The secretion of exopolysaccharide in an axenic culture of the marine benthic diatom Cylindrotheca closterium was investigated. The central question of the experiments was if polysaccharide secretion was dependent on light and photosynthesis. Cells were incubated in the Light, in the dark, or in the light with addition of the inhibitor of Photosystem II, 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU). These treatments were also applied to a population of benthic diatoms on an intertidal mudflat in the Westerschelde (Scheldt estuary, The Netherlands). In the light (60 mu mol photons m(-2) s(-1)) C, closterium showed high rates of polysaccharide secretion, while no secretion was observed in the dark or in the presence of DCMU. No intracellular carbohydrate was converted to exopolysaccharide in the dark or in the Light with DCMU added. This indicated that secretion of exopolysaccharide was dependent on oxygenic photosynthesis. Similarly, high rates of exopolysaccharide accumulation were observed during daytime emersion on the mudflat, but not in darkened or DCMU-treated sediment. This demonstrated that the pattern observed in cultures of C, closterium was reproducible in situ. It was observed that during daytime emersion patterns of vertical migration in the dark and DCMU-treated plots did not differ from those in the light. This implies that motility was not the steering factor for the observed accumulation of exopolysaccharide in the Light. When an axenic culture of C. closterium was incubated under an alternating 12 h Light:12 h dark cycle, exopolysaccharide concentrations decreased in the dark. Degradation of exopolysaccharide was also observed in the natural population on the mudflat during emersion at night. Because no bacteria were present in the C, closterium cultures, it was conceived that the degradation of exopolysaccharide observed in cultures was due to secretion of hydrolytic enzymes by C, closterium.
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