22)(23)(24)(25)(26) and, in the case of EPS, also form physico-chemical buffers between the organisms and the surrounding brines and ice matrix (9,14,17).…”

“…However, there is increasing evidence that the processes of seawater freezing can be biologically mediated by ice-binding proteins and EPS secreted by bacteria and algae. These compounds can alter ice structure (14,15,(21)(22)(23)(24)(25)(26) and, in the case of EPS, also form physico-chemical buffers between the organisms and the surrounding brines and ice matrix (9,14,17).When sea ice melts, its dissolved and particulate constituents are released into the surface waters (27, 28), contributing to the microbial dynamics in both the melting ice and melt waters (19,(29)(30)(31). Physical aggregation of EPS in seawater to form larger particles may promote the sinking of particulate organic matter from the surface waters (19, 32), or produce EPS foams that are Significance Many marine microalgae and bacteria secrete polysaccharide gels (exopolymers) in response to environmental stresses, such as the freezing temperatures and salt concentrations that organisms experience when in sea ice.…”

“…1). dCHO are concentrated from sea ice DOC by dialysis (>8 kDa), with subsequent treatment allowing the definition of four subcomponents of the total dCHO pool: (i) dissolved uronic acids (dUA), produced by ice diatoms and ice bacteria (14-16), that confer strong cross-linkages between polymer chains (8), forming low solubility EPS complexes within brine channels (8,14,17); (ii) dEPS, produced by sea ice algae (9,12,18,19) and isolated from dCHO by 70% (vol/vol) alcohol precipitation; (iii) a low solubility fraction of dEPS obtained by 30% (vol/vol) alcohol precipitation, containing complex EPS molecules (dEPS complex ), often produced by algae with reduced biological activity or when under physiological stress (9, 13, 19); and (iv) a fraction of highly soluble carbohydrates that are not considered EPS (dCHO non-EPS ), do not precipitate in alcohol, and are produced by many actively growing ice algae (9, 14).The bacteria and algae that successfully colonize sea ice habitats have mechanisms that enable them to survive temperatures less than −20°C and salinities >100 in the sea ice brines (17,20). However, there is increasing evidence that the processes of seawater freezing can be biologically mediated by ice-binding proteins and EPS secreted by bacteria and algae.…”