Carbohydrate secretion by phototrophic communities in tidal sediments

Two different benthic phototrophic communities on tidal flats were investig ated for their carbohydrate content and distribution. Carbohydrates were an alysed as two operationally defined fractions, related to the difficulty of extraction from the sediment matrix. Water-soluble (colloidal) and EDTA-ex tractable (capsular) carbohydrates were measured in a cyanobacterial mat an d a diatom biofilm, The chlorophyll-specific carbohydrate content of the tw o communities was very different. The diatom biofilm contained Lip to 100 t imes more colloidal carbohydrate than the cyanobacterial mat. The concentra tions of colloidal carbohydrates in the diatom biofilm correlated with biom ass (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 m at correlated with chlorophyll-a. This was probably an artefact caused by t he fact that approximately 50% of the chlorophyll-a in the mat was attribut ed to diatoms. The characteristics of extracellular polysaccharides were in vestigated 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, carboxyl ated 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 incr ease in carbohydrate concentration was found for the cyanobacterial mat, Th e cyanobacteria formed a mat in which the filamentous organisms entangled s and grains and attached firmly to the substratum. The interparticle spaces were completely occluded by polymers, whereas in the diatom biofilm the org anic matrix was less well developed and void spaces could still be discerne d. It is conceived that the properties of extracellular polysaccharides inf luence the stability of the sediment bed. (C) 1999 Elsevier Science B.V. Al l rights reserved.