Picocyanobacterial community structure and space-time dynamics in the subalpine Lake Maggiore (N. Italy)

We investigated the community structure of freshwater picocyanobacteria (Pcy) in the oligotrophic subalpine Lake Maggiore (Northern Italy) using the least conserved sequences of the Internal Transcribed Spacer (ITS-1) fragment. The aim was to characterize the succession of Pcy Operational Taxonomic Units (OTUs) during a seasonal cycle at two environmental niches with distinctive light, temperature and mixing characteristics. ITS fragment length outlined a succession of OTUs from spring to summer, even in the absence of a pronounced spring peak. The vertical partitioning of Pcy OTUs down the water column, more evident in summer, indicates different community composition at the two depths. Redundancy Data Analysis (RDA) was carried out using temperature, pH and depth of mixing layer (Zm), which passed the Monte Carlo test (p<0.05). A correlation between axis 1 and temperature, pH and Zm of respectively 0.85, 0.64 and 0.61 was found. Our results indicate that lake thermal structure, defined by temperature and Zm, has a significant influence on the succession of Pcy OTUs and on their dynamics in colonizing different ecological niches in the vertical gradient

Effects of predation pressure on bacterial abundance, diversity, and size-structure distribution in an oligotrophic system

We designed microcosm experiments to study the response of bacterial communities to altered predation pressure, in phosphorus-limited conditions. Different-sized predators were removed through filtration, yielding the following treatments: bacteria only (no predation, NP; <1 ?m filtrate); small-sized predators (10P, <10?m filtrate) and small- to medium-sized predators present (50P, <50 ?m filtrate). Natural control (NC) included predators of all sizes. Thus we compared the relative impact of differential predation on abundance, biovolume, community composition and size-structure distribution of bacterial assemblages subject to grazing by different-sized predators. The relative diversity of microbial communities was estimated by a fingerprinting based approach for both prey and predators. The results showed that the presence of grazers preserved the Shannon diversity of the bacterial community and shifted the size-structure distribution towards grazing- resistant forms. Absence of predation promoted competition for resources and resulted in a constant reduction of the relative diversity of the bacterial community. The change in the size-structure distribution of the bacterial communities in the treatments was accompanied by alterations in the relative operational taxonomic unit (OTU) composition of the eukaryotic and bacterial communities. Bacterial OTUs grouped in two distinct fractions linked to their size-structure distribution, in dependence to the presence of the predators: Small and Edible cells were favoured by low grazing pressure whereas Filaments and Aggregates were stimulated by predator presence. Eukaryotic OTUs successful at high grazing activity resulted as rather different than OTUs successful at lower degree of grazing. Under high protistan grazing pressure, there was a clear shift in bacterial community composition regarding both size-structure distribution and genotypes. Nevertheless, diversity was preserved. The opposite situation haracterized the predator-free bacterial communities; a clear and constant reduction of the community diversity was indicated, confirming that moderate top-down control is fundamental to the shaping and preservation of natural bacterial communities, even in oligotrophic systems

Photosynthetic characteristics and diversity of freshwater Synechococcus at two depths during different mixing conditions in a deep oligotrophic lake

We studied the photosynthetic characteristics and genetic diversity of Synechococcus assemblages at two different light and mixing conditions in a deep oligotrophic lake (Lake Maggiore, Northern Italy). Sampling was performed at the beginning of summer stratification, when the base of the photic zone was isolated by the presence of a shallower thermocline, and during late summer, when the mixing layer extended below the photic zone. Two depths were sampled (15 % and 1 % of surface PAR). To study the diversity of Synechococcus, we used a classical molecular fingerprinting technique (Denaturing Gradient Gel Electrophoresis, DGGE) with ambient samples and sequencing of the prominent bands. The ecotypes selected within the samples from different depths were different in photosynthetic characteristics and grew in a range of nutrient concentrations. The OTUs colonizing different niches in the water column did not show any significant clustering. However, the OTU richness was significantly different at the two depths in spring, reaching the highest values at 15 % of surface PAR. Cluster analysis of DGGE lanes provided evidence of different community compositions between spring and late summer. Sequencing of the most prominent bands showed one spring OTU affiliated to the Synechococcus subalpine cluster I, with 100% similarity to LM94, MW76B2 and MW15#2 (sensu Crosbie et al. 2003a), not present in late summer. Another OTU present both in spring and late summer showed 100% similarity to MH301, a strain near to subalpine cluster II. Our results support the coexistence of ecotypes in the vertical gradients, rapidly acclimating and performing differently in the microhabitats

Biodiversity and eco-physiological characteristics of freshwater picocyanobacteria

