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

Structural and Functional Characterization of Mature Forms of Metalloprotease E495 from Arctic Sea-Ice Bacterium Pseudoalteromonas sp. SM495

E495 is the most abundant protease secreted by the Arctic sea-ice bacterium Pseudoalteromonas sp. SM495. As a thermolysin family metalloprotease, E495 was found to have multiple active forms in the culture of strain SM495. E495-M (containing only the catalytic domain) and E495-M-C1 (containing the catalytic domain and one PPC domain) were two stable mature forms, and E495-M-C1-C2 (containing the catalytic domain and two PPC domains) might be an intermediate. Compared to E495-M, E495-M-C1 had similar affinity and catalytic efficiency to oligopeptides, but higher affinity and catalytic efficiency to proteins. The PPC domains from E495 were expressed as GST-fused proteins. Both of the recombinant PPC domains were shown to have binding ability to proteins C-phycocyanin and casein, and domain PPC1 had higher affinity to C-phycocyanin than domain PPC2. These results indicated that the domain PPC1 in E495-M-C1 could be helpful in binding protein substrate, and therefore, improving the catalytic efficiency. Site-directed mutagenesis on the PPC domains showed that the conserved polar and aromatic residues, D26, D28, Y30, Y/W65, in the PPC domains played key roles in protein binding. Our study may shed light on the mechanism of organic nitrogen degradation in the Arctic sea ice

Dissimilar responses of fungal and bacterial communities to soil transplantation simulating abrupt climate changes.

Both fungi and bacteria play essential roles in regulating soil carbon cycling. To predict future carbon stability, it is imperative to understand their responses to environmental changes, which is subject to large uncertainty. As current global warming is causing range shifts toward higher latitudes, we conducted three reciprocal soil transplantation experiments over large transects in 2005 to simulate abrupt climate changes. Six years after soil transplantation, fungal biomass of transplanted soils showed a general pattern of changes from donor sites to destination, which were more obvious in bare fallow soils than in maize cropped soils. Strikingly, fungal community compositions were clustered by sites, demonstrating that fungi of transplanted soils acclimatized to the destination environment. Several fungal taxa displayed sharp changes in relative abundance, including Podospora, Chaetomium, Mortierella and Phialemonium. In contrast, bacterial communities remained largely unchanged. Consistent with the important role of fungi in affecting soil carbon cycling, 8.1%-10.0% of fungal genes encoding carbon-decomposing enzymes were significantly (p < 0.01) increased as compared with those from bacteria (5.7%-8.4%). To explain these observations, we found that fungal occupancy across samples was mainly determined by annual average air temperature and rainfall, whereas bacterial occupancy was more closely related to soil conditions, which remained stable 6 years after soil transplantation. Together, these results demonstrate dissimilar response patterns and resource partitioning between fungi and bacteria, which may have considerable consequences for ecosystem-scale carbon cycling

Remineralization of particulate organic carbon in an ocean oxygen minimum zone

This work was funded by a NERC standard grant NE/E01559X/1

The role of iron in the bacterial degradation of organic matter derived from <i>Phaeocystis antarctica</i>

In high-nutrient low-chlorophyll areas, bacterial degradation of organic matter may be iron-limited. The response of heterotrophic bacteria to Fe addition may be directly controlled by Fe availability and/or indirectly controlled through the effect of enhanced phytoplankton productivity and the subsequent supply of organic matter suitable for bacteria. In the present study, the role of Fe on bacterial carbon degradation was investigated through regrowth experiments by monitoring bacterial response to organic substrates derived from Phaeocystis antarctica cultures set up in P. antarctica and on the quality of Phaeocystis-derived organic matter. Fe addition leaded to a decrease of C/N ratio of Phaeocystis material. The bacterial community composition was modified as observed from denaturing gradient gel electrophoresis (DGGE) profiles in LFe as compared to HFe bioassays. The percentage of active bacteria as well as their specific metabolic activities (ectoenzymatic hydrolysis, growth rates and bacterial growth efficiency) were enhanced in HFe bioassays. As a consequence, the lability of Phaeocystis-derived organic matter was altered, i.e., after seven days more than 90% was degraded in HFe and only 9% (dissolved) and 55% (total) organic carbon were degraded in LFe bioassays. By inducing increased bacterial degradation and preventing the accumulation of dissolved organic carbon, the positive effect of Fe supply on the carbon biological pump may partly be counteracted

