Characterizing the Feeding Habits of the Testate Amoebae Hyalosphenia papilio and Nebela tincta along a Narrow "Fen-Bog" Gradient Using Digestive Vacuole Content and (13)C and (15)N Isotopic Analyses.

International audiencePopulation dynamics and feeding habits of the testate amoebae Nebela tincta and Hyalosphenia papilio were studied along a short "fen" to "bog" gradient in a Sphagnum-dominated mire (Jura, France). Samples were collected in living "top segments" (0-3cm) and early declining "bottom segments" (3-6cm) of Sphagnum fallax peat. Observations of digestive vacuole content and stable isotope analyses ((13)C and (15)N) were used to establish the feeding behavior of both testate amoeba species. Owing to their vertical distribution, the feeding habit of H. papilio was described from top segments, and that of N. tincta from bottom segments. Among identified food sources, those most frequently ingested by N. tincta were spores and mycelia of fungi (55%), microalgae (25%) and cyanobacteria (8.5%). For H. papilio, the most frequently ingested prey were ciliates (55%) and microalgae (35%). Nonmetric Multidimensional Scaling analysis clearly demonstrated that the two species did not have the same feeding habit along the "fen-bog" gradient, and furthermore that a significant spatial split exists in the feeding behavior of H. papilio. Additionally, isotope analyses suggested that H. papilio and N. tincta did not have the same trophic position in the microbial food web, probably resulting from their different feeding strategies

Phenoloxidase and peroxidase activities in Sphagnum-dominated peatland in a warming climate

International audiencePeatlands still suffer from the scarcity of available data about the characterization and the response to climate forcing of the main oxidative enzymes that occur over the seasons. In the present study, phenoloxidase and peroxidase activities were examined in Sphagnum lawns along a narrow fen-bog gradient under experimental elevated temperatures. We showed that peroxidase activities from Sphagnum mosses were 1000-fold higher than those of phenoloxidases irrespective of seasons and sampling areas. Peroxidase activities increased (+30%) with the rise of air temperatures (an average of 1 °C), while warming did not alter phenoloxidase activities. These results suggest that the monitoring of peroxidase activities in peatlands may represent a suitable and forward indicator of the impact of climate warming on carbon cycle in peatlands

Effect of a temperature gradient on Sphagnum fallax and its associated living microbial communities: a study under controlled conditions.

International audienceMicrobial communities living in Sphagnum are known to constitute early indicators of ecosystem disturbances, but little is known about their response (including their trophic relationships) to climate change. A microcosm experiment was designed to test the effects of a temperature gradient (15, 20, and 25°C) on microbial communities including different trophic groups (primary producers, decomposers, and unicellular predators) in Sphagnum segments (0-3 cm and 3-6 cm of the capitulum). Relationships between microbial communities and abiotic factors (pH, conductivity, temperature, and polyphenols) were also studied. The density and the biomass of testate amoebae in Sphagnum upper segments increased and their community structure changed in heated treatments. The biomass of testate amoebae was linked to the biomass of bacteria and to the total biomass of other groups added and, thus, suggests that indirect effects on the food web structure occurred. Redundancy analysis revealed that microbial assemblages differed strongly in Sphagnum upper segments along a temperature gradient in relation to abiotic factors. The sensitivity of these assemblages made them interesting indicators of climate change. Phenolic compounds represented an important explicative factor in microbial assemblages and outlined the potential direct and (or) indirect effects of phenolics on microbial communities

Occurrence and distribution of glycerol dialkyl glycerol tetraethers in a French peat bog

