Carbohydrate secretion by phototrophic communities in tidal sediments

Abstract

Two different benthic phototrophic communities on tidal flats were investigated for their carbohydrate content and distribution. Carbohydrates were analysed as two operationally defined fractions, related to the difficulty of extraction from the sediment matrix. Water-soluble (colloidal) and EDTA-extractable (capsular) carbohydrates were measured in a cyanobacterial mat and a diatom biofilm. The chlorophyll-specific carbohydrate content of the two communities was very different. The diatom biofilm contained up to 100 times more colloidal carbohydrate than the cyanobacterial mat. The concentrations of colloidal carbohydrates in the diatom biofilm correlated with biomass (chlorophyll-a), but this was not the case with the carbohydrate in the EDTA extract. It is proposed that the capsular carbohydrates were probably recalcitrant to mineralisation and therefore accumulated in the sediment. Neither colloidal nor EDTA-extractable carbohydrate in the cyanobacterial mat correlated with chlorophyll-a. This was probably an artefact caused by the fact that approximately 50% of the chlorophyll-a in the mat was attributed to diatoms. The characteristics of extracellular polysaccharides were investigated in laboratory cultures of the dominant organisms. Extracellular polysaccharides of the cyanobacterium Microcoleus chthonoplastes and of the diatom Navicula menisculus did not contain uronic acids. However, carboxylated sugars were found in large quantities in the capsular polysaccharides of the cyanobacterium and were present in equal ratios in the extracellular and capsular carbohydrate of the diatom Cylindrotheca closterium. Both in laboratory model systems of diatom biofilms and in situ, enhanced colloidal carbohydrate production was observed in the light. No light-dependent increase in carbohydrate concentration was found for the cyanobacterial mat. The cyanobacteria formed a mat in which the filamentous organisms entangled sand grains and attached firmly to the substratum. The interparticle spaces were completely occluded by polymers, whereas in the diatom biofilm the organic matrix was less well developed and void spaces could still be discerned. It is conceived that the properties of extracellular polysaccharides influence the stability of the sediment bed.