Phaeocystis blooms in the global ocean and their controlling mechanisms: A review

Phaeocystis is a genus of marine phytoplankton with a world-wide distribution. It has a polymorphic life cycle alternating free-living cells and colonies but develops massive blooms under the colony form in nutrient (major)-enriched areas (mostly nitrates) of the global ocean. Among the 6 species, only 3 (P. pouchetii, P. antarctica, P. globosa) have been reported as blooming species. However information on the present-day contribution of Phaeocystis-dominated ecosystems to regional and global marine production as well as to global C and S cycles and how these might change in the future is lacking. As a first step in this direction this review synthesises knowledge on the physiology, growth and fate of Phaeocystis in the global ocean. Emphasis is given on identifying environmental and biological factors favourable to bloom formation with the aim of selecting relevant parameters for implementing a mechanistic model describing Phaeocystis blooms in the global ocean. It also aims to identify missing information concerning regulation of most important bottom-up and top-down processes. The synthesis of published data suggests that it is possible t

Modeling phytoplankton blooms and carbon export production in the Southern Ocean:dominant controls by light and iron in the Atlantic sector in Austral spring 1992

The high nutrient low chlorophyll (HNLC) conditions of the Southern Ocean were explored with an ecological model (SWAMCO) describing the cycling of C, N, P, Si and Fe through different, aggregated, chemical and biological compartments of the plankton ecosystem. The structure of the model was chosen to take explicitly into account biological processes of importance in the formation and mineralization of carbon biomass in surface waters and in carbon export production. State variables include major inorganic nutrients (NO3, NH4, PO4, Si(OH)4), dissolved Fe, two groups of phytoplankton (diatoms and nanoflagellates), bacteria, heterotrophic nanoflagellates, microzooplankton, labile DOC and two classes of dissolved and particulate organic polymers with specific biodegradability. The model is closed by export production of particulate organic matter out of the surface layer and, when relevant, by metazooplanton, the grazing pressure of which is described as a forcing function. Parameterization was derived from the current knowledge on the kinetics of biological processes in the Southern Ocean and in other 'HNLC' areas. For its application in the Atlantic sector in spring 1992, the SWAMCO model was coupled 'off-line' to a 1D physical model forced by in situ meteorological and sea-ice conditions. The predictions of the model were successfully compared with chemical and biological observations recorded in the Antarctic circumpolar current (ACC) during the 1992 cruise ANTX/6 of RV Polarstern. In particular, the model simulates quite well the diatom bloom and carbon export event observed in the iron-enriched Polar Frontal region and the lack of ice-edge phytoplankton blooms in the marginal zone (MIZ) of the ACC area. Model analysis shows that sufficient light and iron concentrations above 0.5 μmol m-3 are the necessary conditions for enhancing diatom blooms and particulate carbon export production in the Southern Ocean. Low iron availability prevents diatom growth but is still adequate for nanophytoplankton, the biomass of which is, however, kept to Chl a levels less than 1 mg m-3 due to the loss by the ubiquitous micrograzers. Little carbon export is predicted under iron-limitation conditions. Sensitivity tests conducted on the parameters describing iron and silicon uptake by diatoms reveal the complex nature of Fe and Si limitation in regulating the magnitude and extent of diatom blooms and carbon and opal export production in the Southern Ocean. (C) 2000 Elsevier Science Ltd.info:eu-repo/semantics/publishe