Mesure de la production bactérienne par incorporation de thymidine tritiée

Au cours des dix dernières années, de nombreuses méthodes ont été proposées dans la littérature, afin de mesurer l'activité des bactéries hétérotrophes en milieu aquatique. Parmi celles-ci, la mesure de l'incorporation de thymidine tritiée dans le DNA bactérien semble être, à l'heure actuelle, la méthode la plus utilisée. Elle offre, en effet, l'avantage de sa spécificité et d'un protocole expérimental simple. Néanmoins, la conversion des résultats expérimentaux en production de biomasse bactérienne pose un certain nombre de problèmes quant à l'interprétation correcte de cette méthode. Cet article fait le point sur les réponses théoriques et expérimentales qui peuvent être apportées à ces problèmes, ainsi que sur les diverses possibilités d'utilisation de cette méthode.During the fast ten years, numerous methods have been proposed in the literature to mesure the activity of heterotrophic bacteria in aquatic ecosystems. Among these methods, the measurement of tritiated thymidine incorporation proposed by FUHRMAN and AZAM (1980) seems to be, at the present time, the most useful. It offers in fact the advantage of its specificity for bacteria and its simple experimental procedure.This method is based on the fact that, in bacteria, DNA synthesis is directly proportional to the division rate. The close relation between growth and DNA synthesis means that measurement of the rate of DNA synthesis is a good way to measure the bacterial growth rate. The DNA synthesis rate is estimated from the incorporation rate of methyl-3H thymidine. Thymidine is one of the four nucleoside precursors of DNA, but it is not a precursor of RNA. At the nanomolar concentrations of (methyl-3H) thymidine used in this experiment only heterotrophic bacteria utilize exogenous thymidine and all active heterotrophic bacteria utilize thymidine. The usual experimental procedure used by the various authors working with this method is that of the FUHRMAN and AZAM (1982). A 5 nM thymidine concentration has been recommended by these authors for the marine environment, but it has been shown that in more eutrophic ecosystems higher concentrations are needed to saturate the incorporation process.In fact, the conversion of experimental data into bacterial production raises some problems. Among these, two questions are important :- Which is the relative part of exogenous and endogenous thymidine used for DNA synthesis? In other words, which is the isotopic dilution factor?- The cold trichloroacetic acid (TCA) fraction collected in this experiment includes, besides DNA, proteins and RNA. As some catabolic products of thymidine could be incorporated into RNA or proteins, how mach radioactivity is really incorporated into DNA?Some theoretical and experimental answers can be given to these questions.- Comparing their results of thymidine and H32PO4= incorporation in DNA of marine bacteria, FUHRMAN and AZAM (1982) found an isotopic dilution factor in the 3-7 range. MORIARTY and POLLARD (1981) have proposed a kinetic approach for estimating the internal pool of thymidine but the accuracy of this method was critized by RIEMANN et al., (1982) and FUHRMAN and AZAM (1982).- Usually, in order to determine the part of radioactivity incorporated into DNA, RNA and proteins, the biochemical procedure of LURIA (1960) is used. SERVAIS et al., (1987), after testing this procedure with labelled macromolecules, have concluded that this method is not accurate for such determinations. These authors have proposed an enzymatic procedure based on the use of DNase tell breakage. With this method, they have showed important fluctuations in the percentage of thymidine incorporated into DNA from one ecosystem to another.The first authors using thymidine incorporation calculated conversion factors based on theoretical data and various assumptions to convert thymidine incorporation data into bacterial cell production. These factors were in the range of 0.2 to 4 1018 bacteria produced per mole of thymidine incorporated in the cold TCA insoluble material. More recently, because of the uncertainty or the difficulty in estimating some of the parameters (isotopic dilution, part of thymidine incorporated into DNA, part of thymidine residues in DNA, quantity of DNA per bacterial cells) required for the calculation of the conversion factor, most of the authors have used an experimental conversion factor. It was estimated from the comparison of cell number increase and thymidine incorporation in sterilized, and reinoculated samples. These experimental conversion factors were usually in the range of 0.5 to 10 1018 bacteria produced per mole of thymidine incorporated.The conversion of cells production into bacterial biomass production expressed in carbon - which is finally the important flux to know for the study of the first trophic levels dynamic - requires a knowledge of the average bacterial biovolume and the carbon content par unit of cell volume. The first parameters can be estimated from the observation of bacteria by epifluorescence microscopy. Most authors use 1.2 10-13 gC.µm-3 to calculate carbon content of bacteria proposed by WATSON et al., (1977).The use of the tritiated thymidine incorporation method is not limited to measure bacterial production in the water column; it is also used to measure the activity of fixed bacteria, to study the grazing of bacteria by microzooplankton and in ecotoxicological studies