International audienceThe present study was aimed at examining the distribution and abundance of glycerol dialkyl glycerol tetraethers (GDGTs) of archaeal and bacterial origin in peat samples from surface and deep (ca. 50 cm) horizons of a peat bog in the Jura Mountains (northeastern France). Two principal types of GDGTs are present: extractable GDGTs, recoverable by solvent extraction, and non-extractable GDGTs, linked to the soil matrix. Within the extractable pool, “free” (i.e. core lipids) and “bound” (i.e. intact polar and/or ester-bound lipids) GDGTs can be distinguished. Extractable “free” and “bound” GDGTs were extracted using both accelerated solvent extraction (ASE) and a modified Bligh and Dyer technique. Both methods were shown to allow adequate extraction of “free” archaeal and bacterial GDGTs from soil samples. Both extraction protocols afforded similar relative distributions of archaeal and bacterial GDGTs, although poorer extraction of “bound” GDGTs was observed for ASE relative to Bligh and Dyer. Even though only low amounts of bacterial GDGTs were released after acid hydrolysis of solvent-extracted samples, non-extractable and total extractable GDGTs showed different distribution patterns in some samples. Consequently, these two lipid pools potentially reflect different proxy records of mean annual air temperature (MAAT) and pH. Last, the distribution of bacterial GDGTs differed between the different samples. Samples from deep horizons gave lower GDGT-derived MAAT values than those from surficial horizons, in agreement with measured soil temperatures at 7 cm and 50 cm depths from April to September. MAAT estimates more closely resemble spring and summer temperatures rather than annual soil temperature. The variability in bacterial GDGT distribution and resulting MAAT estimates probably also reflects the heterogeneity of peat samples and the variation in several environmental factors such as peat moisture level and oxygen availability

Experimental climate effect on seasonal variability of polyphenol/phenoloxidase interplay along a narrow fen-bog ecological gradient in Sphagnum fallax

International audienceExtracellular phenoloxidase enzymes play an important role in the stability of soil carbon storage by contributing to the cycling of complex recalcitrant phenolic compounds. Climate warming could affect peatland functioning through an alteration of polyphenol/phenoloxidase interplay, which could lead them to becoming weaker sinks of carbon. Here, we assessed the seasonal variability of total phenolics and phenoloxidases subjected to 2-3 °C increase in air temperature using open-top chambers. The measurements were performed along a narrow fen-bog ecological gradient over one growing season. Climate warming had a weak effect on phenoloxidases, but reduced phenolics in both fen and bog areas. Multivariate analyses revealed a split between the areas and also showed that climate warming exacerbated the seasonal variability of polyphenols, culminating in a destabilization of the carbon cycle. A negative relationship between polyphenols and phenoloxidases was recorded in controls and climate treatments suggesting an inhibitory effect of phenolics on phenoloxidases. Any significant decrease of phenolics through repeatedly elevated temperature would greatly impact the ecosystem functioning and carbon cycle through an alteration of the interaction of polyphenols with microbial communities and the production of extracellular enzymes. Our climate treatments did not have the same impact along the fen-bog gradient and suggested that not all the peatland habitats would respond similarly to climate forcing

Above- and belowground linkages in Sphagnum peatland: climate warming affects plant-microbial interactions

International audiencePeatlands contain approximately one third of all soil organic carbon (SOC). Warming can alter above- and belowground linkages that regulate soil organic carbon dynamics and C-balance in peatlands. Here we examine the multiyear impact of in situ experimental warming on the microbial food web, vegetation, and their feedbacks with soil chemistry. We provide evidence of both positive and negative impacts of warming on specific microbial functional groups, leading to destabilization of the microbial food web. We observed a strong reduction (70%) in the biomass of top-predators (testate amoebae) in warmed plots. Such a loss caused a shortening of microbial food chains, which in turn stimulated microbial activity, leading to slight increases in levels of nutrients and labile C in water. We further show that warming altered the regulatory role of Sphagnum-polyphenols on microbial community structure with a potential inhibition of top predators. In addition, warming caused a decrease in Sphagnum cover and an increase in vascular plant cover. Using structural equation modelling, we show that changes in the microbial food web affected the relationships between plants, soil water chemistry, and microbial communities. These results suggest that warming will destabilize C and nutrient recycling of peatlands via changes in above- and belowground linkages, and therefore, the microbial food web associated with mosses will feedback positively to global warming by destabilizing the carbon cycle. This study confirms that microbial food webs thus constitute a key element in the functioning of peatland ecosystems. Their study can help understand how mosses, as ecosystem engineers, tightly regulate biogeochemical cycling and climate feedback in peatland

To What Extent Do Food Preferences Explain the Trophic Position of Heterotrophic and Mixotrophic Microbial Consumers in a Sphagnum Peatland?

