Environmental effects on exopolymer production by marine benthic diatoms: Dynamics, changes in composition, and pathways of production

Marine benthic diatoms excrete large quantities of extracellular polymeric substances (EPS), both as a function of their motility system and as a response to environmental conditions. Diatom EPS consists predominantly of carbohydrate-rich polymers and is important in the ecology of cells living on marine sediments. Production rates, production pathways, and monosaccharide composition of water-soluble (colloidal) carbohydrates, EPS, and intracellular storage carbohydrate (glucans) were investigated in the epipelic (mud-inhabiting) diatoms Cylindrotheca closterium (Ehrenburg), Navicula perminta (Grun.) in Van Heurck, and Amphora exigua Greg. under a range of experimental conditions simulating aspects of the natural environment. Cellular rates of colloidal carbohydrate, EPS, and glucan production were significantly higher during nutrient-replete compared with nutrient-limited growth for all three species. The proportion of EPS in the extracellular carbohydrate pool increased significantly (to 44%-69%) as cells became nutrient limited. Cylindrotheca closterium produced two types of EPS differing in sugar composition and production patterns. Nutrient-replete cells produced a complex EPS containing rhamnose, fucose, xylose, mannose, galactose, glucose, and uronic acids. Nutrient-limited cells produced an additional EPS containing mannose, galactose, glucose, and uronic acids. Both EPS types were produced under illuminated and darkened conditions. C-14-labeling revealed immediate production of C-14-glucan and significant increases in C-14-EPS between 3 and 4 h after addition of label. The glucan synthesis inhibitor 2,6-dichlorobenzonitrile significantly reduced C-14-colloidal carbohydrate and C-14-EPS. The glucanase inhibitor P-nitrophenyl beta-D-glucopyranoside resulted in accumulation of glucan within cells and lowered rates of C-14-colloidal and C-14-EPS production. Cycloheximide prevented glucan catabolism, but glucan production and EPS synthesis were unaffected.