La biodiversità dei picocianobatteri d’acqua dolce (0.2-2 µm) è stata studiata utilizzando approcci molecolare basati sulle Fingerprinting Techniques (FT). Le FT utilizzate in questo studio (DGGE, T-RFLP, ARISA) sono state confrontate progettando accanto allo studio della comunità naturale di picocianobatteri del Lago Maggiore un esperimento su una comunità artificiale semplificata. Dai dati di entrambe le comunità si nota una maggiore risoluzione dell’ITS-1 rispetto al 16S rDNA, ma tra le tecniche utilizzate non se ne osserva generalamente una più sensibile di un’altra. T-RFLP e ARISA, sono state successivamente utilizzate per l’analisi ecologica della comunità picocianobatterica del Lago Maggiore. La composizione dei campioni ha permesso di valutare come ARISA (ITS-1) abbia una maggiore risoluzione rispetto alla T-RFLP (16S rDNA) nella descrizione della dinamica spazio-temporale della biocenosi. La successione di ceppi di picocianobatteri osservabile durante l’evoluzione stagionale della comunità è sia spaziale che temporale. Dall’analisi multivariata (RDA) la temperatura è risultata la principale variabile ambientale (P<0.001) nel determinare i cambiamenti osservati nella comunità picocianobatterica. Infine, ARISA è stata utilizzata per lo studio della diversità picocianobatterica di un distretto di laghi andini con caratteristiche limnologiche molto peculiari. Accanto a studi di diversità di questi laghi sono state misurate le caratteristiche fotosintetiche delle comunità picocianobatteriche. Non si è osservata una differenza genotipica tra i popolamenti del gruppo di laghi, alle due profondità, ma si osserva una risposta simile tra le diverse composizioni delle comunità con i tratti fotosintetici misurati. E’ stata trovata una correlazione positiva tra una OTU, presente soltanto in un cluster (formato da quattro laghi) e la produzione specifica per unità di clorofilla a. Questo dato indica le effettive potenzialità dell’analisi ITS-1 di ottenere informazioni ecologiche di supporto alle informazione sulla diversità ed è un indizio del ritrovamento di particolari adattamenti eco-fisiologici che le OTU possono sviluppare in determinate condizioni ambientali

Dinamica stagionale e distribuzione orizzontale di fitoplancton, carbonio organico e batterioplancton. Dinamica stagionale dei batteri e studio della loro distribuzione orizzontale

No abstract availabl

Dinamica stagionale e distribuzione orizzontale di fitoplancton, carbonio organico e batterioplancton. Dinamica stagionale dei batteri e studio della loro distribuzione orizzontale

No abstract availabl

Bacteria, Archaea, and Crenarchaeota in the Epilimnion and Hypolimnion of a Deep Holo-Oligomictic Lake ▿

In a deep, subalpine holo-oligomictic lake, the relative abundance of Archaea and Crenarchaeota, but not that of Bacteria, increases significantly with depth and varies seasonally. Cell-specific prokaryotic productivity is homogeneous along the water column. The concept of active Archaea observed in the deep ocean can therefore be extended to a deep oxic lake

Long-term trends of epilimnetic bacteria and organic carbon in a deep holo-oligomictic lake

International audienceWe analysed the long-term dynamics (1980―2007) of hypolimnetic and epilimnetic bacterial abundances and organic carbon concentrations, both dissolved (DOC) and particulate (POC), in the deep holo-oligomictic Lake Maggiore, included in the Southern Alpine Lakes Long-Term Ecological Research (LTER) site. During the 28 years of investigation, bacterial abundance and POC concentrations did not decrease with declining phosphorus concentrations, while DOC concentrations showed a pronounced decrease in the epi- and hypolimnion. We used the annual mean total lake heat content and total annual precipitation as climate-related variables, and in-lake total phosphorus as a proxy for trophic state. The model (forward stepwise regression, FSR) showed that reduced anthropogenic pressure was more significant than climate change in driving the trend in DOC concentrations. Bacterial dynamics in the hypolimnion mirrored the fluctuations observed in the epilimnion, but average cell abundance was three times lower. The FSR model indicates that bacterial number variability was dependent on POC in the epilimnion and DOC in the hypolimnion. In the hypolimnion, cell biovolumes for rod and coccal morphotypes were significantly larger than in the epilimnion

Long-term trends of epilimnetic and hypolimnetic bacteria and organic carbon in a deep holo-oligomictic lake.

International audienc

Indagini sull\u27ambiente pelagico. Carbonio organico e popolamenti batterici eterotrofi

No abstract availableIndagini sull\u27ambiente pelagico. Carbonio organico e popolamenti batterici eterotrof