Estimating bacterial DNA synthesis from [3H] thymidine incorporation : discrepancies among macromolecular extraction procedures

The estimation of bacterial DNA synthesis in trophic studies with [3H] thymidine requires quantitative extraction of labeled DNA. To determine the DNA contribution to total macromolecular labeling in a eutrophic ecosystem, we tested three extraction procedures currently used to estimate DNA labeling from thymidine incorportation : enzymatic fractionation, acid-base hydrolysis, and phenol-chloroform extraction. Because labeled macromolecular fractions are generally defined as DNA, RNA, and proteins, we used incorporation of tritiated thymidine, uridine, and leucine to preferentially label DNA, RNA, and proteins. Our data showed that each fractionation method yielded different apparent macromolecular distributions of the same radiolabeled substrates. Enzymatic digestions of the fractions obtained by acid-base hydrolysis and phenol-chloroform extraction showed that these two procedures are inadequate for estimating bacterial DNA labeling in our ecosystem. Finally, using the enzymatic procedure at different sites, DNA labeling appeared to represent a nearly constant proportion of the labeled macromolecules (20,1%, r=0.952, n=101) over a wide range of incorporations rates. (Résumé d'auteur

Bacterioplankton production determined by DNA synthesis, protein synthesis, and frequency of dividing cells in Tuamotu atoll lagoons and surrounding ocean

This study compares three independent methods used for estimating bacterioplankton production in waters from the lagoon (mesotrophic) and the surrounding ocean (oligotrophic) of two atolls from the Tuamotu archipelago (French Polynesia). Thymidine and leucine incorporation were calibrated in dilution cultures and gave consistent results when the first was calibrated against cell multiplication and the second against protein synthesis. This study demonstrates that determining conversion factors strongly depends on the selected calculation method (modified derivative, integrative, and cumulative). These different estimates are reconciled when the very low proportion of active cells is accounted for. Frequency of dividing-divided cells (FDDC) calibrated using the same dilution cultures led to unrealistically high estimates of bacterial production. However, highly significant correlations between FDDC and either thymidine- or leucine-specific incorporation per cell were found in lagoon waters in situ. These correlations became more positive when oceanic data were added. This suggests that the FDDC method is also potentially valid to determine bacterioplankton growth rates after cross calibration with thymidine or leucine methods. If recommended precautions are observed, the three methods tested in the present study would give reliable production estimates. (Résumé d'auteur

Advantages of distinguishing the active fraction in bacterioplankton assemblages : some examples

La difficulté qu'il y a à distinguer les cellules bactériennes actives des cellules en dormance ou mortes est un problème important pour les microbiologistes des milieux aquatiques. L'INT (2- (p-iodophenyl)-3- (p-nitrophenyl)-5-phenyl tetrazolium chloride) incolore est réduit par la respiration cellulaire en INT-Formazan opaque. Les cellules bactériennes actives peuvent ainsi être distinguées, par examen au microscope, des cellules bactériennes mortes ou en dormance. Cette méthode appliquée dans le lac Léman et dans un bassin d'aquaculture de Côte d'Ivoire montre que la proportion des cellules bactériennes actives varie entre quelques pour cents et près de 100% selon le lieu, la saison, le niveau et l'heure de la journée. Dans tous les cas, l'activité bactérienne globale, évaluée par assimilation de Thymidine 3h, de glucose 14C ou par la fréquence des divisions cellulaires, est mieux corrélée au nombre de cellules réduisant l'INT qu'au nombre total de cellules. Il est ainsi prouvé que l'abondance de cellules bactériennes réduisant l'INT a une meilleure signification métabolique que l'abondance totale des cellules, qui peuvent être présentes, inactives en grande proportion dans les milieux aquatiques naturels. (Résumé d'auteur

Comparing taxonomic and morphological biodiversity of tintinnids (planktonic ciliates) of New Caledonia

