Marc Elskens

Metabolism and functions of glutathione in micro-organisms.

Publisher Summary The chapter discusses the biologically relevant chemistry of glutathione (GSH) and its occurrence in microbial cells. The GSH-related biochemical reactions and the physiological roles of GSH are summarized. The biosynthesis of GSH is remarkable in two ways: it is mRNA independent, and the glutamic residue is joined in an unusual peptide linkage of the γ -carbon atom to the cysteine residue. Due to this structural peculiarity, GSH is protected against proteolytic cleavage. The GSH status of cells is defined by the total concentration of GSH and the nature and distribution of the possible forms of occurrence of the tripeptide in the cell. GSH and related compounds are widespread in the microbial world, especially amongst organisms with an aerobic lifestyle. This observation emphasizes the role of GSH in cellular protection against by-products generated by oxidative metabolism, but it does not limit its functions to this role. Glutathione acts as an enzyme cofactor, transport component, nucleophilic substrate, and sulphur reservoir; and participates in key cellular processes such as protein synthesis and degradation, regulation of enzyme activity, synthesis of DNA, and maintenance of the integrity of cell membranes and organelles. Having a functional diversity, GSH is interrelated with a number of metabolic pathways and its intracellular modulation could have an impact on the entire cell, making it extremely difficult to associate directly a given cellular end-point with one molecule or system. Both in Escherichia coli and Saccharomyces cerevisiae, GSH plays an important role in cellular protection during chemical stresses in spite of the fact that key enzymes of detoxification, such as GSH peroxidase and GSH S-transferase, remain at a low level.

Accumulation of trace metals in the muscle and liver tissues of five fish species from the Persian Gulf

In this study, concentrations of 16 elements were quantified in muscles and livers of 141 fishes belonging to five commercially species. It was also our intention to evaluate potential risks to human health associated with seafood consumption. The grunt, flathead, greasy grouper, tiger-tooth croaker and silver pomfret fish species were obtained from Abadan, Deylam, Bushehr-Nirogah, Dayyer port, Lengeh port and Abbas port in Hormozgan, Bushehr and Khozesran provinces at the Iranian waters of the Persian Gulf. The contents of Al, As, Be, Cd, Cr, Co, Cu, Fe, Pb, Mn, Mo, Ni, Sb, Tl, V and Zn in fish muscles and livers were determined by Inductively Coupled Plasma Mass Spectrometry (ICP MS), after digestion in a CEM (Mars 5) microwave oven using nitric acid and hydrogen peroxide. Our results indicated that almost all metals were more accumulated in younger flathead, greasy grouper and tiger-tooth fishes. Contrary to the other fish species, grunt seems to stronger accumulating elements in the older fishes. Strong and positive correlations were observed in three or more of the fish species between V, Al, Fe, Tl, Co and Pb. The results confirmed that fish muscle and liver tissues appeared to be good bio-indicators for identification of coastal areas exposed to metallic contaminants. The results also showed that the element levels in the muscles of all fishes in our study were lower than the maximum allowable concentrations and pose no threat to public health, except for arsenic.

Glutathione as an endogenous sulphur source in the yeast Saccharomyces cerevisiae

Glutathione-deficient mutants (gshA) of the yeast Saccharomyces cerevisiae, impaired in the first step of glutathione (GSH) biosynthesis were studied with respect to the regulation of enzymes involved in GSH catabolism and cysteine biosynthesis. Striking differences were observed in the content of the sulphur amino acids when gshA mutants were compared to wild-type strains growing on the same minimal medium. Furthermore, all mutants examined showed a derepression of gamma-glutamyltranspeptidase (gamm-GT), the enzyme initiating GSH degradation. However, gamma-cystathionase and cysteine synthase were unaffected by the GSH deficiency as long as the nutrient sulphate source was not exhausted. The results suggest that the mutants are probably not impaired in the sulphate assimilation pathway, but that the gamma-glutamyl cycle could play a leading role in the regulation of the sulphur fluxes. Studies of enzyme regulation showed that the derepression of gamma-GT observed in the gshA strains was most probably due to an alteration of the thiol status. The effectors governing the biosynthesis of cysteine synthase and gamma-cystathionase seemed different from those playing a role in gamma-GT regulation and it was only under conditions of total sulphate deprivation that all these enzymes were derepressed. As a consequence the endogenous pool of GSH was used in the synthesis of cysteine. GSH might, therefore, fulfil the role of a storage compound.

