Assessing the importance of a self-generated detachment process in river biofilm models

1. Epilithic biofilm biomass was measured for 14 months in two sites, located up- and downstream of the city of Toulouse in the Garonne River (south-west France). Periodical sampling provided a biomass data set to compare with simulations from the model of Uehlinger, Bürher and Reichert (1996: Freshwater Biology, 36, 249–263.), in order to evaluate the impact of hydraulic disturbance. 2. Despite differences in application conditions (e.g. river size, discharge, frequency of disturbance), the base equation satisfactorily predicted biomass between low and high water periods of the year, suggesting that the flood disturbance regime may be considered a universal mechanism controlling periphyton biomass. 3. However modelling gave no agreement with biomass dynamics during the 7-month long low water period that the river experienced. The influence of other biomass-regulating factors (temperature, light and soluble reactive phosphorus) on temporal biomass dynamics was weak. 4. Implementing a supplementary mechanism corresponding to a temperature-dependent self-generated loss because of heterotrophic processes allowed us to accurately reproduce the observed pattern: a succession of two peaks. This case study suggests that during typical summer low water periods (flow stability and favourable temperature) river biofilm modelling requires self-generated detachment to be considered

Development of Bacterial Biofilms on Artificial Corals in Comparison to Surface-Associated Microbes of Hard Corals

Numerous studies have demonstrated the differences in bacterial communities associated with corals versus those in their surrounding environment. However, these environmental samples often represent vastly different microbial micro-environments with few studies having looked at the settlement and growth of bacteria on surfaces similar to corals. As a result, it is difficult to determine which bacteria are associated specifically with coral tissue surfaces. In this study, early stages of passive settlement from the water column to artificial coral surfaces (formation of a biofilm) were assessed. Changes in bacterial diversity (16S rRNA gene), were studied on artificially created resin nubbins that were modelled from the skeleton of the reef building coral Acropora muricata. These models were dip-coated in sterile agar, mounted in situ on the reef and followed over time to monitor bacterial community succession. The bacterial community forming the biofilms remained significantly different (R = 0.864 p<0.05) from that of the water column and from the surface mucus layer (SML) of the coral at all times from 30 min to 96 h. The water column was dominated by members of the α-proteobacteria, the developed community on the biofilms dominated by γ-proteobacteria, whereas that within the SML was composed of a more diverse array of groups. Bacterial communities present within the SML do not appear to arise from passive settlement from the water column, but instead appear to have become established through a selection process. This selection process was shown to be dependent on some aspects of the physico-chemical structure of the settlement surface, since agar-coated slides showed distinct communities to coral-shaped surfaces. However, no significant differences were found between different surface coatings, including plain agar and agar enhanced with coral mucus exudates. Therefore future work should consider physico-chemical surface properties as factors governing change in microbial diversity

The Athena X-ray Integral Field Unit: a consolidated design for the system requirement review of the preliminary definition phase

Instrumentatio

Réponse structurelle et fonctionnelle des communautés microbiennes hétérotrophes benthiques aux contaminants : quelle conséquence pour le fonctionnement de l'écosystème ?

National audienceBien que non exhaustives, les études citées dans ce chapitre illustrent la large palette d'outils dont disposent les écotoxicologues microbiens pour appréhender les effets des contaminants sur les communautés hétérotrophes benthiques, tant d'un point de vue structurel que fonctionnel. Cependant, une des finalités de l'écotoxicologie est d'appréhender le risque et l'impact des contaminants sur le fonctionnement des écosystèmes. Même s'il est établi que les communautés microbiennes benthiques jouent un rôle fonctionnel majeur dans les milieux aquatiques, il existe encore un besoin de recherche important pour transposer les conséquences des impacts écotoxicologiques observés à l'échelle de ces communautés à celle des écosystèmes et de leur fonctionnement

Viability of differentiated epilithic bacterial communities in the River Garonne (SW France)

National audienceEpilithic bacterial community viability was assessed on natural biofilm assemblages from environmentally contrasting locations over a 17-months period to determine if it reflects environmental conditions or conditions within the biofilm assemblage. Vital state was assessed by membrane integrity using LIVE/DEAD® BacLight staining kit. Samples were regularly collected in a large river, up and downstream of a large urban centre. Epilithic biomasses were similar between sites irrespective of the distinct water quality but varied temporarily, peaking up to 48 g AFDM m−2. Bacterial community composition assessed by 16S rDNA based PCR-DGGE significantly differed between sites. Bacterial densities (median of 2.5 × 1011 cell g AFDM−1) were stable whatever the sample origin or biomass. Viable bacterial fractions ranged between 13 and 83% of the total bacterial densities and were correlated with hydrological stability indicators (average of 41.9% during stable water periods, 62.4% during intermediate flow regimes and 50.0% during flow instability) and seasonal parameters. At the river section and epilithic community scales, consistent bacterial densities per unit of biomass could reflect a biofilm assemblage carrying capacity while variable membrane integrity likely integrates changes in the vital state of the community under changing environmental conditions