Mesure de la biomasse et de l'activité bactérienne dans l'eau de distribution

Afin d'étudier la reviviscence bactérienne dans les réseaux de distribution, des méthodes de mesures de la biomasse et de l'activité bactérienne ont été investiguées sur des eaux provenant d'un réseau de distribution. Trois méthodes d'estimation de la biomasse bactérienne ont été comparées : le comptage sur gélose, selon la norme française d'examen bactériologique des eaux de consommation, le dosage de l'ADN contenu dans les particules retenues sur une membrane de porosité de 0.2 µm et le comptage direct au microscope à épifluorescence après coloration des bactéries à l'acridine orange. Les comptages sur gélose, tout comme en milieu aquatique naturel, sous-estiment très largement le nombre de bactéries; ceci semble principalement lié à la présence de bactéries viables mais non cultivables. Le dosage de l'ADN et les comptages directs corrèlent assez bien avec en moyenne un contenu en ADN par bactérie de 4,1 x 10-15 g d'ADN, mais la première méthode semble moins précise. Le comptage direct semble donc la méthode la plus adaptée à l'estimation du nombre total de bactéries dans ce type de milieu.Afin d'estimer l'activité bactérienne, les protocoles expérimentaux de deux méthodes utilisées en écologie bactérienne ont été adaptés aux conditions particulières de l'eau de distribution : l'incorporation de thymidine tritiée dans l'ADN bactérien et l'incorporation de leucine tritiée dans les protéines. La comparaison des deux méthodes sur une série d'échantillons montre une bonne corrélation, avec un rapport molaire entre incorporation de leucine et de thymidine compatible avec les facteurs de conversion des deux méthodes cités dans la littérature et établis pour les milieux aquatiques naturels. Les deux méthodes sont utilisables pour mesurer l'activité bactérienne dans l'eau potable, néanmoins l'incorporation de thymidine est plus aisée à mettre en oeuvre, car elle ne nécessite de travailler qu'à une seule concentration en traceur radioactif.Bacterial regrowth in distribution systems is an important problem for drinking water producers. It is linked to the more and more frequent utilization of low quality surface waters, containing high concentration of organic matter, as raw water, and also to the increase in size and complexity of the distribution networks with high residence time of the water between its production and utilization. At the present time chlorination of treated water, with sometimes rechlorination in the network, is the usual way to limit growth in distribution systems. This solution however presents disadvantages, the major one is the formation of unpleasant organochlorine compounds which are responsible for tastes and odours of water. An alternative strategy consists of developing treatment lines in which biodegradable dissolved organic carbon is removed. It allows through a reduction of the chlorine demand of the water to increase the stability of the chlorine residual of the water. In this context, it is important to get a good knowledge of the factors controlling bacterial development in distribution networks. Up to now, studies on this subject have met some methodological problems linked to the fact that classical bacteriological methods are inadequate to study this kind of systems.In this paper, various methods have been investigated to estimate bacterial biomass and activity in tap water. For this study, the analyzed water samples have been collected in the distribution system of the Parisian suburbs.Three methods have been tested for the determination of bacterial biomass : plate count, measurement of DNA associated with particles with a size higher than 0.2 µm and direct microscopic enumeration. Heterotrophic plate counts have been performed following the French standard and results are expressed in CFU (Colony Forming Units) per ml; the DNA collected after filtration of 500 ml to 1500 ml of water on a 0.2 µm pore size membrane was estimated using a fluorimetric method, as proposed by Mc COY and OLSON (1985); direct enumerations were performed by epifluorescence microscopy after acridine orange staining (AODC) following the procedure proposed by HOBBIE et al. (1977), the comparison between plate counts and AODC (fig. 1) shows the important underestimation of the bacterial numbers when estimated by the CFU (up to 3 orders of magnitude). Such discrepancy has already been observed in natural aquatic ecosystems and is usually explained by the presence of numerous dead cells enumerated by microscopy. Now, it seems that the difference between, plate counts and direct counts may rather be explained by the presence in water of « viable but non culturable » bacteria.A comparison between DNA estimation and direct counts have also been performed. Figure 2 shows the results of this comparison. In spite of the dispersion, the correlation between both methods is significant and the correlation straight line indicates an average DNA content per bacteria of 4.1 x 10-15g DNA in good accordance with the values quoted in the literature. The dispersion of the data around this average can be explained by various ways : the variability of DNA per cell content for the different bacterial strains present in the water samples, the precision of the DNA method which is not higher than 20 % and possible contamination by other organisms than bacteria, as flagellates or ciliates, which are retained on the 0.2 µm pore size membrane.On the basis of these tests, it seems that the direct count by epifluorescence microscopy is the most adapted method for studying the bacterial regrowth in distribution system.The understanding of bacterial dynamics in a distribution system requires measurements of bacterial activity. Various methods have been developed in order to estimate bacterial activity in natural aquatic ecosystems. They are primarily based on the use of radioactive tracers. At the present time, the tritiated thymidine incorporation method, which measures the replication of bacterial DNA, is the most usually used one, but the incorporation of tritiated leucine into proteins, which measures increase in bacterial biomass, seems to be also an interesting method. These methods have been selected, on one hand, because of their specificity towards bacteria and, on the other hand, because of their high sensibility which is required for measurements of bacterial activity in the conditions of drinking water. Up to now, these methods have never been applied to drinking water. We have modified the experimental procedure of both methods : incubation time, radioactive tracers concentrations and volume of the sample have been tested and adapted in order to allow measurement in the conditions of drinking water samples. For thymidine incorporation, the volume of sample, incubated during 20 hours in the presence of 20 nM concentration of 3H-thymidine, was 100 ml. The incorporation was measured in the DNA, using the biochemical procedure proposed by WICKS and ROBARTS (1977), rather than in the total macromolecules. For leucine incorporation, we measured the incorporation rate at four leucine concentrations (2, 27, 52, 77 nM : 2 nM of 3H-leucine + non radioactive leucine) in 25 ml samples and the incubation lasted 3 to 4 hours. The incorporation rate was calculated as the reciprocal of the angular coefficient of the correlation straight fine obtained when the reciprocal of the fraction of leucine incorporated per hour was plotted against leucine concentration (fig. 3). Comparison of both methods on samples of drinking is presented at figure 4, a good linear correlation was found. The equation of the correlation straigth line is :log [Inc.leu (pmol/l.h)] = 0,97 log [Inc.thy (pmol/l.h)] + 1.35(n = 69, r = 0.84)The molar ratio between leucine and thymidine incorporation found in these samples (20 to 25) seems to be in good agreement with the usual conversion factors found for both methods in natural aquatic ecosystems. Bath methods seem to be available to bacterial activity estimations in drinking water, the triatiated thymidine incorporation method which requires working with only one concentration of radioactive tracer seems easier to use

Comparaison de deux méthodes d'estimation du broutage des bactéries par les protozoaires en milieux aquatiques [Courte note]