New production of the NW Iberian shelf during the upwelling season over the period 1982–1999

New production (NP) is calculated for NW Iberian shelf waters from 421 to 431N (3500 km 2 ), at the fortnight, upwelling-season (March–October) and inter-annual time-scales. The time series used are (1) upwelling rates (daily values of offshore Ekman transport from 1982 to 1999), (2) bottom shelf temperatures (twice a week values from 1987 to 1999), and (3) the nutrient–temperature relationships ofupwelled Eastern North Atlantic Central Water (ENACW) obtained during 14 hydrographic cruises to the study area (between 1977 and 1998). Marked inter-annual variability is observed, both at the fortnight and the seasonal time-scales. Average NP over the upwelling-season ranged from 330 to 815 mg C m � 2 d � 1 (mean, 4907145 mg C m � 2 d � 1 ) in the 1982–1999 period. Large inter-annual changes ofupwelling rates are the reason behind the NP fluctuations: 83% ofthe variability ofNP can be explained by the offshore Ekman Transport ð� QX Þ: NP is compared with satellite-derived net microbial community production (NCP) during the 1998– 1999 upwelling seasons, when SeaWiFS images are available. An average upwelling-season NP/NCP ratio of0.33 was obtained, indicating that 67% of NCP is respired in situ and 33% is exported off-shelf to the surrounding oligotrophic ocean.

Phytoplankton and microzooplankton variability between the Subtropical and Polar Fronts south of Australia: Thriving under regenerative and new production in late summer

Phytoplankton, microzooplankton were studied along a 42°-55°S, 141°-143°E transect in March 1998 and compared with production-related parameters (carbon biomass, chlorophyll a, nitrogen and carbon uptake, and f ratios). The transect crossed the Subtropical Front (STF), the Subantarctic Front (SAF), and the Polar Front (PF). Phytoplankton assemblages were dominated by nano- and pico-sized flagellates; their peak numbers (nanoflagellates: 8.2 x 10 5 cells L -1 ) occurred in the areas of STF and within the Subantarctic Zone (SAZ). North of the SAF, dinoflagellates were next in abundance. Diatoms exceeded dinoflagellates in the PF area (maximum 1.26 x 10 5 cells L -1 ). Dinoflagellates were dominated by nano-sized gymnodinioid forms with microplanktonic species increasing in numbers in SAZ and STF. Diatoms contained mainly Fragilariopsis pseudonana and Pseudonitzschia lineola; several abundant species exibited a latitudinally restricted distribution. Phytoplankton carbon biomass was dominated by dinoflagellates (including >20 pm heterotrophs) representing 48 to 84% of total cell carbon. Maxima of 18-26 pg C L -1 occured both at STF and PF. Heterotrophic dinoflagellates and ciliates showed similar distributions. Their peaks of cell densities and carbon in STF and SAZ were associated with phytoplankton maxima. Microzooplankton cell distribution and biomass suggest they are major grazers and contributors to carbon flow. Phytoplankton assemblages represented at least three stages with different relative contributions of regenerative and new production. Production related parameters (e.g., low f ratio and high NH 4 + uptake) point to the presence of regenerative community at the STF. It attracted the highest concentration of microzooplankton. The phytoplankton community associated with a frontal feature (46°-47°S) within the SAZ, thrived under increased new production (e.g., relatively higher f ratio and NO 3 - uptake). The community along 47°-55°S was characterized by intermediate f ratios, with slight predominance of regenerated production. Southward of 47°S, the relative contribution of new production increased.

