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Dive into the research topics where Graham J. C. Underwood is active.

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Featured researches published by Graham J. C. Underwood.


Journal of the Marine Biological Association of the United Kingdom | 1993

Seasonal changes in diatom biomass, sediment stability and biogenic stabilization in the Severn Estuary

Graham J. C. Underwood; David M. Paterson

Epipelic diatoms represented the dominant microphy tobenthos on the intertidal mudflats of the Severn Estuary, south-western Britain. Algal biomass (measured as chlorophyll a concentration) varied over a seasonal cycle and was strongly correlated with sediment shear strength and critical shear stress and therefore with position on the shore. High levels of diatom biomass were positively correlated with the concentration of colloidal carbohydrate within the surface sediments. The critical shear strength for incipient erosion was significantly correlated with position on the shore (moisture content) and with both chlorophyll a and colloidal carbohydrate, the latter being the best biochemical predictor for the incipient erosion threshold. The range of stress required to cause incipient erosion varied from 1.0 to 8.0 N m -2 , with the sediment increasing in resistance landwards. Two-way analysis of variance using both moisture content and colloidal carbohydrate as variables explained the stability of the sediment better than individual pair-wise comparisons.


Journal of Phycology | 2001

The production of extracellular carbohydrates by estuarine benthic diatoms: the effects of growth phase and light and dark treatment

Graham J. C. Underwood

Epipelic diatoms are important constituents of estuarine microphytobenthic biofilms. Field‐based investigations have shown that the production of carbohydrates by such taxa is ecologically important. However, limited information exists on the dynamics of carbohydrate production by individual species of epipelic diatoms. The production of low and high molecular weight extracellular carbohydrates in axenic cultures of five species of benthic estuarine diatoms, Cylindrotheca closterium (Ehrenberg), Navicula perminuta (Grun.) in Van Heurck, Nitzschia frustulum (Kütz.) Grunow, Nitzschia sigma (Kütz.) Grunow, and Surirella ovata (Kütz.) Grunow, were investigated. All species produced colloidal (water‐soluble) carbohydrates during growth, with maximal production occurring during stationary phase. During logarithmic growth, approximately 20% of extracellular carbohydrates consisted of polymeric material (extracellular polymeric substances [EPS]), but during stationary phase, EPS content increased to 34%–50%. Pyrolysis–mass spectrophotometry analysis showed differences in the composition of EPS produced during logarithmic and stationary phase. All species synthesized glucan as a storage carbohydrate, with maximum glucan accumulation during the transition from log to stationary phase. Short‐term labeling with 14C‐bicarbonate found that between 30 and 60% of photoassimilates were released as colloidal carbohydrate, with EPS consisting of approximately 16% of this colloidal fraction. When cells were placed in darkness, EPS production increased, and between 85 and 99% of extracellular carbohydrate produced was polymeric. Glucan reserves were utilized in dark conditions, with significant negative correlations between EPS and glucan for N. perminuta and S. ovata. Under dark conditions, cells continued to produce EPS for up to 3 days, although release of low molecular weight carbohydrates rapidly ceased when cells were dark treated. Three aspects of EPS production have been identified during this investigation: (1) production during rapid growth, which differs in composition from (2) EPS directly produced as a result of photosynthetic overflow during growth limiting conditions and (3) EPS produced for up to 3 days in the dark using intracellular storage reserves (glucans). The ecological implications of these patterns of production and utilization are discussed.


Journal of Phycology | 2004

Environmental effects on exopolymer production by marine benthic diatoms: Dynamics, changes in composition, and pathways of production

Graham J. C. Underwood; Matthew Boulcott; Christine A. Raines; Keith W. Waldron

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


Diatom Research | 2004

THE UPS AND DOWNS OF LIFE IN A BENTHIC BIOFILM: MIGRATION OF BENTHIC DIATOMS

Mireille Consalvey; David M. Paterson; Graham J. C. Underwood

Benthic diatom communities have been documented many times to exhibit a predictable pattern of migration entrained to diurnal and tidal cycles forming transient biofilms in the estuarine intertidal. The highly visible nature of these biofilms means that many anecdotal descriptions of this behaviour exist, yet much still remains to be elucidated about these behaviours and what drives them. There has been a resurgence in work examining the migratory patterns of benthic diatoms because of the important roles they play in the overall functioning of the ecosystem (e.g. primary production). It is the aim of this paper to provide a comprehensive overview of the migratory literature to date. The paper will review the methodology that was historically used to examine patterns in diatom migration as well as introduce new technologies (e.g. remote sensing). The patterns and cues for migration are discussed, highlighting the fact that whilst a “classic” pattern of migration is often found (migration of cells to the sediment surface as the tide leaves the site followed by a downwards migration in anticipation of the incoming tide), these cannot be considered to be consistent, with many site and taxon specific idiosyncrasies. Furthermore, it has been suggested that cells also migrate within the biofilm, cycling their position at the sediment surface therefore maximising their overall fitness, and this hypothesis is discussed. The vertical migration strategy of the benthic diatoms is critical to their pivotal role in the structure and functioning of estuarine intertidal mudflats and the costs and benefits of a microphytobenthic existence are discussed in this context.