L'objectif du présent travail est de comparer deux méthodes indépendantes permettant d'estimer, dans les milieux aquatiques, le flux de carbone transitant du compartiment bactérien vers les protozoaires. Les deux méthodes utilisées sont, d'une part, celle basée sur le suivi de la décroissance de radioactivité du matériel génétique bactérien après marquage à la thymidine tritiée (SERVAIS et al., 1985) et, d'autre part, celle de mesure du taux d'ingestion de bactéries fluorescentes (FLB) par les protozoaires. Elles ont été appliquées en parallèle sur des échantillons de la rivière Meuse (Belgique). L'emploi de la première méthode a montré des taux de broutage compris entre 0.002 h-1 et 0.016 h-1 qui représentent en moyenne 72 % des taux de mortalité totale. Une excellente corrélation entre les estimations de flux de broutage obtenues par les deux techniques a été trouvée, mais les valeurs estimées à partir de la méthode FLB sont systématiquement inférieures (d'environ 30% en moyenne) à celles obtenues par l'autre méthode. Une part de cette différence peut vraisemblablement s'expliquer par la non prise en compte par la méthode FLB du broutage par des organismes de taille supérieure à 100 µm.The goal of the present work was to compare two methods allowing to estimate, in aquatic ecosystems, the carbon flux due to grazing of bacteria by protozoa. The first method follows the decrease of labeling in the DNA of natural assemblages of bacteria previously labeled with tritiated thymidine (SERVAIS et al., 1985) and the second procedure is based on the estimation of bacterial ingestion rate by protozoa using fluorescently labeled bacteria (FLB). Both methods were applied in parallel on river Meuse (Belgium) samples. Using the first method, grazing rates in the range 0.002 h-1 to 0.016 h-1 were observed; they represented in average 72 % of the total bacterial mortality rates. A very good correlation between both estimates of the grazing fluxes was found but the data obtained by the FLB method were systematically lower (around 30% in average) than those estimated with the other method. A part of this difference is probably due to he fact that the FLB method does not take into account grazing by organism higher than 100 µm

Estimation de la biomasse bactérienne dans les effluents urbains par mesure de l'activité exoprotéolytique potentielle

Des études récentes ont montré que, lors du rejet d'eaux usées dans une rivière, la quantité de biomasse bactérienne hétérotrophe amenée par les effluents influence considérablement la cinétique de biodégradation de la matière organique dans la rivière et donc les caractéristiques du déficit d'oxygène généralement observé dans le milieu naturel en aval du rejet. La mesure de la biomasse bactérienne contenue dans un rejet domestique est donc nécessaire afin de bien comprendre la cinétique de biodégradation. Cette biomasse peut être estimée en microscopie à épifluorescence après coloration des cellules bactériennes par un fluorochrome. Cette technique appliquée aux eaux usées est néanmoins difficile et fastidieuse. Dans cette étude, une méthode alternative à l'estimation de la biomasse bactérienne dans les eaux usées a été testée ; elle consiste à mesurer l'activité exoprotéolytique potentielle (AEP) des bactéries. Nous avons montré qu'il existait, dans les eaux usées, une corrélation significative entre l'AEP et la biomasse bactérienne estimée en microscopie à épifluorescence ce qui permet d'utiliser l'AEP pour estimer facilement et rapidement la biomasse bactérienne dans ce type d'échantillon. Comme exemple d'application, des mesures d'AEP nous ont permis d'étudier l'impact de divers types de traitement dans plusieurs stations d'épuration sur la biomasse bactérienne hétérotrophe des effluents urbains. Sur base de ces mesures, les charges spécifiques en biomasse bactérienne (charge par habitant et par jour) des eaux brutes et traitées ont pu être calculées.Recent studies have shown that when wastewaters are discharged into a river, heterotrophic bacterial biomass in the effluent exerts considerable influence on the biodegradation kinetics of organic matter and provokes the oxygen deficits often observed downstream from outfalls. Quantification of bacterial biomass in wastewaters is required for a good understanding the biodegradation kinetics. Bacterial biomass can be estimated by epifluorescence microscopy after staining cells with fluorochrome. Applying this technique to wastewater samples, however, is painstaking and difficult. In this study, an alternative method for estimating bacterial biomass in wastewaters was tested; it consists of measuring the potential exoproteolytic activity (PEA) of bacteria. Previous studies on other types of aquatic systems have shown that bacterial biomass can be estimated from PEA. After optimising the PEA procedure for use with wastewater, we found a significant correlation between PEA and the bacterial biomass estimated by epifluorescence microscopy. This opens the way for PEA measurements for rapid and easy estimates of bacterial biomass in wastewaters. As an example of the application of this technique, the impact of wastewater treatment on bacterial biomass in effluent was investigated at various treatment plants. On the basis of our procedure, specific loads of bacterial biomass (load per inhabitant and per day) were calculated for raw and treated wastewaters

