Structure and ecological roles of a novel exopolysaccharide from the arctic sea ice bacterium Pseudoalteromonas sp. Strain SM20310

Abstract

The structure and ecological roles of the exopolysaccharides (EPSs) from sea ice microorganisms are poorly studied. Here we show that strain SM20310, with an EPS production of 567 mg liter(-1), was screened from 110 Arctic sea ice isolates and identified as a Pseudoalteromonas strain. The EPS secreted by SM20310 was purified, and its structural characteristics were studied. The predominant repeating unit of this EPS is a highly complicated α-mannan with a molecular mass greater than 2 × 10(6) Da. The backbone of the EPS consists of 2-α-, 6-α-mannosyl residues, in which a considerable part of the 6-α-mannosyl residues are branched at the 2 position with either single t-mannosyl residues or two mannosyl residues. The structure of the described EPS is different from the structures of EPSs secreted by other marine bacteria. Analysis of the ecological roles of the identified EPS showed that the EPS could significantly enhance the high-salinity tolerance of SM20310 and improve the survival of SM20310 after freeze-thaw cycles. These results suggest that the EPS secreted by strain SM20310 enables the strain to adapt to the sea ice environment, which is characterized by low temperature, high salinity, and repeated freeze-thaw cycles. In addition to its functions in strain SM20310, this EPS also significantly improved the tolerance of Escherichia coli to freeze-thaw cycles, suggesting that it may have a universal impact on microorganism cryoprotection.

Fig 1

EPS production of 13 bacterial strains. EPS production was determined by quantifying the carbohydrate content as d-glucose equivalents using the phenol-sulfuric acid method. The values shown are means ± SDs from three experimental repeats.

Fig 2

Structure of the predominant repeating units of the EPS from strain SM20310.

Fig 3

Effect of the EPS on the high-salinity tolerance of strain SM20310 (A) and E. coli (B). (A1, B1) Incubation for 24 h; (A2, B2) incubation for 48 h; (A3, B3) incubation for 72 h. The values shown are means ± SDs from three experimental repeats. Black bars, absence of EPS; red bars, 0.5 mg ml⁻¹ EPS; blue bars, 1.0 mg ml⁻¹ EPS.

Fig 4

Cryoprotective effect of the EPS on strain SM20310 (A) and E. coli (B) after 3, 5, and 7 freeze-thaw cycles. An equal volume of seawater (A) or 0.9% (wt/vol) NaCl solution (B) in place of EPS was used as the control. The values shown are means ± SDs from three experimental repeats. Black bars, absence of EPS; red bars, 0.2 mg ml⁻¹ EPS; blue bars, 0.5 mg ml⁻¹ EPS; dark cyan bars, 2.5 mg ml⁻¹ EPS; magenta bars, 5.0 mg ml⁻¹ EPS; dark yellow bars, 10 mg ml⁻¹ EPS; navy blue bars, 20 mg ml⁻¹ EPS; wine-colored bars, 30 mg ml⁻¹ EPS.