Export production in the Bay of Biscay as estimated from barium – barite in settling material: a comparison with new production

We present barium data for sediment traps deployed in a northeast Atlantic margin environment (Bay of Biscay). Fluxes of excess barium were measured with the objective of calculating carbon export production rates from the surface mixed layer and thus contribute to the understanding of organic carbon transport in a margin environment. Therefore, it was necessary to properly understand the different processes that affected the barium fluxes in this margin environment. Seasonal variability of POC/Ba flux ratios and decrease of barium solubilisation in the trap cups with increasing depth in the water column probably indicate that the efficiency of barite formation in the organic micro-environment varies with season and that the process is relatively slow and not yet completed in the upper 600 m of water column. Thus barite presence in biogenic aggregates will significantly depend on water column transit time of these aggregates. Furthermore, it was observed that significant lateral input of excess-Ba can occur, probably associated with residual currents leaving the margin. This advected excess-Ba affected especially the recorded fluxes in the deeper traps (>1000 m) of the outer slope region. We have attempted to correct for this advected excess-Ba component, using Th (reported by others for the same samples) as an indicator of enhanced lateral flux and assigning a characteristic Ba/Th ratio to advected material. Using transfer functions relating excess-Ba flux with export production characteristic of margin areas, observed Ba fluxes indicate an export production between 7 and 18 g C m−2 yr−1. Such values are 3–7 times lower than estimates based on N-nutrient uptake and nutrient mass balances, but larger and more realistic than is obtained when a transfer function characteristic of open ocean systems is applied. The discrepancy between export production estimates based on excess-Ba fluxes and nutrient uptake could be resolved if part of the carbon is exported as dissolved organic matter. Results suggest that margin systems function differently from open ocean systems, and therefore Ba-proxy rationales developed for open ocean sites might not be applicable in margin areas.

Seasonal variation of phytoplankton community structure and nitrogen uptake regime in the Indian Sector of the Southern Ocean

Abstract This study investigates the dynamics of phytoplankton communities and nitrogen uptake in the Indian sector of the Southern Ocean during spring and summer. The study area is oligotrophic (Chl a stocks <50 mg m−2); nevertheless, a large spatial variation of phytoplankton biomass and community structure was observed. During both seasons the phytoplankton community in the seasonal ice zone showed higher biomasses and was mainly composed of large diatom cells. However, in the permanently open ocean zone the community had low biomass and was chiefly composed of nano- and picoflagellates. In the polar front zone, although biomass was higher, the community structure was similar to the open ocean zone. The results suggest that the variation in phytoplankton community structure on a larger scale resonates with gradients in water column stability and nutrient distribution. However, significant changes in biomass and nutrient stocks but little change in community structure were observed. Absolute nitrogen uptake rates were generally low, but their seasonal variations were highly significant. During spring the communities displayed high specific nitrate uptake (mean rate = 0.0048 h−1), and diatoms (in the seasonal ice zone) as well as nano- and picoflagellates (in the permanently open ocean zone and polar front zone) were mainly based on new production (mean ƒ-ratio = 0.69). The transition to summer was accompanied by a significant reduction in nitrate uptake rate (0.0048 h−1 → 0.0011 h−1) and a shift from predominantly new to regenerated production (ƒ-ratio 0.69 → 0.39). Ammonium played a major role in the seasonal dynamics of phytoplankton nutrition. The results emphasize that, despite a large contrast in community structure, the seasonal dynamics of the nitrogen uptake regime and phytoplankton community structure in all three subsystems were similar. Additionally, this study supports our previous conclusion that the seasonal shift in nitrogen uptake regime can occur with, as well as without, marked changes in community structure.

Biogeochemical behaviour of Cd, Cu, Pb and Zn in the Scheldt estuary during the period 1981-1983

