Impacts climat/écosystèmes : fiche de synthèse 03

L'impact des changements climatiques sur les écosystèmes est avéré et aura des répercussions importantes dans la région Languedoc-Roussillon qui est une des plus exposées à l'augmentation de la température et à la fréquence des sécheresses et des événements extrêmes (précipitations, incendies). Le fonctionnement des écosystèmes naturels et anthropisés sera perturbé en termes de phénologie, de répartition et de dynamiques d'évolution (dont flux d'énergie, migration, espèces envahissantes et maladies émergentes, etc.). La complexité de ces interactions nécessite de pouvoir comprendre et appréhender les évolutions à venir et pouvoir proposer des mesures d'atténuation et d'adaptation qui devront être mises en oeuvre dès que possible. Cela nécessitera une appropriation des enjeux par tous les acteurs et la mise en oeuvre des outils de gestion ad hoc: développement des connaissances, observatoire de l'environnement, conditionnalité des actions de développement au changement climatique. (Résumé d'auteur

Taxonomic and Environmental Variability in the Elemental Composition and Stoichiometry of Individual Dinoflagellate and Diatom Cells from the NW Mediterranean Sea

Here we present, for the first time, the elemental concentration, including C, N and O, of single phytoplankton cells collected from the sea. Plankton elemental concentration and stoichiometry are key variables in phytoplankton ecophysiology and ocean biogeochemistry, and are used to link cells and ecosystems. However, most field studies rely on bulk techniques that overestimate carbon and nitrogen because the samples include organic matter other than plankton organisms. Here we used X-ray microanalysis (XRMA), a technique that, unlike bulk analyses, gives simultaneous quotas of C, N, O, Mg, Si, P, and S, in single-cell organisms that can be collected directly from the sea. We analysed the elemental composition of dinoflagellates and diatoms (largely Chaetoceros spp.) collected from different sites of the Catalan coast (NW Mediterranean Sea). As expected, a lower C content is found in our cells compared to historical values of cultured cells. Our results indicate that, except for Si and O in diatoms, the mass of all elements is not a constant fraction of cell volume but rather decreases with increasing cell volume. Also, diatoms are significantly less dense in all the measured elements, except Si, compared to dinoflagellates. The N:P ratio of both groups is higher than the Redfield ratio, as it is the N:P nutrient ratio in deep NW Mediterranean Sea waters (N:P = 20–23). The results suggest that the P requirement is highest for bacterioplankton, followed by dinoflagellates, and lowest for diatoms, giving them a clear ecological advantage in P-limited environments like the Mediterranean Sea. Finally, the P concentration of cells of the same genera but growing under different nutrient conditions was the same, suggesting that the P quota of these cells is at a critical level. Our results indicate that XRMA is an accurate technique to determine single cell elemental quotas and derived conversion factors used to understand and model ocean biogeochemical cycles

Maximum in the Middle: Nonlinear Response of Microbial Plankton to Ultraviolet Radiation and Phosphorus

The responses of heterotrophic microbial food webs (HMFW) to the joint action of abiotic stressors related to global change have been studied in an oligotrophic high-mountain lake. A 2×5 factorial design field experiment performed with large mesocosms for >2 months was used to quantify the dynamics of the entire HMFW (bacteria, heterotrophic nanoflagellates, ciliates, and viruses) after an experimental P-enrichment gradient which approximated or surpassed current atmospheric P pulses in the presence vs. absence of ultraviolet radiation. HMFW underwent a mid-term (<20 days) acute development following a noticeable unimodal response to P enrichment, which peaked at intermediate P-enrichment levels and, unexpectedly, was more accentuated under ultraviolet radiation. However, after depletion of dissolved inorganic P, the HMFW collapsed and was outcompeted by a low-diversity autotrophic compartment, which constrained the development of HMFW and caused a significant loss of functional biodiversity. The dynamics and relationships among variables, and the response patterns found, suggest the importance of biotic interactions (predation/parasitism and competition) in restricting HMFW development, in contrast to the role of abiotic factors as main drivers of autotrophic compartment. The response of HMFW may contribute to ecosystem resilience by favoring the maintenance of the peculiar paths of energy and nutrient-mobilization in these pristine ecosystems, which are vulnerable to threats by the joint action of abiotic stressors related to global change.This research was supported by Junta de Andalucía (Excelencia P07-CVI-02598 to PC, and P09-RNM-5376 to JMMS), the Spanish Ministries of Medio Ambiente, Rural y Marino (PN2009/067 to PC) and Ciencia e Innovación (GLC2008-01127/BOS and CGL2011-23681 to PC), the ERC Advanced Grant project number 250254 “MINOS” (to GB), and two Spanish government grants (to JADM and FJB)

Seasonal variations of pico- and nano-detrital particles (DAPI Yellow Particles, DYP) in the Ligurian Sea (NW Mediterranean)

International audienceSeasonal variations of pico- and nano-sized marine detrital particles (DAPI Yellow Particles, DYP) and their relationships with components of the microbial food web were studied from April 1993 to March 1994 in the NW Mediterranean Sea. A hierarchical flexible clustering distinguished 2 major groups of DYP: less than or equal to 10 mu m and 10-20 mu m. Average abundance and total surface area of less than or equal to 10 mu m DYP in surface waters were 21.6 +/- 3.2 x 10(6) l(-1) and 153 +/- 55 mm(2) l(-1), respectively, approximately an order of magnitude greater than abundances or total surface areas reported for larger detrital particles. Relationships of DYP, chlorophyll a and micro-organisms were investigated within and among 5 different hydrographic periods distinguished via vertical temperature gradients. Peak concentrations of less than or equal to 10 mu m DYP occurred in early autumn at the end of stratification. In contrast, DYP 10-20 mu m peaked at the beginning of the stratification period in early June. Correlation analysis revealed only weak relationships between DYP and abundance of bacteria, heterotrophic microflagellates and ciliates. However, declines in chlorophyll were generally followed by increases in DYP concentrations

