Some mixotrophic flagellate species selectively graze on Archaea

Altres ajuts: Spanish Ministry of the Environment project EGALA (OAPN-124/2010)Many phototrophic flagellates ingest prokaryotes. This mixotrophic trait becomes a critical aspect of the microbial loop in planktonic food webs because of the typical high abundance of these flagellates. Our knowledge of their selective feeding upon different groups of prokaryotes, particularly under field conditions, is still quite limited. In this study, we investigated the feeding behavior of three species (Rhodomonas sp., Cryptomonas ovata, and Dinobryon cylindricum) via their food vacuole content in field populations of a high mountain lake. We used the catalyzed reporter deposition-fluorescence in situ hybridization (CARD-FISH) protocol with probes specific for the domain Archaea and three groups of Eubacteria: Betaproteobacteria, Actinobacteria, and Cytophaga-Flavobacteria of Bacteroidetes. Our results provide field evidence that contrasting selective feeding exists between coexisting mixotrophic flagellates under the same environmental conditions and that some prokaryotic groups may be preferentially impacted by phagotrophic pressure in aquatic microbial food webs. In our study, Archaea were the preferred prey, chiefly in the case of Rhodomonas sp., which rarely fed on any other prokaryotic group. In general, prey selection did not relate to prey size among the grazed groups. However, Actinobacteria, which were clearly avoided, mostly showed a size of <0.5 μm, markedly smaller than cells from the other groups

Efectos de la actividad ganadera en los lagos del Parque Nacional de Aigüestortes i Estany de Sant Maurici: aportes de nitrógeno y riesgo de eutrofización

Este capítulo contiene 16 páginas, 2 tablas, 11 figuras.El proyecto EGALA estudió el efecto de la actividad ganadera en los lagos del Parque Nacional de Aigüestortes i Estany de Sant Maurici especialmente por los aportes de N que representa y el riesgo de eutrofización que éste puede provocar. La cuestión se planteó a tres escalas espacio-temporales distintas: a escala de paisaje lacustre (en un conjunto de 19 cuencas lacustres) a escala de cuenca lacustre (en un conjunto de 12 lagos de la Vall de Sant Nicolau) y a escala de lago (valorando los efectos a lo largo de un transecto horizontal de la orilla al centro del lago). La concentración de N de los lagos no depende de manera simple de unos pocos factores. Aunque las variables relacionadas con la ganadería explican una fracción marginal de la variancia de las concentraciones de N, no parecen tener un papel significativo en la biogeoquímica de los lagos a escala de paisaje lacustre. Los lagos de mayor altitud con cuencas pequeñas y poco desarrollo de la vegetación y el suelo, acumulan una mayor cantidad de nitrato proveniente del depósito atmosférico. De manera opuesta los lagos situados en el fondo de los valles, con cuencas mayores cubiertas de pastos donde se concentra el ganado, presentan una menor concentración de nitrato en sus aguas, debido a una mayor retención del depósito en los suelos y la vegetación. Aunque la estima de la carga de N que moviliza anualmente la actividad ganadera en las distintas cuencas raramente supera el 20% del depósito atmosférico, se comprobó que es de una magnitud similar a la carga que realmente entra al lago. Esto podría indicar la importancia de la presencia de ganado en la movilización de un N que permanece retenido en la cuenca. Los lagos son sumideros de N, retienen las formas más oxidadas y exportan en menor medida las formas reducidas. El efecto directo de la presencia de ganado es detectable en la orilla del lago, no solo por el impacto físico y la limitación del desarrollo de la comunidad de macrófitos propia del sistema; sino porque se localiza una zona con mayores concentraciones de N, composición isotópica diferenciada y elevadas actividades microbianas. Por dilución y por mezcla con la masa de agua del lago, el efecto se diluye rá- pidamente al aumentar la profundidad de la columna de agua. En este proyecto se han encontrado evidencias de que el ganado tiene influencia sobre el estado trófico de los lagos, y aunque actualmente en general sea baja, se conoce que hay ciertas masas de agua más sensibles y que en tiempos pasados la presencia humana fue mayor y pudo producir un mayor grado de eutrofización de las aguas.El proyecto EGALA ha estado financiado por el Organismo Autónomo Parques Nacionales (nú- mero de referencia 124/2010).Peer reviewe

Deployment of ENEX enclosures in high‐mountain lake Redon (Spain)

The ENEX experiment in Lake Redon (Pyrenees) investigated the relationship between the changes in plankton stoichiometry, productivity, and community structure occurring during nutrient fluctuations in P‐limited conditions. Columnar enclosures were used for treatments varying P availability, N:P imbalance, and N source. C:N:P seston ratios were stable in P‐limited conditions, with loose coupling with productivity, nutrient supply ratios, and species dominance. The stoichiometric ratios shifted to Redfield proportions in P‐repleted conditions. The results suggest a complex regulation of P scarcity in planktonic communities that goes beyond immediate acclimation growth responses and might include alternative physiological and biogeochemical states