Biodegradable dissolved organic carbon removal during biological filtration on granular actived carbon

En production d'eau potable, la nature bactérienne de l'abattement du carbone organique dissous biodégradable (CODB) observé dans les filtres à charbon actif en grains (CAG) a été démontrée. Les performances de fonctionnement de ce type de contacteur biologique ont été principalement étudiées sur pilotes. Dans la présente étude, elles sont vérifiées et transposées en condition d'exploitation sur une usine de production d'eau potable de la. banlieue parisienne. La colonisation bactérienne du CAG a été suivie et montre que l'équilibre biologique est atteint après filtration d'environ 12500m3 d'eau/m3 de CAG. Durant cette phase de colonisation, la biodégradation se substitue progressivement à l'adsorption pour abattre le COD. Après colonisation, l'efficacité des filtres biologiques, exprimée en terme d'abattement de CODB, est fonction du temps de contact quelle que soit la vitesse de filtration (dans la gamme de 2 à 18 m/h). Les résultats de suivis de deux filtres sur deux ans montrent que l'efficacité a été globalement meilleure en 1989 qu'en 1990, cette différence s'explique par les fluctuations plus importantes de CODB dans l'influent en 1989. Un modèle mathématique, établi à partir des équations cinétiques des processus bactériens dans les filtres à CAG (modèle CHABROL), développé sur base d'observations antérieures, permet de simuler correctement les observations faites au cours de la présente étude. Avec la mesure du CODB, le modèle CHABROL constitue un outil très bien adapté pour contrôler les performances des contacteurs biologiques. Ils permettent, entre autre, de définir le temps de contact optimal de l'eau dans le filtre en fonction d'une température et d'une qualité d'eau donnée dans l'influent et d'une qualité d'eau souhaitée dans l'effluent.In drinking water production, filtration on granular activated carton (GAC) is generally used in order to remove by adsorption the dissolved organic matter. Nevertheless, the adsorption capacity of GAC is rapidly saturated and it is so necessary to regenerate the GAC. An interesting alternate has been applied in some treatment plants. It consists to use GAC filtration without regeneration taking benefit of the activity of the microbial community which colonize the GAC particles (RITTMAN and HUCK, 1989). In fact, this biological filtration offers the advantage to specially remove the biodegradable fraction of the dissolved organic carbon (BDOC), which is responsible for the problem of bacterial growth into the distribution networks.The bacterial nature of the BDOC removal achieved by the biological filtration on GAC has been now clearly demonstrated (SERVAIS et al., 1991) and some important results of the functioning of these filters has been obtained in studies conducted on pilots filters (BOUILLOT et al., 1990; SERVAIS et al., 1992). These studies have for example shown that only a very small part of the bacterial biomass produced in the filter is exported with the outflow.In the present study, biological filtration has been investigated in a full scale treatment line at Choisy-le-Roi in the Parisian suburbs and the results compared with those gained on pilot filters.The working conditions of the three GAC filter studied are presented in table 1 and compared with those of pilot filters used in a previous study conducted al Neuilly-sur-Marne (table 2). The microbial colonization has been followed in two of the liners. If lasted roughly 3 months to reach biological equilibration, it corresponds to a water volume filtrated of 12 500 m3 per m3 of GAC. Efficiency of the removal during this period is presented in figure 2. Progressively, biological processes take turn with adsorption (fig. 1).As already demonstrated by SERVAIS et al. (1992), the efficiency of biological filtration, calculated in percentage of BDOC removal, increases with increasing contact time whatever the filtration velocity could be in the range 2 m/h to 18 m/h (fig. 3). However, the percentage of BDOC, at similar temperature, is higher in the GAC filters at Choisy-le-Roi than at Neuilly-sur-Marne. The fixed bacterial biomass is also higher at Choisy-le-Roi (average 7.5 µgC/cm3) than at Neuilly-sur-Marne (average 2 µC/cm3).Following during two years the functioning of the n° 56 and 38 filters (tables 3, 4 and fig. 5, 7), it seems that the global efficiency of filtration is better in 1990 than in 1989. This can be linked to the greater fluctuations in BDOC in the influent water in 1989 than in 1990, as shown on figure 8. Fluctuations in the quality of the influent water requires a period to reach the equilibrium during which the effluent is charchacterized by a lower quality (fig. 8). This period is longer at low temperature. The mathematical modal based on the kinetics of the basic microbiological processes involved in biological filtration (the CHABROL model) has been previously developed (BILLEN et al., 1992) in order la simulate the performances of the filtration. It can be used to simulate the vertical profiles of BDOC and bacterial biomass in the filters of the Choisy-le-Roi treatment plant, with modifying only one parameter in the model, the average bacterial mortality “kd” (fig. 4). BDOC decreases versus empty bed contact time (EBCT) calculated by the modal are presented on figure 6 for the Choisy-le-Roi and Neuilly-sur-Marne treatment plants and for two temperatures.From a management point of view, the minimum BDOC is reached for contact time between 15 and 20 minutes at Neuilly-sur-Marne, while at Choisy-le-Roi it is rather between 10 and 15 minutes.In conclusion, BDOC measurements and CHABROL modal constitute powerful tools for management and design of biological GAC filters