European Journal of Phycology | 1998

Distribution of estuarine benthic diatom species along salinity and nutrient gradients

Graham J. C. Underwood; James Phillips; Karen Saunders

Changes in relative abundance of estuarine epipelic microalgae along an estuarine gradient were investigated, and population densities of different species along a small-scale nutrient gradient generated by a sewage treatment outfall and in laboratory mesocosms enriched with ammonium were also studied. The relative abundance of certain species of epipelic diatoms was related to location along the estuarine salinity and nutrient gradient: Navicula gregaria and N. phyllepta were abundant at oligo- and mesohaline sites respectively, and Pleurosigma angulatum and Plagiotropis vitrea were abundant at polyhaline sites. On a smaller spatial scale, though there were no significant patterns in microalgal biomass in relation to nutrient enrichment, there were significant differences in the population densities of different epipelic species along a multivariate nutrient gradient (decreasing concentrations of ammonium, nitrite, silicate, organic content, and increasing salinity and pore-water nitrate concentrations) ...


Microbial Ecology | 1998

Predicting Epipelic Diatom Exopolymer Concentrations in Intertidal Sediments from Sediment Chlorophyll a

Graham J. C. Underwood

A bstractIn many intertidal cohesive—sediment habitats, epipelic diatoms are the dominant microphytobenthic organisms. In such sediments, concentrations of colloidal carbohydrate [including the exopolymeric substances (EPS) produced by diatoms during motility] are closely correlated with the biomass (chlorophyll a) of epipelic diatoms. A model describing this relationship (log (conc. coll. carbo. + 1) = 1.40 + 1.02(log (chl. a conc. + 1)) was derived from published data. It was validated against published and unpublished data from 6 different estuaries, and accounted for 64.6% of the variation in sediment colloidal carbohydrate concentrations. The model was valid for intertidal habitats with cohesive sediments where epipelic diatoms constituted >50% of the microphytobenthic assemblage. In sites with noncohesive sediments, or where the microphytobenthic assemblage was dominated by other algal groups, the model was not applicable. The mean percentage of EPS in colloidal carbohydrate extracts varied between 11 and 37% for axenic cultures of epipelic diatoms (with higher values obtained during stationary phase), and between 22.7% and 24.3% for natural sediments dominated by epipelic diatoms. Assuming an EPS percentage of 25% in colloidal extracts yielded an EPS chl. a ratio of 2.62:1. Maximum rates of EPS production in diatom cultures occurred at the beginning of stationary phase (1.6–5.09 μg EPS μg−1 chl a d−1), with Nitzschia sigma having a significantly (P < 0.05) higher rate of production than N. frustulum, Navicula perminuta and Surirella ovata. Similar rates of EPS production were measured in the field. The dynamics of EPS production and loss on mudflats is discussed, with reference to the model and these production rates.


Advances in Botanical Research | 2003

The importance of extracellular carbohydrate productionby marine epipelic diatoms

Graham J. C. Underwood; David M. Paterson

Abstract Soft-sediment habitats in intertidal and shallow subtidal marine ecosystems frequently support extensive populations of benthic microalgae (microphytobenthos). These algal assemblages are dominated by species of motile benthic diatoms and form biofilms, a matrix of cells, sediments and extracellular polymeric substances (EPS), that create a complex microhabitat and act to stabilise sediments. Diatom EPS consists of a relatively undefined complex mixture of proteins, proteoglycans and carbohydrates. This complexity causes problems in extracting and analysing EPS and in the intercomparison of studies. This chapter reviews our current knowledge on the production rates, patterns and composition of benthic diatom EPS in culture and field studies. Production patterns are dynamic, changing with cell growth phase, photosynthesis and irradiance, nutrient conditions, and are also linked to endogenous cell rhythms. Meta-analysis of published monosaccharide composition data identified at least four major types of EPS produced by benthic diatoms, with varying patterns of production and composition. It is clear that more detailed research on the structural and physical properties of EPS are needed to understand its role in the environment. The natural occurrence of EPS is closely linked to diatom biomass, a pattern consistent over both macro (km) and micro ( μ m) scales. EPS is lost from sediments by various routes, solubilisation and removal by overlying water, bacterial degradation and consumption by deposit-feeding invertebrates. Work is needed to quantify these pathways and clarify the importance of EPS in coastal carbon cycles. Diatom EPS is a widely cited mechanism for increasing sediment stability and stabilisation by biofilms is well described. However, data are not consistent and developments in our knowledge of the structure and function of EPS are needed to explain how EPS binds and interacts within the sediment- biofilm matrix and affects the theology of sediment.