Although microorganisms are the primary drivers of biogeochemical cycles, the structure and functioning of microbial food webs are poorly studied. This is the case in Sphagnum peatlands, where microbial communities play a key role in the global carbon cycle. Here, we explored the structure of the microbial food web from a Sphagnum peatland by analyzing (1) the density and biomass of different microbial functional groups, (2) the natural stable isotope (δ 13C and δ 15N) signatures of key microbial consumers (testate amoebae), and (3) the digestive vacuole contents of Hyalosphenia papilio, the dominant testate amoeba species in our system. Our results showed that the feeding type of testate amoeba species (bacterivory, algivory, or both) translates into their trophic position as assessed by isotopic signatures. Our study further demonstrates, for H. papilio, the energetic benefits of mixotrophy when the density of its preferential prey is low. Overall, our results show that testate amoebae occupy different trophic levels within the microbial food web, depending on their feeding behavior, the density of their food resources, and their metabolism (i.e., mixotrophy vs. heterotrophy). Combined analyses of predation, community structure, and stable isotopes now allow the structure of microbial food webs to be more completely described, which should lead to improved models of microbial community functio

Chapitre 7 - Les relations entre ville et agriculture au prisme de l’innovation territoriale

Résumé. Le concept d’innovation territoriale est mobilisé dans la littérature pour analyser les rapports entre centre et périphérie, la qualité des milieux et la gouvernance territoriale. Nos recherches reprennent ce concept pour saisir les multiples dimensions des relations entre ville et agriculture et pour comprendre ainsi les transformations de l’agriculture dans le contexte de la société urbaine. Nous analysons pour cela les agencements sociaux, spatiaux et organisationnels qui s’opèrent dans les initiatives agri-urbaines locales. À partir d’une chronique de la place prise par l’agriculture dans l’aménagement urbain et dans les politiques locales, l’exemple de Montpellier permet d’illustrer comment ces agencements agri-urbains sont sources d’innovation territoriale. En effet, l’innovation devient territoriale par accumulation de micro-changements, qui finissent par infléchir des fonctionnements établis dans les usages et les normes qui régulent les relations entre ville et agriculture. Ce processus de passage à une plus grande échelle (scaling up) ouvre un champ de recherche sur les relations entre innovations territoriales et transitions globales

Innovation et développement dans les systèmes agricoles et alimentaires

L’innovation est souvent présentée comme l’un des principaux leviers pour promouvoir un développement plus durable et plus inclusif. Dans les domaines de l’agriculture et de l’alimentation, l’innovation est marquée par des spécificités liées à sa relation à la nature, mais aussi à la grande diversité d’acteurs concernés, depuis les agriculteurs jusqu’aux consommateurs, en passant par les services de recherche et de développement. L’innovation émerge des interactions entre ces acteurs, qui mobilisent des ressources et produisent des connaissances dans des dispositifs collaboratifs, afin de générer des changements. Elle recouvre des domaines aussi variés que les pratiques de production, l’organisation des marchés, ou les pratiques alimentaires. L’innovation est reliée aux grands enjeux de développement : innovation agro-écologique, innovation sociale, innovation territoriale, etc. Cet ouvrage porte un regard sur l’innovation dans les systèmes agricoles et alimentaires. Il met un accent particulier sur l’accompagnement de l’innovation, en interrogeant les méthodes et les organisations, et sur l’évaluation de l’innovation au regard de différents critères. Il s’appuie sur des réflexions portées par différentes disciplines scientifiques, sur des travaux de terrain conduits tant en France que dans de nombreux pays du Sud, et enfin sur les expériences acquises en accompagnant des acteurs qui innovent. Il combine des synthèses sur l’innovation et des études de cas emblématiques pour illustrer les propos. L’ouvrage est destiné aux enseignants, professionnels, étudiants et chercheurs