[Show abstract][Hide abstract] ABSTRACT: We investigated the community structure of freshwater picocyanobacteria (Pcy) in the oligotrophic subalpine Lake Maggiore (North-ern 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 (Z m), which passed the Monte Carlo test (p<0.05). A correlation between axis 1 and temperature, pH and Z m of respectively 0.85, 0.64 and 0.61 was found. Our results indicate that lake thermal structure, defined by temperature and Z m , has a significant influence on the succession of Pcy OTUs and on their dynamics in colonizing different ecological niches in the vertical gradient.
[Show abstract][Hide abstract] ABSTRACT: Through laboratory experiments, we tested whether UV radiation (UVR) induces filamentation in natural bacteria assemblages from North Andean Patagonian lakes. We incubated water from three different lakes for 72 h in four separate treatments: (1) UVR + PAR (photosynthetically active radiation), (2) 50% UVR + PAR, (3) PAR and (4) 50% PAR. The irradiance levels used in the experiments were equivalent to those registered at the epilimnion of the lakes. In the UVR treatments filamentation was induced after the first 24 h and the proportion continued to increase for the next 48-72 h. A comparison of the gross composition and diversity of the entire community (cells >0.2 microm) with bacterial filaments alone (>5.0 microm) showed that UVR-induced filamentation is not a feature of any particular cluster. By sequencing part of the 16S rRNA gene of the taxonomic units obtained using denaturing gels, we observed that strains in the beta-Proteobacteria group were of relatively high importance in filament formation, followed by Cytophaga-Flavobacterium-Bacteroides, gamma-Proteobacteria and alpha-Proteobacteria, whereas Actinobacteria were almost nonexistent in the filaments. We propose that UVR doses equivalent to those of Andean lakes produce bacterial morphological changes, and that all bacterial groups except Actinobacteria can potentially form filaments.
Full-text · Article · Jul 2010 · Photochemistry and Photobiology
[Show abstract][Hide abstract] ABSTRACT: We 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.
[Show abstract][Hide abstract] ABSTRACT: Picocyanobacteria (Pcy) diversity was studied in a set of ultraoligotrophic North Patagonian Andean lakes. The glacial lake
system includes a central lake (Lake Nahuel Huapi, with its side basin, Lake Moreno) and four satellite lakes (Lakes Espejo,
Correntoso, Gutiérrez and Mascardi) derived from a larger paleolake. Automated ribosomal intergenic spacer analysis showed
a Pcy community structure composed of 18 operational taxonomic units (OTUs), of which only one (OTU 1094) was widely distributed,
being found at all depths in all of the lakes. Most of the others were observed in a few of the lakes. The principal component
analysis revealed the habitat specificity of some Pcy OTUs: the satellite lakes have a highly irradiated epilimnion, while
the central lake has a well-mixed euphotic zone that does not extend beyond the epilimnion. We also observed a distinctive
vertical distribution of the OTUs and a significant correlation between Pcy chlorophyll-specific primary production and OTU
738, which was only found in the satellite lakes. Lastly, the high ß-diversity of this lake district supports the hypothesis
that microdiversity is higher in glacier-derived lake systems where habitat fragmentation due to geographic barriers results
in rapid speciation.
Full-text · Article · Mar 2010 · Journal of Plankton Research
[Show abstract][Hide abstract] ABSTRACT: 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.
[Show abstract][Hide abstract] ABSTRACT: We investigated the community composition and the relative size-shape distribution of bacterial assemblages in deep transparent lakes, where bacteria may be affected by harmful ultraviolet radiation (UVR). Summer samples of the euphotic zones of nine ultraoligotrophic lakes located in different drainage basins between 40u279 and 42u499S in the North Andean Patagonia were analyzed for relative diversity by denaturating gradient gel electrophoresis and the bacterial morphology (free-living cocci and rods vs. filamentous forms) in relation to environmental factors (UVR, photosynthetically active radiation, temperature, nutrients, nanoflagellates, and ciliate abundances). The overall bacterial community composition was similar in all lakes and over depth in each lake. In contrast, the relative proportion of filaments to total bacterial biovolume was higher in the upper layers, which have higher UVR intensities (305-340 nm). Lakes with lower Kd305 (diffuse extinction coefficient, 305 nm) showed a greater proportion of filaments. Filament mean length in the upper layers was also significantly greater than at deeper levels. There was a significant correlation between UVR and filament proportion, whereas neither nanopredator abundances (nanoflagellates and ciliates), nor predation pressure, nor resources (dissolved nutrients) could explain the high proportion of filamentation in the near-surface layers. We propose that UVR plays a decisive role stimulating bacterial filamentation, particularly in lakes with high UVR penetration.
Full-text · Article · Jul 2009 · Limnology and Oceanography
[Show abstract][Hide abstract] ABSTRACT: 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 µm filtrate); small-sized predators (10P, <10µm filtrate) and small-to medium-sized predators present (50P, <50 µm 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 characterized 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.
[Show abstract][Hide abstract] ABSTRACT: During the year 2002, the size variability of most of the species found in Lago Maggiore was analysed in detail, measuring
through image analysis the main morphological parameters (maximum linear dimension, surface, volume) of the algal cells. Many
individuals belonging to the same species were measured sampling by sampling, collecting about 28,000 data.
This data set allowed us to evaluate the morphological plasticity of many species across the seasonal succession: through
multivariate statistical analysis we compared the changes of cell volume, cell surface, maximum linear dimension and surface-to-volume
ratio to the fluctuations of the main physical and chemical parameters.
The responses we observed were variable, depending on the different taxonomic groups or species as well as on the morphometric
parameter considered. As a general pattern, a strong seasonality of the size changes was observed, mainly dependent on the
gradients of nutrients and temperature.
[Show abstract][Hide abstract] ABSTRACT: Summary1. We applied Reynolds's approach to the study of phytoplankton ecology through functional associations of species to identify possible algal species, which associate with the freshwater cyanobacterium Synechococcus. Previously an association among Synechococcus spp. and small-celled chlorophytes (association Z) has been recognised by evaluating phytoplankton associations according to functional criteria.2. Biomass data for phytoplankton and picocyanobacteria from Lago Maggiore spanning more years were organised in a matrix and a cluster analysis was performed. The results showed four groups separated at a linkage distance of 0.20. Mixotrophic species which clustered with Synechococcus spp. were Ceratium hirundinella, Chrysochromulina parva, Cryptomonas erosa, Cryptomonas ovata, Dinobryon bavaricum, Dinobryon sociale, Rhodomonas minuta and Uroglena americana. The redundancy analysis (RDA) consolidated the association of Synechococcus with C. hirundinella and R. minuta showing greater probability of occurrence than random aggregates of species.3. The association Synechococcus spp. –C. hirundinella also appeared from temporal variation of their biomass. In early summer both these species increased at the same time; later, the peak of C. hirundinella (potential predator) coincided with a minimum of Synechococcus spp. suggesting a possible predator-prey interaction. This implied that phytoplankton assemblages which form a functional group cannot only have similar adaptations and requirements but can also exhibit trophic interactions.4. We propose to enlarge the association Z and create an association ZMX (where MX stands for mixotrophs) which would include Synechococcus spp. and C. hirundinella as the most representative of the mixotrophic species found in the oligotrophic Lago Maggiore.
Full-text · Article · Dec 2005 · Freshwater Biology