Cu, Zn, Cd and Pb have been analysed in suspendedmatter and water samples from the Scheldt estuary,collected during five cruises between May 1982 and May1983. In order to evaluate the overall metal behaviourin the estuary, continuous longitudinal profiles ofthe total, the particulate and the dissolved metalconcentrations are discussed in relation to variousspecific physico-chemical parameters. By means ofcorrelation and multi-regression techniques, severalestuarine processes could be identified which dominateand/or control the behaviour of the individual tracemetals in three distinct areas of the Scheldt estuary.The behaviour of the four selected metals was foundto show some common features, but is besides this alsocharacterised by many individual and specificproperties. The total metal concentrations seem toco-vary well with the turbidity in the maximumturbidity zone between salinities 2 and 10 psu. In thedownstream area a dilution profile is observed for thetotal metal concentrations of Cu and Cd as well as forthe dissolved concentrations of all metals.Dissolved metal production rates are high for Cu andCd, but much lower for Zn and Pb, confirming thedifferent longitudinal dissolved metal profiles. ForCu and Cd these dissolved metal profiles arecharacterised by a broad mobilisation area, coveringthe whole middle estuary. Pb on the other hand, showsa more confined mobilisation area, and for Zn no sucharea was found. During winter the dissolved metalproduction rates seem to be controlled, predominantly,by desorption processes. During summer dissolved metalproduction is correlated mainly to the dissolvedoxygen content, suggesting the domination of redoxprocesses solely or in combination with desorptionprocesses. In some cases, co-existent with nearly zerooxygen concentrations, removal of dissolved metals wasobserved, possibly reflecting precipitation reactions.Longitudinal particulate metal profiles all show anegative deviation versus the dilution profile. Cu andCd provide the higher deviations, characterised by anarrow enrichment zone preceding a broad mobilisationarea. Finally, all particulate metal profiles appearedto correlate well with POC, when expressed involumetric units.

Nitrogen uptake regime and phytoplankton community structure in the Atlantic and Indian sectors of the Southern Ocean

Abstract Phytoplankton nitrogen uptake is studied in relation to the biomass and structure of phytoplankton community in the Atlantic and Indian sectors of the Southern Ocean. Two scenarios of seasonal evolution of uptake regime and phytoplankton community structure are described. The first scenario includes the Marginal Ice Zone areas of the Weddell Sea and adjacent areas where a predominantly nitrate based, diatom dominated assemblage, thriving in a stable water column at the beginning of the season was transformed into a mainly ammonium based, flagellate dominated assemblage, towards the end of the season. The change in phytoplankton community structure was caused by selective grazing by large grazers and reduced stability of the water column and the shift in uptake regime was due to increased ammonium availability and changes in community structure. In the second scenario, in the Coastal and Continental Shelf Zone (CCSZ) and Open Oceanic Zone (OOZ) of the Indian sector, a shift in uptake regime occurred without a big change in phytoplankton community structure. These areas were sampled late in the growth season and were characterized by prolonged water column stability, less grazing pressure on large diatoms and high ammonium availability. Diatoms dominated the assemblage and about 80% phytoplankton biomass was on the >10 μm size fraction. Unlike the first scenario, diatoms were largely based on ammonium. Thus, in areas of persistent water column stability and less selective grazing pressure, a shift in uptake regime can occur without change in community structure. The dominance of diatoms under regenerated production provides a physiological evidence for the excess net removal of silicate over nitrate occurring in certain provinces of the Southern Ocean.

Effect of organic complexation on the behaviour of dissolved Cd, Cu and Zn in the Scheldt estuary

Samples from the Scheldt estuary have been assayed for dissolved Cd, Cuand Zn using differential pulse anodic stripping voltammetry, either astotal (after UV irradiation) or labile concentrations. Under theseexperimental conditions, labile concentrations ranged between51–65% of total Cu, 16–66% of total Zn and53–91% of total Cd. The metal–organic interactions wereassessed by evaluating (a) the distribution coefficient Kd forthe distribution of the metals between the liquid phase (complexation) andtheir binding to particulate matter, and (b) the competitive effect exertedby inorganic complexing ligands using a multi–element interactionmodel. The proportion of organically bound metals (strong and labile) wasestimated, in this speciation scheme, to range from 86 to 99% for Cu,from 90 to 96% for Zn, and from 10 to 35% for Cd. From thedissolved organic carbon distribution in the Scheldt (≤ 10 mg Cl-1 and taking into account competition from major cations Caand Mg, free ligand concentrations available for heavy metal complexationwere estimated to be ≤ 0.15 mg C l-1. With these values,conditional stability constants for the chelation of Cu, Zn and Cd werecalculated assuming either a single-step or a two-step complexation in thedissolved phase. Given the assumptions made in these models, stabilityconstants in the range of 107.8–1010.6 forCu, 107.0–109.1 for Zn and106.9–108.9 for Cd were obtained. Therelevance of these data to previous in vitro and in situ studies isdiscussed taking into consideration current concepts of metal bindingaffinity for organic ligands.