Rassoulzadegan@Villefranche : 3 Decades of Aquatic Microbial Ecology

International audienceFereidoun Rassoulzadegan began his career working on ciliate microzooplankton in the 1970's as one of the pioneers bringing microbial food webs into the limelight. His contributions over the past 30 odd years have spanned from the nutrition of tintinnid ciliates to the relationships between limiting nutrients and dissolved organic carbon dynamics. This year he will be assuming a new role, that of 'Emeritus', and to mark the occasion a special issue of Aquatic Microbial Ecology, composed of contributions from leading researchers in the field of microbial ecology, has been published

A simple method for the quantification of a class of labile marine pico- and nano-sized detritus: DAPI Yellow Particles (DYP)

International audienceA simple and rapid method for microscopic quantification of an abundant class of labile pico- and nano-sized detrital particles (0.2 to 20 mu m), using the fluorochrome stain DAPI, is described. Using an appropriate UV-filter set, examination of DAPI-stained samples revealed not only blue (DNA-containing) but also yellow particulate matter. We showed that DAPI yellow fluorescing particles resembled detritus made from plankton tow material or copepod faecal pellets and were almost exclusively organic, enzyme-degradable matter. Quantification of the total surface area of these particles in the Ligurian Sea (NW Mediterranean) revealed stocks averaging 80 mm(2) l(-1) in surface layer waters. The possible origin and fate of these pico- and nano-sized DAPI Yellow Particles (DYP) are discussed

Predation on marine picoplankton populations examined with an `add-in' approach

NW Mediterranean surface water was spiked with picoplankton prey (heterotrophic bacteria or cyanobacteria) or predators (bacterivorous microflagellates or ciliates) to investigate differential grazing pressure on picoplankton populations. Adding a particular prey type did not yield different growth patterns for heterotrophic bacteria and cyanobacteria, but save either similar, positive, effects on both picoplankton types or similar negative effects. Natural populations of both predator types increased with additions of cyanobacteria, but not heterotrophic bacteria. Ciliate additions gave marked decreases in cyanobacteria. While individual groups of grazers may preferentially consume cyanobacteria, selective grazing is probably not responsible for the maintenance of apparently stable populations of different groups of picoplankters during the summer

Pigment dynamics associated with the grazing of a ciliate and a flagellate feeding on a cyanobacterium

The egestion of particulate material as well as pigment degradation during microzooplankton grazing on phytoplankton are poorly known processes. In an attempt to evaluate these processes, changes in pigment concentrations within various size fractions were monitored in batch cultures of an assemblage of a pelagic ciliate (Strombidium sulcatum) and a heterotrophic flagellate (Paraphysomonas sp.) feeding on a cyanobacterium (Synechococcus sp.) over a 10-day period. Chlorophyll a, carotenoids and phaeopigments were not found in the 0.1-0.7 mu m fraction while the pigments originally in the 0.7-3.0 mu m fraction (prey) were transferred into the > 3.0 mu m size fraction (predator). During this transfer, the carotenoids (zeaxanthin and B-carotene) were not degraded significantly. In contrast, chlorophyll a was degraded into phaeophytin-like compounds which accounted for almost 100 % of the recorded phaeopigments. The destruction of chlorophyll a varied with time ranging from 4 % (day 3) to almost 100 % tend of the experiment) and this destruction was inversely related to micro-grazer ingestion rates. Microscopic examinations of samples did not reveal any large egested particles > 3.0 mu m, suggesting that phaeopigments and carotenoids measured in this size fraction were accumulated inside the protozoa. Zeaxanthin was very stable even when it was within the mice-grazer.L'égestion de matériel particulaire et la dégradation des pigments par suite du broutage microhétérotrophe sur le phytoplancton sont des processus peu documentés. Afin de les évaluer, les changements des concentrations pigmentaires dans différentes fractions de taille sont, étudiés, sur une période de dix jours, dans des cultures en batch d'un assemblage de cyanobactéries (Synechococcus sp.) de ciliés pélagiques (Strombidium sulcatum) et de flagellés hétérotrophes (Paraphysomonas sp.). Aucun pigment (chlorophylle a, caroténoïdes et phéopigments) n'est détecté dans la fraction de taille 0,1–0,7 μm. Les pigments initialement présents dans la fraction de taille 0,7–3,0 μm (proies) ont été transférés vers la fraction > 3,0 μm (prédateurs). Pendant le transfert, les caroténoïdes (zéaxanthine et β-carotène) ne sont pas dégradés de façon significative, alors que la chlorophylle a est dégradée principalement en composés de la famille des phéophytines qui représentent près de 100 % des phéopigments. La destruction de la chlorophylle a varie dans le temps de 4 % (jour 3) à 100 % (fin de l'expérience). Cette destruction est inversement liée aux taux d'ingestion des micro-brouteurs. L'étude microscopique des échantillons ne montre aucune éjesta particulaire > 3,0 μm. Cela suggère que les phéopigments et les caroténoïdes measurés dans cette fraction de taille sont accumulés à l'intérieur de protozoaires. Le zéaxanthine est très stable, même lorsqu'il se trouve à l'intérieur des micro-brouteurs