MEPS 379:45-58 (2009)  -  DOI: https://doi.org/10.3354/meps07875

ABSTRACT: 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.

KEY WORDS: Biofilms · EPS · Microphytobenthos · Exopolymers · Degradation · Mudflats