Diatom Research | 1994

SEASONAL AND SPATIAL VARIATION IN EPIPELIC DIATOM ASSEMBLAGES IN THE SEVERN ESTUARY

Graham J. C. Underwood

The species composition of epipelic algal assemblages on intertidal mudflats was investigated at three sites in the Severn estuary, England, between March 1990 and February 1991. Diatoms were the dominant algal group, comprising greater than 95% of living cells in 100 out of 106 lens tissue samples. Euglenoid algae were abundant at one site during the summer months. Although over 60 diatom taxa were identified, only 15–20 species occurred regularly, with 12 species being dominant in the samples. Upper and middle shore diatom assemblages were dominated by Navicula pargemina during the spring and autumn, and by Nitzschia epithemioides during the early summer months. High relative abundances of Rhaphoneis minutissima were recorded at lower shore stations throughout the year and on upper and middle shores during the winter months. These lower shore assemblages contained high proportions of epipsammic species (Coscinodiscus sp., Cymatosira belgica). Principal component scores (obtained from assemblage composit...


Water Research | 2001

The potential for phosphorus release across the sediment–water interface in an eutrophic reservoir dosed with ferric sulphate

Rupert Gordon Perkins; Graham J. C. Underwood

Alton Water, Suffolk, UK is a pumped storage reservoir that has a history of cyanobacterial blooms. Dosing of the input water with ferric sulphate to control external phosphorus loading has occurred since 1983. A detailed study of the sediment chemistry of the site was carried out between May 1995 and July 1997. Sequential phosphorus fraction analysis indicated a decrease along the length of the reservoir in sediment labile phosphorus content from 0.62 to 0.08 mg P g-1 dw and iron-bound phosphorus content from 3.22 to 0.46 mg P g-1 dw. These gradients positively correlated with water column chlorophyll a concentrations reported in a parallel study. Labile and iron-bound sediment phosphorus contents were in a dynamic equilibrium due to diffusional release, contributing to internal loading to the water column. Equilibrium phosphorus concentrations (EPC) determined from phosphorus adsorption capacity (PAC) experiments were lower inside the bunded region (0.01-0.03 mg P-PO4 l-1) where iron content was greatest compared to outside the bund (0.15-0.20 mg P-PO4 l-1) suggesting greater potential for diffusional release of phosphorus at the latter site. PAC experiments indicated that anaerobic and pH-mediated loadings were of less importance than diffusional release, although the latter may have contributed to internal loading in the main reservoir. Sulphate concentrations may act to increase the potential for anaerobic internal loading near to the pumped input in microstratified sediment. Sediment iron content decreased from 250 +/- 13.1 to 51 +/- 4.0 mg Fe g-1 dw across the line of a constructed bund at the north-west end near to the pumped input, which indicated successful control of dispersal of the fine ferric floc. The management implications with regard to phosphorus loadings indicated by these results are discussed.


Journal of Phycology | 2006

Extracellular matrix assembly in diatoms (Bacillariophyceae). V. Environmental effects on polysaccharide synthesis in the model diatom, Phaeodactylum tricornutum

Abass S. Abdullahi; Graham J. C. Underwood; Michael R. Gretz

The effects of phosphate (P) limitation, varying salinity (5–65 psu), and solid media growth conditions on the polysaccharides produced by the model diatom, Phaeodactylum tricornutum Bohlin were determined. Sequential extraction was used to separate polymers into colloidal (CL), colloidal extracellular polymeric substances (cEPS), hot water soluble (HW), hot bicarbonate soluble (HB), and hot alkali (HA) soluble fractions. Media‐soluble polymers (CL and cEPS) were enriched in 4‐linked mannosyl, glucosyl, and galactosyl residues as well as terminal and 3‐linked xylosyl residues, whereas HW polymers consisted mainly of 3‐linked glucosyl as well as terminal and 2,4‐linked glucuronosyl residues. The HB fraction was enriched in terminal and 2‐linked rhamnosyl residues derived from the mucilage coating solubilized by this treatment. Hot alkali treatment resulted in the complete dissolution of the frustule releasing 2,3‐ and 3‐linked mannosyl residues. The fusiform morphotype predominated in standard and P‐limited cultures and cultures subjected to salinity variations, but growth on solid media resulted in an enrichment of the oval morphotype. The proportion and linkages of 15 residues, including neutral, uronic acid, and O‐methylated sugars, varied with environmental conditions. P limitation and salinity changes resulted in 1.5‐ to 2.5–fold increase in carbohydrate production, with enrichment of highly branched/substituted and terminal rhamnose, xylose, and fucose as well as O‐methylated sugars, uronic acids, and sulfate. The increased deoxy‐ and O‐methylated sugar content under unfavorable environments enhances the hydrophobicity of the polymers, whereas the anionic components may play important roles in ionic cross‐linking, suggesting that these changes could ameliorate the effects of salinity or P‐stress and that these altered polysaccharide characteristics may be useful as bioindicators for environmental stress.

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David N. Thomas

Finnish Environment Institute

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