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)

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

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

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

Elemental composition of individual pico- and nano-sized marine detrital particles in the northwestern Mediterranean Sea

The elemental composition of individual < 10 mu m detrital particles from Mediterranean surface waters was analysed using a Transmission Electron Microscope (TEM) equipped with an energy dispersive X-Ray microanalyser. Results show that carbon and phosphorus content per detritus volume are much higher in pico-detrital particles < 2 mu m (42 kg C m(-3) and 1 kg P m(-3)) than in 5-10 mu m detrital particles (20 kg C m-3 and 0.1 kg P m(-3)). The C:N:P atomic ratios for different sized fractions of the detrital particles were found to be 82:10:1 for < 2 mu m particles, 120:29:1 for 2-5 mu m particles and 308:37:1 for 5-10 mu m particles. The average ratio for all size classes of detrital particles (< 10 mu m) was 132:23:1. The differences in elementary compositions of the detrital particles studied here suggest that the different size fractions probably have different origins. The role and origins of < 10 mu m detrital particles within the biogeochemical cycles are discussed.La composition élémentaire des particules détritiques < 10 μm provenant de l'eau de surface de la Méditerranée a été analysée à l'aide d'un microscope électronique à transmission, équipé d'un micro-analyseur aux rayons X. Les résultats montrent que les quantités de carbone et de phosphore par volume de détritus sont plus importantes pour les pico-détritus < 2 μm (42 kg C m−3 et 1 kg P m−3) par rapport à la classe de taille 5–10 μm (20 kg C m−3 et 0,1 kg P m−3). Le rapport atomique de C:N:P pour les différentes fractions de taille des particules détritiques est de 82:10:1 pour les détritus < 2 μm, de 120:29:1 pour les détritus 2–5 μm et de 308:37:1 pour la classe de taille de 5–10 μm. Le rapport moyen pour toutes les classes de taille de détritus < 10 μm est de 132:23:1. Les différences de composition élémentaire des détritus étudiés ici suggère que les différences fractions de taille ont probablement différentes origines. Le rôle et l'origine des détritus < 10 μm sont discutés dans le cadre des cycles biogéochimiques