Tintinnid ciliates are planktonic grazers of nanoplankton. They have a lorica (or shell) into which the ciliate cell can withdraw. The lorica provides information on both the identity and the ecology of the organism because characteristics of the lorica distinguish species and the diameter of the oral opening is related to the size of prey ingested. We examined the relationship between biodiversity estimates on the basis of classifying specimens as belonging to a species or a simple morphological group defined by lorica oral diameter (LOD) in a presumably species-rich area, a tropical lagoon. Two sites were sampled in the lagoon off Noumea over an annual cycle. The tintinnid fauna was species-rich (76 species) and represented a relatively even distribution of LOD sizes compared to other tropical and temperate sites. Median LOD varied with the fraction of the chlorophyll concentration by > 10 Am. Total chlorophyll concentration was related to tintinnid concentration and, in turn, weakly correlated with numbers of species and LOD size-classes. Numbers of species were closely related to numbers of LOD size-classes as were H' of species and H' (Shannon index) of LOD size-classes. Thus, metrics of a morphological characteristic, related to the ecology of the organisms, can be used to estimate species diversity

Natural recolonization of a productive tropical pond : day to day variations in the photosynthetic parameters

Chlorophyll pigments (CHL), primary productivity (PP) and particulate nitrogen (Np) in relation to several environmental factors were monitored during planktonic colonization of an aquaculture pond (Layo, Côte d'Ivoire). How interactions between the organisms are established in an initially azoic environment were investigated ... Overall, photosynthetic activity appeared to be closely linked to algal biomass. The study of autotrophic biomass and activity in different size classes in relation to the other parameters allowed us to precise the origin of the biomass fluctuations. The first bloom appeared to be controlled by selective grazing on small algae. The second algal development ended when N requirement represented at least 69% of N supply (in the N - NH4 form). This control was enhanced by the appearance of rotifers, leading to a more complex equilibrium. (D'après résumé d'auteur

Diel fluctuations of bacterial abundance and productivity in a shallow eutrophic tropical lagoon

Abundance and productivity (3H thymidine incorporation) of free-living and attached bacteria from surface waters were studied from hourly samples during typical hydrological conditions (rainy and flood seasons) in three different bays of the Ebrié lagoon (Côte d'Ivoire). Several additional biological variables (particulate organic carbon, phytoplankton biomass, zooplankton density) were also recorded in order to establish possible relation with bacterial parameters. (D'après résumé d'auteur

Conséquences biogéochimiques de l'atténuation de stratification dans une baie eutrophe : la baie de Biétri en lagune Ebrié (Côte d'Ivoire)

L'influence de l'affaiblissement annuel du gradient de densité sur l'hydrologie et la géochimie de la baie de Biétri (lagune Ebrié, Côte d'Ivoire) a été étudiée en suivant l'évolution des caractéristiques physiques et chimiques des eaux au cours de 45 sorties réparties sur 5 mois. A chaque sortie, température, salinité, transparence, oxygène dissous, potentiel redox et concentrations de N-NH4, N-NO2 et N-NO3 et P-PO4 ont été déterminés sur un profil vertical, au centre de la baie. La confrontation des données recueillies lors de cette étude avec les cycles de marées et la température de surface de la mer a permis de préciser les conditions requises pour un affaiblissement maximal du gradient de densité. Elles correspondent à l'atténuation du gradient de salinité, au début de la grande saison sèche, lors de la période de forte influence marine, et à l'installation d'une remontée côtière océanique refroidissant les eaux de surface, par le jeu des marée

Eutrophisation en milieu lagonaire : compte-rendu subvention CORDET no 94 T 09

La finalité du programme ANTROPIC est de dresser un bilan des conditions du milieu (eutrophisation, hypersédimentation) dans la zone la plus urbanisée du lagon de Tahiti, de proposer des outils de diagnostic de l'état de l'environnement lagonaire utiles pour la gestion du milie