Studies of BDOC and bacterial dynamics in the drinking water distribution system of the Northern Parisian suburbs

La dégradation de la qualité de l'eau dans les réseaux de distribution, due à la reviviscence bactérienne, est, à l'heure actuelle, un souci majeur pour les producteurs d'eau potable. Dans ce contexte, une bonne connaissance des facteurs de contrôle du développement bactérien dans ce type de milieu s'avère nécessaire. Le but de la présente étude est de comprendre le rôle du carbone organique dissous biodégradable (CODB) dans la dynamique bactérienne en réseau de distribution. Cet article présente les résultats d'une étude en cours, lancée à l'initiative du Syndicat des Eaux de l'Ile de France, sur le réseau de distribution de la banlieue nord de Paris alimenté par l'usine de production de Méry-sur-Oise.Le CODB a été déterminé par la méthode de bioessai proposée par SERVAIS et al. (1987, 1989). La biomasse bactérienne libre a été estimée par microscopie à épifluorescence après coloration des bactéries à l'acridine orange, et la méthode d'incorporation de thymidine tritiée utilisée en écologie bactérienne a été adaptée, afin d'estimer la production bactérienne des bactéries présentes dans l'eau du réseau. De plus, la biomasse et l'activité des bactéries fixées ont été étudiées. Une méthode d'estimation de la biomasse, basée sur la mesure de l'activité exoprotéolytique potentielle des bactéries, a été développée. Pour l'estimation de la production bactérienne, la méthode d'incorporation de thymidine tritiée a été adaptée pour être utilisée pour les bactéries fixées.Les résultats obtenus mettent clairement en évidence une décroissance significative de la teneur en CODB dans les canalisations de faible diamètre dans la plupart des situations. Lorsque l'on porte la décroissance du CODB entre l'eau refoulée et l'eau présente dans les canalisations de faible diamètre en fonction du CODB dans l'eau refoulée, une corrélation significative est observée (fig. 1); l'intersection de la droite de corrélation avec l'abscisse indique la présence d'un seuil (environ 0,16 mgC.L-1) en-dessous duquel aucune décroissance de CODB n'est observée. Ce résultat, qui doit encore être confirmé, est important en vue de définir un objectif à atteindre en fin de filière, en terme de teneur en CODB.Dans l'eau refoulée, l'abondance bactérienne est proche de 1 x 104 cellules par mL. Dans le réseau de distribution, elle est toujours supérieure avec des valeurs observées allant jusqu'à 7 x 105 bact.mL-1; elle semble surtout liée à l'absence d'un résiduel de chlore libre (fig. 2). La température et la concentration en CODB dans l'eau refoulée sont aussi déterminantes comme le montre la figure 3 où l'abondance bactérienne dans les canalisations de faibles dia-mètres a été portée en fonction de la température pour deux gammes de concentration en CODB dans l'eau refoulée. Les taux de croissance des bactéries (calculés à .partir des estimations de production bactérienne et de biomasse) sont dans la gamme 0,005 à 0,1 h-1 (fig. 4), en l'absence de chlore libre ce qui correspond à des temps de génération compris entre 7 et 140 heures. La température semble fixer la valeur maximale du taux de croissance, sous ce maximum une large gamme de valeurs est observée traduisant la variabilité des conditions nutritionnelles. Les plus hauts taux de croissance observés dans le réseau sont proches des taux de croissance de bactéries mesurés dans les milieux aquatiques naturels.Les résultats obtenus sur les bactéries fixées montrent une biomasse bactérienne fixée dans la gamme 0,25 à 0,65 µgC.cm-2, ce qui correspond à une abondance de 1 x 107 à 2,6 x 107 bact.cm-2 (tableau 1). Ainsi donc, dans une canalisation de 100mm de diamètre, on