Microbial food web components, bulk metabolism, and single-cell physiology of piconeuston in surface microlayers of high-altitude lakes

12 páginas, 4 tablas, 3 figuras.Sharp boundaries in the physical environment are usually associated with abrupt shifts in organism abundance, activity ,and diversity. Aquatic surface microlayers (SML) from a steep gradient between two contrasted environments, the atmosphere and surface waters, where they regulate the gas exchange between both environments. They usually harbor an abundant and active microbial life: the neuston. Few ecosystems are subjected to such a high UVR regime as high altitude lakes during summer. Here, we measured bulk estimates of heterotrophic activity, community structure and single-cell physiological properties by flow cytometry in 19 high-altitude remote Pyrenean lakes and compared the biological processes in the SML with those in the underlying surface waters. Phototrophic picoplankton (PPP) populations, were generally present in high abundances and in those lakes containing PPP populations with phycoerythrin (PE), total PPP abundance was higher at the SML .Heterotrophic nanoflagellates (HNF) were also more abundant in the SML. Bacteria in the SML had lower leucine incorporation rates, lower percentages of “live” cells, and higher numbers of highly-respiring cells ,likely resulting in a lower growth efficiency. No simple and direct linea rrelationships could be found between microbial abundances or activities and environmental variables, but factor analysis revealed that, despite their physical proximity ,microbial life in SML and underlyingwaters was governed by different and independent processes. Overall, we demonstrate that piconeuston in high altitude lakes has specific features different from those of the picoplankton ,and that they are highly affected by potential stressful environmental factors, such as high UVR radiation.This work was supported by grants 2009SGR/1177 “Grup d’estructura i funcio de xarxes tròfiques microbianes planctòniques” and 2009SGR/361 “Grup d’Ecologia dels Canvis Ambientals, GECA” from Generalitat de Catalunya, AERBAC-2 178/2010 (to EOC) and EGALA 124/2010 (MF, LC) from the Ministerio de Medio Ambiente- Red de Parques Nacionales, and PIRENA CGL2009-13318 (EOC) and NITROPIR CGL2010-19373 (MF, LC) from MINECO. HS’s work was supported by CNPq and FAPESP (Process: 2014/ 14139-3).Peer reviewe

Episodic nutrient enrichments stabilise protists coexistence in planktonic oligotrophic conditions

1. Seasonal compositional changes in plankton communities are usually considered as species replacements. Provided the enormous number of individuals integrating the communities and our limited capacity for counting and determining a substantial proportion of them, it may happen that we would only be observing alternative population peaks of a large number of coexisting species. The contemporary coexistence theory addresses coexistence in communities of competing species considering simultaneously relative fitness inequalities and stabilising niche differences as components of average long-term growth rates. Here, we experimentally show that response patterns predicted by the theory occur when varying nutrient pulses fertilise the planktonic community. 2. We used gently self-filling 100 L enclosures to minimise disturbance of the initial community and different pulse P and N additions to manipulate the apparently species-poor epilimnetic community of an ultraoligotrophic P-limited lake. We measured and compared protist species growth response to a gradient of P enrichment and another of N stoichiometric imbalance. The P and N levels selected were within the oligotrophic seasonal and interannual variation of the lake, plus some extreme treatments providing mesotrophic conditions of remote regions affected by N atmospheric contamination. All treatments were replicated using ammonium or nitrate alternatively as N source. 3. Most protist species ¿ recorded across seasons in previous studies in this lake ¿ were recovered, indicating a persistent assemblage of species that is seasonally-hidden to observation. Recovery included some rare species only observed in the slush layers of the seasonal snow and ice cover. Coexistence stabilising mechanisms were indicated by treatment response features such as frequency-dependent growth, inverse relationship between fitness inequality and niche differentiation proxies, high-rank taxonomic levels clustering across the limiting nutrient gradient but segregation at the species level according to the type of nitrogen supply, and resting stage development depending on nutrient conditions. Response similarities between autotrophic and heterotrophic organisms indicate a network of interactions that may reinforce coexistence. 4. Synthesis. The results indicate that many planktonic protist species in oligotrophic waters can show stable long-term non-equilibrium coexistence by alternately recovering from very low densities when episodic nutrient enrichments ¿ of varying P and N amounts and composition ¿ occur

Homeostasis and non-linear shift in the stoichiometry of P-limited planktonic communities

