Dynamics and compositional changes in extracellular carbohydrates in estuarine sediments during degradation

Microphytobenthic biofilms in estuarine sediments, consisting of photosynthetic microalgae, heterotrophic bacteria and their associated exopolymers, contain high concentrations of water-soluble colloidal carbohydrates that can be separated using ethanol precipitation into low molecular weight (LMW) carbohydrate and colloidal extracellular polymeric substances (cEPS). These carbohydrate-rich components are potentially a significant carbon source, but utilisation processes are not well described. Short-term (24 h) degradation studies using sediment slurries with enhanced colloidal carbohydrate and cEPS concentrations found rapid increases in LMW carbohydrate concentrations during the first 4 h, with subsequent utilisation of LMW carbohydrate and cEPS after 8 to 24 h. The cEPS contribution to the colloidal carbohydrate pool increased (16 to between 67 and 97%) in control and colloidal carbohydrate-enriched slurries, with decreasing glucose and increasing mannose and xylose contents within the EPS fraction after 24 h, suggesting initial utilisation of glucose-rich components in preference to more complex EPS. When cEPS concentrations were increased (cEPS-enrichment), there was greater EPS utilisation, and a decline in the relative contribution of EPS (72 to 31% of the colloidal carbohydrate fraction), without significant change in the monosaccharide composition in the cEPS fraction. Using selective media with cEPS as a sole carbon source, a β-proteobacterium Variovorax sp. Alr1 was isolated. Variovorax sp. Alr1 grew rapidly on cEPS as a sole carbon source, with increasing β-glucosidase activity and the production of new EPS rich in rhamnose, galactose and fucose, but showed only limited growth on EPS extracted from sediment using a hot-bicarbonate extraction procedure, despite increases in β-glucosidase and aminopeptidase activity. Differences in the short-term lability of different fractions of sediment carbohydrate and the ability of a heterotroph to utilise these fractions suggests a range of different heterotrophic bacteria may be involved in the complete degradation of EPS in situ.