ENVIRONMENTAL EFFECTS ON EXOPOLYMER PRODUCTION BY MARINE BENTHIC DIATOMS: DYNAMICS, CHANGES IN COMPOSITION, AND PATHWAYS OF PRODUCTION¹

<jats:p>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 <jats:italic>Cylindrotheca closterium</jats:italic> (Ehrenburg), <jats:italic>Navicula perminta</jats:italic> (Grün.) in Van Heurck, and <jats:italic>Amphora exigua</jats:italic> 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. <jats:italic>Cylindrotheca closterium</jats:italic> 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. <jats:sup>14</jats:sup>C‐labeling revealed immediate production of <jats:sup>14</jats:sup>C‐glucan and significant increases in <jats:sup>14</jats:sup>C‐EPS between 3 and 4 h after addition of label. The glucan synthesis inhibitor 2,6‐dichlorobenzonitrile significantly reduced <jats:sup>14</jats:sup>C‐colloidal carbohydrate and <jats:sup>14</jats:sup>C‐EPS. The glucanase inhibitor P‐nitrophenyl β‐<jats:sc>d</jats:sc>‐glucopyranoside resulted in accumulation of glucan within cells and lowered rates of <jats:sup>14</jats:sup>C‐colloidal and <jats:sup>14</jats:sup>C‐EPS production. Cycloheximide prevented glucan catabolism, but glucan production and EPS synthesis were unaffected.</jats:p>