[Show abstract][Hide abstract] ABSTRACT: The chlorophyll a content and two operational fractions of carbohydrate (water extractable and EDTA extractable) were measured every three months during one year along transects on a tidal flat in the Ems-Dollard estuary (The Netherlands). Chlorophyll a was used as an indicator of microphytobenthos biomass, which was composed predominantly of epipelic diatoms. Both carbohydrate fractions correlated significantly with chlorophyll a. EDTA extractable carbohydrates were more resistant towards degradation than the water extractable fraction. During most of the year, concentrations of chlorophyll a and carbohydrates were low, but in June, high concentrations of up to 90 g chlorophyll a/g sediment were found in a narrow zone running parallel to the channel. Maximum concentrations of water extractable carbohydrates and EDTA extractable carbohydrates ranged between 800–1200 and 600–800 g/g sediment, respectively. The mud content was high ( 90%) at the margin of the tidal flat. This was not limited to the growth season of the diatoms, but was observed throughout the year. This indicated that the high mud content at the mudflat margin was mainly caused by hydrodynamic factors, and not by biostabilization. In June, exceptionally high diatom densities were found in sediment with a high mud content. There was only minor evidence that biostabilization by epipelic diatoms lead to a further increase in the mud content of the sediment.
[Show abstract][Hide abstract] ABSTRACT: Three years of combined physical and biological measurements in the Dollard tidal basin showed that in the early spring of 1996, suspended particulate matter (SPM) concentrations were low compared to observations in spring 1995 and 1997. Since in 1996 the duration of ice formation on the tidal flats in winter was much longer than in 1995 or 1997 (until late February), it is proposed that ice cover reduced resuspension of bed material by enhancing consolidation of the sediment and thus increasing the critical shear stress for erosion (τcrit), and also by preventing the occurrence of high waves. Both phenomena led to low suspended particulate matter (SPM) concentrations. After the ice cover vanished, the clear water phase was followed by a microalgal bloom both in the water column and on the sediment. During the algal bloom, SPM concentrations remained low. Floc size measurements indicated that low SPM concentrations during the benthic and pelagic algal blooms were not caused by increased flocculation. On the sediment bed however benthic microalgae produced large amounts of extracellular carbohydrates, which led to an increase in bed strength (τcrit) and a reduction of resuspension, and thus kept SPM concentrations low during the algal bloom.
[Show abstract][Hide abstract] ABSTRACT: During the stationary phase of a batch culture of the epipelic diatom Cylindrotheca closterium, accumulation of exopolysaccharides and intracellular carbohydrates was observed. When nitrogen was added to the culture in the stationary phase, growth was resumed and the accumulation of exopolysaccharides was delayed. This indicated that nitrogen depletion caused cessation of growth, and stimulated exopolysaccharide accumulation. Exopolysaccharide accumulation was also stimulated when cells were either resuspended in medium lacking N or P, or when they were inoculated in medium with low concentrations of N or P. Growth was not immediately affected by low N or P concentrations. S depletion only resulted in exopolysaccharide accumulation when growth was affected. Si or Fe depletion did not stimulate exopolysaccharide accumulation, even when growth rates were lowered. Apparently, stimulation of exopolysaccharide accumulation is dependent on the type of nutrient depletion. Intracellular storage carbohydrates did not accumulate when cells were incubated at low N or P concentrations. Cells grown with ammonium as nitrogen source produced more carbohydrates (both extracellular and intracellular) than cells grown with nitrate as nitrogen source, indicating that both exopolysaccharides and intracellular carbohydrates accumulated as a result of overflow metabolism.
Journal of Experimental Marine Biology and Ecology 07/2000; 249(1):13-27. · 2.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: 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. [KEYWORDS: Cylindrotheca closterium; diatoms; exopolysaccharide; extracellular polymeric substances; intertidal sediments; microphytobenthos; photosynthesis; polysaccharide Intertidal cohesive sediments; epipelic diatoms; chlorophyll-a; erodibility; adhesion]
Marine Ecology-progress Series - MAR ECOL-PROGR SER. 01/2000; 193:261-269.
[Show abstract][Hide abstract] ABSTRACT: The production and composition of extracellular polymeric substances (EPS) in axenic batch cultures of the benthic marine epipelic diatoms Navicula salinarum and Cylindrotheca closterium were investigated. EPS was secreted into the medium and the bulk was loosely associated with the cells. Neither N. salinarum nor C. closterium formed a well-defined polysaccharide capsule. EPS of both N. salinarum and C. closterium consisted predominantly of polysaccharide but small quantities of protein were present as well. EPS also contained uronic acids and SO4 groups. Analysis of monosaccharides using gas chromatography showed that for both species glucose and xylose were the main constituents, but several other monosaccharides were present in smaller quantities. Two fractions of EPS were distinguished: a small amount was secreted into the medium and a second fraction was extracted in water at 30 °C. For both species the two fractions differed somewhat in composition, indicating that they represented two different types of EPS. The EPS produced by N. salinarum and by C. closterium differed in their composition. The rate of EPS production in batch culture was highest during the transition from exponential growth to stationary growth. Negatively charged groups such as uronic acids and sulphated sugars determine the adhesion capacity of EPS and probably play an important role in the stabilization of intertidal sediments on which these diatoms grow and produce biofilms.
European Journal of Phycology 01/1999; 34(2):161-169. · 1.91 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The interacting effects of cadmium toxicity and food limitation on the midge, Chironomus riparius, were studied during chronic exposure in laboratory experiments. If the food was supplied ad libitum, both larval developmental time and mortality of the larvae were negatively affected by cadmium concentrations of 2.0-16.2 micrograms/L. The number of eggs deposited per female and the mean life span of the imagines were not affected by cadmium. Integration of these separate effects into a population growth rate showed a clear reduction with increasing cadmium concentrations. Food limitation of unexposed larvae at high population density reduced fitness, judged on all parameters studied and consequently reduced the population growth rate (up to 85%). The effects on larvae, which were exposed to both cadmium and food limitation, differed considerably from the response to the individual stress factors. Exposure to cadmium increased mortality among food-limited first and second instar larvae. Consequently, the amount of food available for each surviving larva increased. At the two lowest concentrations studied (2.0 and 5.6 micrograms Cd/L), these indirect positive effects of cadmium overruled the direct negative effects and caused an increase of the fitness of the food-limited exposed larvae compared to the food-limited, unexposed controls. At a concentration of 16.2 micrograms Cd/L, the negative effects of cadmium on food-limited midges balanced the positive effects of reduced food limitation. At this concentration, the population growth rate did not differ significantly from the food-limited control any more. It is concluded that the indirect positive effects of cadmium on food limitation could eliminate negative, direct effects of low cadmium concentrations on food-limited chironomid populations.
Archives of Environmental Contamination and Toxicology 03/1994; 26(2):143-8. · 2.01 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The phosphorus-limited growth of cultures of the nitrogen-fixing cyanobacteria Aphanizomenon and Anabaena was investigated. In conditions of nutnent and light excess Anabaena has a competitive advantage. The lower the light intensity conditions at which Aphanizomenon populations dominate are indicated for future study.