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

Abstract

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 (Grün.) 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. 14 C-labeling revealed immediate production of 14 C-glucan and significant increases in 14 C-EPS between 3 and 4 h after addition of label. The glucan synthesis inhibitor 2,6-dichlorobenzonitrile significantly reduced 14 C-colloidal carbohydrate and 14 C-EPS. The glucanase inhibitor P-nitrophenyl b-D-glucopyranoside resulted in accumulation of glucan within cells and lowered rates of 14 C-colloidal and 14 C-EPS production. Cycloheximide prevented glucan catabolism, but glucan production and EPS synthesis were unaffected.

The carbohydrates of Phaeocystis and their degradation in the microbial food web

“…The change in colony shape and appearance at the end of Phaeocystis blooms (e.g., Verity et al 1988;Rousseau et al 1994) is therefore not reflected in the aldose composition of the mucopolysaccharides. This is different from the situation reported in diatoms, where upon nutrient limitation, the benthic diatom Cylindrotheca closterium excretes a separate extracellular polysaccharide with a different monosaccharide composition compared to optimal growth conditions (De Brouwer et al 2002;Underwood et al 2004). Phaeocystis acts as a secretory cell; mucopolysaccharides are excreted in a way similar to animal cells and vascular plants in which the regulated export of cellular products takes place via exocytosis.…”

Release of dissolved organic matter by coastal diatoms

“…However, in many cases, bacteria acting on and influencing the fresh phytoplankton DOM pool creates uncertainties in the magnitude of C and N fluxes to that pool. In the case of the short-term 14 C tracer studies, Underwood et al (2004) found that there was an ,3-h time lag between addition of 14 C label to diatom cultures and the subsequent production of labeled extracellular polysaccharides, a potentially large component of phytoplankton-derived DOM. Thus, short (less than a few hours) 14 C studies may completely miss C dynamics associated with the extracellular polysaccharide pool, a point noted in the earlier studies of Mague et al (1980) and Smith (1982).…”

Dissolved organic matter (DOM) release by phytoplankton in the contemporary and future ocean

“…TEP plays an important role in the biogeochemistry of the biological carbon pump (Passow & Carlson, 2012) and consequently there is a need to better define its composition to determine its origin and biogeochemical significance. Much of the recent detailed work on the composition of EPS produced by diatoms has been conducted with laboratory-grown cultures of benthic epipelic diatoms (Underwood et al, 2004;Bellinger et al, 2005;Abdullahi et al, 2006). There is no reason why the techniques used in this research could not be applied to ecologically relevant taxa of phytoplankton.…”