peut dire que la biomasse fixée est, en moyenne, approximativement de 50 à 75 fois plus élevée que la biomasse moyenne des bactéiesJibres (2 x 105 bact.mL-1) (tableau 2). Le taux de croissance des bactéries fixées apparaît du même ordre de grandeur que celui des bactéries en suspension. Ceci signifie que dans un réseau de distribution, l'essentiel de la production de biomasse bactérienne s'effectue sur les parois des canalisations, les bactéries en suspension résultant principalement d'un décrochage de bactéries.Un modèle mathématique de la dynamique du CODB et des bactéries dans les réseaux de distribution, incluant les connaissances acquises concernant le contrôle de l'activité bactérienne par la matière organique dissoute, est actuellement développé (fig. 5). Il inclut une représentation mathématique des cinétiques des processus d'adsorption-désorption des bactéries (tableau 4), de fixation des bactéries, d'utilisation de la matière organique biodégradable et de la croissance bactérienne (tableau 3), ainsi que de l'impact du chlore libre sur les bactéries libres et fixées (fig. 6). Bien que préliminaire, il permet de simuler l'évolution longitudinale de la biomasse bactérienne libre et fixée, du CODB et du taux de chlore libre dans le cas simplifié d'une canalisation de diamètre fixé parcourue par un flux d'eau à vitesse constante (fig. 7). Une de ses applications permet de simuler l'impact de la teneur en CODB de l'eau injectée dans le réseau sur les biomasses libres et fixées (fig. 8).The deterioration of water quality in distribution systems due to bacterial regrowth is, at the present time, a major concern of drinking water producers. In this context, a good knowledge of the factors controlling bacterial development is required; the aim of the present study is to understand the rote of biodegradable dissolved organic carbon (BDOC) in the bacterial dynamics of the distribution system.This paper discusses the results obtained in a study carried out in order to assess the dynamics of biodegradable dissolved organic carbon and suspended bacteria in the water distribution system of the Northern Parisian suburbs lad by the Méry-sur-Oise treatment plant.The results show clearly that a significant decrease in BDOC occurs within the smallest pipes, when the BDOC level in the finished water is higher than about 0.20 mgC.L-1. However, no decrease in BDOC is observed when the BDOC in the finished water is lower than 0.16 mgC.L-1. The bacterial abundance in the distribution system is primarily linked to the absence of free chicane. Temperature and BDOC concentration in the finished water are also major controlling factors of bacterial numbers. Bacterial growth rates are in the range 0.005 to 0.1 h-1 in the absence of free chlorine, the highest of these values are in the same range as the growth rates measured for bacteria in natural aquatic ecosystems. Fixed biomass to the inner pipes surface are in the range 0.25 to 0.65 µgC.cm-2 and the average growth rate of fixed bacteria seems to be roughly in the same order of magnitude as the average growth rate of the suspended bacteria.A model of the dynamics of BDOC and bacteria in distribution network, incorporating the knowledge gained from this and previous studies concerning the control of bacterial activity by dissolved organic matter, is presented. It involves a mathematical representation of the kinetics of bacterial adsorption-desorption processes, bacterial attachment, bacterial utilization of biodegradable dissolved organic matter and impact of chlorine on free and fixed bacteria. It allows simulation of the impact of reducing the BDOC in the finished water on processes associated with bacterial regrowth in the distribution network.