Planktonic communities are naturally subjected to episodic nutrient enrichments that may stress or redress the imbalances in limiting nutrients. Human‐enhanced atmospheric nitrogen deposition has caused profound N:P imbalance in many remote oligotrophic lakes in which phosphorus has largely become limiting. These lakes offer an opportunity to investigate the relationship between the changes in plankton stoichiometry, productivity, and community structure occurring during nutrient fluctuations in P‐limited conditions. We performed P ( PO3‐4) and N ( NH+4 or NO‐3) pulse additions to the summer epilimnetic community of an ultraoligotrophic lake using self‐filling ~100‐L enclosures and analyzed the response to varying P availability, N:P imbalance, and N source. Seston C:N:P proportions remained fairly unchanged to P additions that were within the range of values seasonally found in the lake. However, the seston N:P ratio abruptly shifted and approached Redfield's proportions at P additions typical of mesotrophic conditions that provided non‐limiting conditions. N surplus did not affect seston C:N:P proportions. The patterns of seston N:P stability and shift were similar for both N sources. In contrast, productivity was highly sensitive to low and medium P additions and decelerated at high P additions. Phytoplankton biomass dominated particulate organic matter. The autotrophic community differentiated almost linearly across the P gradient. Chrysophytes' dominance decreased, and diatoms and cryptophytes relative abundance increased. Nonetheless, the stoichiometry stability and non‐linear shift involved large biomass proportions of the same species, which indicates that the bulk stoichiometry was related to similar physiological behavior of phylogenetically diverse organisms according to the biogeochemical context. The C:N:P seston stability in P‐limited conditions with loose coupling with productivity, nutrient supply ratios, and species dominance and the sudden shift to Redfield proportions in P‐repleted conditions suggest a complex regulation of P scarcity in planktonic communities that goes beyond immediate acclimation growth responses and might include alternative physiological and biogeochemical states

Bacterioplankton seasonality in deep high-mountain lakes

Due to global warming, shorter ice cover duration might drastically affect the ecology of lakes currently undergoing seasonal surface freezing. Highmountain lakes show snow-rich ice covers that determine contrasting conditions between ice-off and ice-on periods. We characterized the bacterioplankton seasonality in a deep high-mountain lake ice-covered for half a year. The lake shows a rich core bacterioplankton community consisting of three components: (i) an assemblage stable throughout the year, dominated by Actinobacteria, resistant to all environmental conditions; (ii) an ice-on-resilient assemblage dominating during the ice-covered period, which is more diverse than the other components and includes a high abundance of Verrucomicrobia; the deep hypolimnion constitutes a refuge for many of the typical under-ice taxa, many of which recover quickly during autumn mixing; and (iii) an ice-off-resilient assemblage, which members peak in summer in epilimnetic waters when the rest decline, characterized by a dominance of Flavobacterium, and Limnohabitans. The rich core community and low random elements compared to other relatively small cold lakes can be attributed to its simple hydrological network in a poorly-vegetated catchment, the long water-residence time (ca. 4 years), and the long ice-cover duration; features common to many headwater deep high-mountain lakes

Bacterioplankton seasonality in deep high-mountain lakes

Due to global warming, shorter ice cover duration might drastically affect the ecology of lakes currently undergoing seasonal surface freezing. High-mountain lakes show snow-rich ice covers that determine contrasting conditions between ice-off and ice-on periods. We characterized the bacterioplankton seasonality in a deep high-mountain lake ice-covered for half a year. The lake shows a rich core bacterioplankton community consisting of three components: (i) an assemblage stable throughout the year, dominated by Actinobacteria, resistant to all environmental conditions; (ii) an ice-on-resilient assemblage dominating during the ice-covered period, which is more diverse than the other components and includes a high abundance of Verrucomicrobia; the deep hypolimnion constitutes a refuge for many of the typical under-ice taxa, many of which recover quickly during autumn mixing; and (iii) an ice-off-resilient assemblage, which members peak in summer in epilimnetic waters when the rest decline, characterized by a dominance of Flavobacterium, and Limnohabitans. The rich core community and low random elements compared to other relatively small cold lakes can be attributed to its simple hydrological network in a poorly-vegetated catchment, the long water-residence time (ca. 4 years), and the long ice-cover duration; features common to many headwater deep high-mountain lakes

Self-filling enclosures to experimentally assess plankton response to pulse nutrient enrichments

Experimental nutrient additions are a fundamental approach to investigating plankton ecology. Possibilities range from whole-lake fertilization to flask assays encompassing a trade-off between closeness to the "real world" and feasibility and replication. Here we describe an enclosure type that minimizes the manipulation of planktonic communities during the enclosure filling. The enclosure (typically ~100 L volume) consists of a narrow translucent cylinder that can comprise the entire photic zone (or a large part of it in clear deep lakes, e.g. 20-m long) and holds a sediment trap at the bottom for recovering the sinking material. The enclosures are inexpensive and straightforward to build. Thus, many can be used in an experiment, favoring the diversity of treatments and the number of replicates. They also are lightweight with easy transport and use in lakes that cannot be reached by road. The enclosures are fundamentally aimed at investigating the short-term response of the planktonic community, integrated across the photic zone, to pulse perturbations using before and after comparisons and multiple replication and treatments. The pros and cons of the enclosure design are evaluated based on experience gained in Lake Redon, a high mountain ultraoligotrophic deep lake in the Pyrenees