Identifying metabolic pathways for production of extracellular polymeric substances by the diatom Fragilariopsis cylindrus inhabiting sea ice

Shazia N Aslam,Thomas Mock,David N Thomas,Graham J C Underwood,Jan Strauss

doi:10.1038/s41396-017-0039-z

Abstract

Diatoms are significant primary producers in sea ice, an ephemeral habitat with steep vertical gradients of temperature and salinity characterizing the ice matrix environment. To cope with the variable and challenging conditions, sea ice diatoms produce polysaccharide-rich extracellular polymeric substances (EPS) that play important roles in adhesion, cell protection, ligand binding and as organic carbon sources. Significant differences in EPS concentrations and chemical composition corresponding to temperature and salinity gradients were present in sea ice from the Weddell Sea and Eastern Antarctic regions of the Southern Ocean. To reconstruct the first metabolic pathway for EPS production in diatoms, we exposed Fragilariopsis cylindrus, a key bi-polar diatom species, to simulated sea ice formation. Transcriptome profiling under varying conditions of EPS production identified a significant number of genes and divergent alleles. Their complex differential expression patterns under simulated sea ice formation was aligned with physiological and biochemical properties of the cells, and with field measurements of sea ice EPS characteristics. Thus, the molecular complexity of the EPS pathway suggests metabolic plasticity in F. cylindrus is required to cope with the challenging conditions of the highly variable and extreme sea ice habitat.

Highlights

The production of polysaccharide-rich extracellular polymeric substances (EPS) by microorganisms is ubiquitous in many environments: water, soils, benign and pathogenic biofilms [1, 2], where EPS play important roles in cell adhesion, cell signalling, ligand binding and as a carbonShazia N
High concentrations of dissolved organic carbon (DOC) and CHOTOTAL were present in samples from the seawater–ice interface, while CHOTOTAL concentrations decreased in colder brine samples
EPS production by polar diatoms is a significant mechanism for survival for these important primary producers [18, 36, 69]

Summary

Introduction

The production of polysaccharide-rich extracellular polymeric substances (EPS) by microorganisms is ubiquitous in many environments: water, soils, benign and pathogenic biofilms [1, 2], where EPS play important roles in cell adhesion, cell signalling, ligand binding and as a carbonShazia N. A biome covering up to 15% of the world’s ocean area and which supports productive microbial communities within the semi-solid ice-water matrix [11,12,13], 40% of the dissolved organic matter (DOM) present is EPS produced by diatoms [14,15,16]. EPS and DOM modify the physical structure of the ice-water matrix [17, 18], provide a rich carbohydrate source [19], and on ice melt, contribute to the stimulation of water-column carbon cycling [13, 20, 21], influencing vertical carbon fluxes to deeper polar waters [22, 23] and the production of atmospherically active polar aerosols [24]

Methods

Results

Conclusion