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.The abundance, activity, and community composition of epilimnetic and hypolimnetic prokaryotes have been less thoroughly investigated in deep lakes than in oceans. Strong evidence that the depth gradient plays a role in modulating the balance between the domains of Bacteria and Archaea has been found in various marine systems (8,12,13,20). It has been shown that the percentage of Bacteria in the deep marine hypolimnion decreases (up to 5,000 m) while, conversely, the percentage of Archaea increases. The percentage of Crenarchaeota is also higher in the mesopelagic zone than in surface waters (10).Although Archaea have been found in a variety of freshwater habitats (3), little has thus far been published on differentiating between Bacteria, Archaea, and Crenarchaeota in the hypolimnion of deep lakes. An exception is a study of the high-altitude ultraoligotrophic Crater Lake (21, 22), where group I marine Crenarchaeota were observed in deep-water populations (22). This study and another study of various lakes from three continents (9) are based on summer sampling, making it impossible to ascertain the effects of temporal variability on the vertical distribution of Archaea and Crenarchaeota, as has been done for marine systems and shallow lakes (for examples, see references 8 and 11).Our primary objective was to follow variations in the relative abundance of Bacteria, Archaea, and Crenarchaeota found in the hypolimnetic waters of a deep holo-oligomictic lake with a permanent oxic hypolimnion and compare them with those in the epilimnetic assemblages. We used the catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH) technique and compared the data thus obtained with prokaryotic productivity.Environmental characteristics and samplings. Lake Maggiore is a large, deep, subalpine lake (surface, 212 km 2 ; maximum depth, 372 m) in Northern Italy included in the LTER network (http://www.ise.cnr.it/lter). The lake has recovered from an eutrophic state (15) and is now oligomesotrophic (2, 14). A particular hydrodynamic feature of Lake Maggiore is that the full winter overturn occurs only at the end of a particularly cold and windy period (1).Temperature profiles (IDRONAUT OS316 multiparameter probe) indicated that stratification began in June (Fig. 1). The oxygen profiles showed that the water column was in a uniform oxic condition. Samples were taken in February, June, August, and October 2007 at 3 m, 10 m, 200 m, and 350 m, immediately fixed, and stored at Ϫ4°C (1 month).Prokaryote community composition. CARD-FISH analyses for Archaea, Bacteria, and Crenarchaeota were performed in two replicates according to Pernthaler et al. (17) and Teira e...

We analysed the long-term dynamics 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.

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.

We investigated the community structure of freshwater picocyanobacteria (Pcy)

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. I N T RO D U C T I O N Picocyanobacteria (Pcy) play a fundamental role in oligotrophic water systems, contributing up to 80% of the total primary production (Bell and Kalff, 2001; Richardson and Jackson, 2007; Callieri, 2008). Pcy are the dominant picophytoplankters in ultraoligotrophic lakes, and are mainly represented by the genus Synechococcus (Stockner et al., 2000; Callieri et al., 2007b). The study of their genetic diversity has received much less attention than marine Pcy, and has been restricted to cultured strains (