Extracellular polymeric substances: quantification and use in erosion experiments

Extracellular polymeric substances (EPS) is a generic term often applied to high molecular weight polymers implicated in the biostabilisation of natural sediments. Quantitative analysis of in situ EPS production rates and sediment contents has usually involved extraction of EPS in saline media prior to precipitation in alcohol and quantification against a glucose standard (phenol-sulphuric acid assay). Extracted and synthetic EPS has also been used to create engineered sediments for erosion experiments. This study investigated two steps in the EPS extraction procedure, saline extraction and alcohol precipitation. Comparisons of the effects of different extracted polymers were made in sediment erosion experiments using engineered sediments. Sediment EPS content decreased as the salinity of the extractant increased, with highest values obtained for extraction in fresh water. Potential errors were observed in the quantification of the soluble colloidal polymer fraction when divided into EPS and lower molecular weight polymers (LMW) as used in many studies. In erosion studies, 15 mg kg(-1) of alcohol (IMS) extracted EPS polymer (in 5 g kg(-1) IMS precipitate, equivalent to approximately 5g salt kg(-1) sediment dry weight) decreased the erosion threshold of cohesive sediments whereas 30 mg kg(-1) (in 10 g kg(-1) IMS precipitate, approximately 10 g salt kg(-1) sediment dry weight) had no effect compared to controls. This could be due to the influence of EPS on water content: low levels of EPS did not bind but prevented desiccation, lowering sediment stability against controls. At higher EPS content, binding effects balanced water content effects. Salt alone (at 10 g kg(-1)) slightly increased the erosion threshold after a 6-h desiccation period. In comparison, carbohydrates produced without alcohol precipitation (rotary evaporation) increased the erosion threshold at both 0.5 and 1.0 g EPS kg(-1) dry weight of sediment. It was concluded that the role of microphytobenthic polymers in biostabilisation of sediments is best determined through the study of natural intact sediment samples. (C) 2004 Elsevier Ltd. All rights reserved.