Carbon Cycling of Lake Kivu (East Africa): Net Autotrophy in the Epilimnion and Emission of CO2 to the Atmosphere Sustained by Geogenic Inputs

We report organic and inorganic carbon distributions and fluxes in a large (>2000 km2) oligotrophic, tropical lake (Lake Kivu, East Africa), acquired during four field surveys, that captured the seasonal variations (March 2007–mid rainy season, September 2007–late dry season, June 2008–early dry season, and April 2009–late rainy season). The partial pressure of CO2 (pCO2) in surface waters of the main basin of Lake Kivu showed modest spatial (coefficient of variation between 3% and 6%), and seasonal variations with an amplitude of 163 ppm (between 579±23 ppm on average in March 2007 and 742±28 ppm on average in September 2007). The most prominent spatial feature of the pCO2 distribution was the very high pCO2 values in Kabuno Bay (a small sub-basin with little connection to the main lake) ranging between 11213 ppm and 14213 ppm (between 18 and 26 times higher than in the main basin). Surface waters of the main basin of Lake Kivu were a net source of CO2 to the atmosphere at an average rate of 10.8 mmol m−2 d−1, which is lower than the global average reported for freshwater, saline, and volcanic lakes. In Kabuno Bay, the CO2 emission to the atmosphere was on average 500.7 mmol m−2 d−1 (~46 times higher than in the main basin). Based on whole-lake mass balance of dissolved inorganic carbon (DIC) bulk concentrations and of its stable carbon isotope composition, we show that the epilimnion of Lake Kivu was net autotrophic. This is due to the modest river inputs of organic carbon owing to the small ratio of catchment area to lake surface area (2.15). The carbon budget implies that the CO2 emission to the atmosphere must be sustained by DIC inputs of geogenic origin from deep geothermal springs.AFRIVA

Finite element modelling of the Scheldt estuary and the adjacent Belgian/Dutch coastal zone with application to the transport of fecal bacteria

A fundamental problem in coastal modelling is the need to simultaneously consider large- and small-scale processes, especially when local dynamics or local environmental issues are of interest. The approach widely resorted to is based on a nesting strategy by which coarse grid large scale model provide boundary conditions to force fine resolution local models. This is probably the best solution for finite difference methods, needing structured grids. However, the use of structured grids leads to a marked lack of flexibility in the spatial resolution. Another solution is to take advantage of the potential of the more modern finite element methods, which allow the use of unstructured grids in which the mesh size may vary over a wide spectrum. With these methods only one model is required to describe both the larger and the smaller scales.Such a model is use herein, namely the Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM, http://www.climate.be/SLIM). For one of its first realistic applications, the Scheldt Estuary area is studied. The hydrodynamics is primarily forced by the tide and the neatest way to take it into account is to fix it at the shelf break. This results in a multi-scale problem since the domain boundary lies at the shelf break, and covers about 1000km of the North Sea and 60km of the actual estuary, and ends with a 100km long section of the Scheldt River until Ghent where the river is not more than 50 m wide.Two-dimensional elements are used to simulate the hydrodynamics from the shelf break to Antwerp (80km upstream of the mouth) and one-dimensional elements for the riverine part between Antwerp and Ghent.For first application we consider the transport of faecal bacteria (Escherichia coli) which is an important water quality indicator.The model will be described in detail and the simulation results will be discussed. This modelling exercise actually falls within the framework of the interdisciplinary project TIMOTHY (http://www.climate.be/TIMOTHY) dedicated to the modelling of ecological indicators in the Scheldt area