Short-term dynamics in microphytobenthos distribution and associated extracellular carbohydrates in surface sediments of an intertidal mudflat

Abstract

Two field studies were conducted to study the in situ net production of extracellular carbohydrates and the distribution of benthic diatoms over a period of 24 h per study. A comparison was made between a situation where a clear surface biofilm of diatoms had developed and a situation where this was not the case. Vertical profiles were made by sampling the top 2 mm of the sediment at depth intervals of 0.2 mm using the 'cryolander' technique. In the presence of a biofilm, diatom distribution showed a consistent pattern when the sediment was emersed. In the light, most of the diatoms were present in the top 0.2 mm while in the dark, diatoms were homogeneously distributed in the upper 2 mm of the sediment. When a biofilm was absent, no clear patterns were observed. Extracellular carbohydrates were extracted from the sediment and separated in 2 operationally defined fractions (colloidal and EDTA-extractable). The 2 carbohydrate fractions showed a different dynamic behaviour. The colloidal carbohydrate fraction was highly variable while the EDTAextractable fraction behaved more conservatively. Only in the light and in the presence of a diatom biofilm, was production of extracellular carbohydrates observed. The maximum rate of chlorophyllnormalized production of extracellular carbohydrates, expressed in glucose equivalents (g g -1), amounted to 20 h -1 in the upper 0.2 mm. The molecular size distribution of both carbohydrate fractions was similar. The monosaccharide composition was also similar, except that the EDTAextractable fraction contained a higher percentage of uronic acids. Carbohydrates produced during tidal emersion were rich in glucose and were rapidly turned over.

The glucans extracted with warm water from diatoms are mainly derived from intracellular chrysolaminaran and not extracellular polysaccharides

“…In contrast to the conclusion reached in these and related studies (e.g. Staats et al, 2000a,b;de Brouwer & Stal, 2001; that the warm-water extracts contained mainly extracellular polysaccharides, we advocate the view that the water-extractable glucans were mainly derived from intracellular chrysolaminaran and that the chrysolaminaran was the major polysaccharide extracted from diatoms with warm water. In discussing the compositional monosaccharide data for C. closterium and N. salinarum, Staats et al (1999) stated that 'glucose is probably mainly present as the b-1,3 glucan chrysolaminaran, which is known to be the general storage carbohydrate in diatoms'.…”

“…During the last five years, there have been a number of studies that have adopted a warm water treatment to extract putative extracellular polysaccharides from marine benthic diatoms (Staats et al, 1999(Staats et al, , 2000ade Brouwer & Stal, 2001. In these studies, the basic protocol first involved the separation by centrifugation of cells from the surrounding medium, which contained what was typically described as the 'unbound' or 'unattached' EPS.…”

“…In these studies, the basic protocol first involved the separation by centrifugation of cells from the surrounding medium, which contained what was typically described as the 'unbound' or 'unattached' EPS. The pelleted cells were resuspended and extracted with tap or distilled water at 308C for 1 h. Centrifugation and recovery of the supernatant gave a fraction described as 'colloidal carbohydrates ' (de Brouwer & Stal, 2001), 'exopolysaccharides' (Staats et al, 2000b), 'attached exopolysaccharides' (Staats et al, 2000a), 'attached EPS' (Staats et al, 1999;, or 'bound EPS' . In studies of cultured diatom species, where the carbohydrate composition of the nominal 'attached exopolysaccharides' or 'bound EPS' was reported, glucose was invariably the dominant component (55 -85% of total monosaccharides, Staats et al, 1999;.…”

Macromolecular and chemical features of the excreted extracellular polysaccharides in induced biological soil crusts of different ages

“…Indeed, C from EPSs was demonstrated to be the primary substrate respired in the pulse of CO 2 typically observed in dry lands after rainfall events (Thomas et al, 2008). In other systems it is documented that, owing to microbial activity, EPSs are eventually reduced to smaller polymers with a broad range of molecular sizes (Arnosti,1995(Arnosti, ,1996De Brouwer and Stal, 2001) and chain lengths (Bender et al, 1994), depending on the structure and composition of the microbial community.…”

“…In some cases, the amount of these substances was reckoned to represent up to 500% of the cell biomass (Chenu, 1993). Complex high molecular weight (HMW) polymers are degraded by the soil biota to simple sugars such as glucose, galactose (Gross et al, 1998;Brüll et al, 2000;De Brouwer and Stal, 2001) and fructose, which can be readily utilized as carbon source or energy reserve (Bertocchi et al, 1990) by the crustal microbial community and/or by adjacent plants. Indeed, C from EPSs was demonstrated to be the primary substrate respired in the pulse of CO 2 typically observed in dry lands after rainfall events (Thomas et al, 2008).…”