Défis méthodologiques et conceptuels autour des mesures de tolérance microbienne aux polluants pour mieux diagnostiquer l'impact de la contamination

National audienceLa biodiversité et le fonctionnement des écosystèmes sont régulièrement menacés par la présence d'une grande variété de polluants à des concentrations variables. Cette diversité des contaminants ainsi que leurs faibles concentrations peut compliquer l'identification des contaminants néfastes pour l'écosystème. Dans ce contexte, l'étude des communautés microbiennes peut fournir des outils et concepts intéressants afin d'identifier les contaminants altérant la structure ou le fonctionnement des communautés et ainsi estimer leur impact potentiel sur l'écosystème. En effet, la pollution chronique des écosystèmes agit comme une pression de sélection sur les communautés microbiennes, entraînant la disparition des espèces sensibles et favorisant les espèces résistantes. In fine, cette selection naturelle entraîne une augmentation de la tolérance de toute la communauté au contaminant auquel elle a été exposée, suivant le principe de l'acquisition de tolérance induite par la contamination (PICT : "Pollution induced community tolerance") popularisé par Blanck et al. (1988). En écotoxicologie des communautés microbiennes (notamment aquatiques), ce phénomène est utilisé comme une méthode a priori de détection des contaminants. Ainsi, plusieurs études ont mis en évidence le lien entre augmentation de la tolérance microbienne à un pesticide (e.g. diuron, Pesce et al., 2016) ou à un métal et la présence de ce contaminant dans l'environnement, en concentration et/ou fréquence suffisante pour induire des changements dans la communauté microbienne. Néanmoins, le développement de cette approche fait face à plusieurs défis méthodologiques et conceptuels. Tout d'abord, l'établissement de niveaux de tolérance de référence semble essentiel pour appliquer l'approche PICT en s'affranchissant de la comparaison avec un site de référence. Cela ne peut se faire sans une campagne à large échelle, incluant de nombreux sites de référence bien caractérisés et basée sur des tests de toxicité aiguë standardisés et adaptés aux communautés étudiées (périphyton, communauté microbienne attachée aux sédiments). Ensuite, les conséquences d'une acquisition de tolérance à un polluant pour le fonctionnement de la communauté microbienne ou de l'écosystème sont inconnues. En effet, l'approche PICT n'apporte aucune information sur l'état des communautés microbiennes ou leur vulnérabilité. D'une part, l'acquisition de tolérance à un contaminant peut protéger la communauté microbienne d'une contamination similaire suivant le principe de co-tolérance. D'autre part, le « coût » de l'adaptation permettant une augmentation de la tolérance de la communauté est généralement associée à une perte de biodiversité qui pourrait augmenter la vulnérabilité des communautés en cas de stress multiple (Loreau & de Mazancourt, 2013). Plusieurs études ont mis en avant le concept de co-tolérance microbienne entre différents stresseurs (Bonnineau et al., 2012; Tlili et al., 2010) mais des connaissances supplémentaires sont nécessaires pour évaluer plus largement la vulnérabilité à d'autres stresseurs d'une communauté microbienne résistantes à la contamination. Cette présentation a pour objectif de présenter le concept du PICT à travers quelques exemples pour ensuite proposer une discussion sur les liens entre adaptation, acquisition de tolérance, biodiversité et vulnérabilité des communautés microbiennes

Des concentrations environnementales de cuivre, seul ou en mélange avec l'arsenic, peuvent impacter la structure et les fonctions des communautés microbiennes du sédiment

International audienceIn many aquatic ecosystems, sediments are an essential compartment, which supports high levels of specific and functional biodiversity thus contributing to ecological functioning. Sediments are exposed to inputs from ground or surface waters and from surrounding watershed that can lead to the accumulation of toxic and persistent contaminants potentially harmful for benthic sediment-living communities, including microbial assemblages. As benthic microbial communities play crucial roles in ecological processes such as organic matter recycling and biomass production, we performed a 21-day laboratory channel experiment to assess the structural and functional impact of metals on natural microbial communities chronically exposed to sediments spiked with copper (Cu) and/or arsenic (As) alone or mixed at environmentally relevant concentrations (40 mg kg-1 for each metal). Heterotrophic microbial community responses to metals were evaluated both in terms of genetic structure (using ARISA analysis) and functional potential (using exoenzymatic, metabolic and functional genes analyses). Exposure to Cu had rapid marked effects on the structure and most of the functions of the exposed communities. Exposure to As had almost undetectable effects, possibly due to both lack of As bioavailability or toxicity toward the exposed communities. However, when the two metals were combined, certain functional responses suggested a possible interaction between Cu and As toxicity on heterotrophic communities. We also observed temporal dynamics in the functional response of sediment communities to chronic Cu exposure, alone or in mixture, with some functions being resilient and others being impacted throughout the experiment or only after several weeks of exposure. Taken together, these findings reveal that metal contamination of sediment could impact both the genetic structure and the functional potential of chronically exposed microbial communities. Given their functional role in aquatic ecosystems, it poses an ecological risk as it may impact ecosystem functioning