Micro-players for macro-roles: aquatic microbes in deep lakes

Despite their relative low volume on a global scale, lakes represent a valuable reserve of accessible freshwaters. Any increase in our understanding of their functioning is a step towards their better exploitation and preservation. Microorganisms drive the metabolic processes that regulate the biogeochemical cycles in lakes; here, I will focus on Archaea and Cyanobacteria as micro-players playing macro-roles in lake ecosystems. In particular, I will present two examples to demonstrate the importance of prokaryotes in lakes: picocyanobacteria and mesophilic archaea. The role of prokaryotes in lakes will be discussed in relation to their taxonomy, dynamics and ecology using these two examples. The challenge, now, is to better understand the relationship between the diversity and the physiology of various ecotypes, how these interact with each other and with environmental factors allowing the proliferation of the most competitive genotypes

Single cells and microcolonies of freshwater picocyanobacteria: a common ecology

Picocyanobacteria (Pcy) single-cells and microcolonies are common in lakes throughout the world, and abundant across a wide spectrum of trophic conditions. The single-celled Pcy populations tend to be predominant in large, deep oligo-mesotrophic lakes, while the microcolonies find optimal conditions in warmer, shallower and more nutrient rich lakes. Microcolonies of different size (from 5 to 50 cells) constitute a gradient without a net separation from single-celled types. Considering microcolonies as transitional forms from single-cells to colonial morphotypes it is conceivable to propose a common ecology where local communities are not isolated but linked by dispersal of multiple, potentially interactive, species. In this review abiotic forcing and biotic regulation of Pcy community structure and dynamics are examined to offer an updated view of Pcy ecology

Daphnia and ciliates: who is the prey?

The fortuitous observation of a Daphnia infested by Gymnostomatida led us to hypothesize that egg predation by ciliates could be an important mechanism regulating cladoceran population dynamics. In light of this hypothesis we reanalysed data on the dynamics of Daphnia eggs and of one Gymnostomatida species from Lago Maggiore (1996). This communication is mainly a plea to examine data where both cladocerans and Gymnostomatida were collected, to verify if the occurrence of this yet undescribed eggvoricity is widespread. Further research is needed to investigate the ecological significance of this phenomenon

Il Carbonio organico nel Lago Maggiore: tendenza evolutiva, origine e caratteristiche qualitative

Not availableRicerche sull\u27evoluzione del Lago Maggiore. Aspetti limnologici. Programma triennale 2013-2015. Campagna 2013. Il Carbonio organico nel Lago Maggiore: tendenza evolutiva, origine e caratteristiche qualitative. TOC (Total Organic Carbon). TEP (Transparent Exopolymeric Particels). CDOM (Chromophoric Dissolved Organic Matter

Susceptibility of bacterioplankton to nutrient enrichment of oligotrophic and ultraoligotrophic lake waters

We carried out laboratory experiments in one ultraoligotrophic pristine Andean lake (Lake Guti?rrez, Argentina) and in one subalpine lake that is now at the edge of the oligo- to mesotrophic condition (Lake Maggiore, Italy). Lake water was amended with phosphorus (+P), organic carbon (+C), alone or in combination (+CP), to test for short-term changes (48 hours) in bacteria activity and community structure (CARD FISH). Experiments were carried out in spring and summer. Results showed that bacterial production increased in the +CP treatment in both lakes, and in the +P treatment in the ultroligotrophic lake. In both lakes the bacterial activity increased more rapidly in summer (within 24 hours). Bacteria composition changed in both seasons in all the treatments. At the beginning of the experiments the subclass of ?-Proteobacteria dominated both lakes, while ?-Proteobacteria showed higher percentage in spring in Lake Maggiore and in summer in Lake Guti?rrez. After incubation, in spring and in particular in the +CP treatment, we observed an increase in the relative importance of ?-Proteobacteria in both lakes, whereas in Lake Maggiore this group declined in the summer experiments following an increase in ?-Proteobacteria. All our results indicate the different response of bacterioplankton in systems at the edges of the oligotrophic range

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

Does a barcoding gap exist in Prokaryotes? Evidences from species delimitation in Cyanobacteria

The amount of information that is available on 16S rRNA sequences for prokaryotes thanks to high-throughput sequencing could allow a better understanding of diversity. Nevertheless, the application of predetermined threshold in genetic distances to identify units of diversity (Operative Taxonomic Units, OTUs) may provide biased results. Here we tests for the existence of a barcoding gap in several groups of Cyanobacteria, defining units of diversity according to clear differences between within-species and among-species genetic distances in 16S rRNA. The application of a tool developed for animal DNA taxonomy, the Automatic Barcode Gap Detector (ABGD), revealed that a barcoding gap could actually be found in almost half of the datasets that we tested. The identification of units of diversity through this method provided results that were not compatible with those obtained with the identification of OTUs with threshold of similarity in genetic distances of 97% or 99%. The main message of our results is a call for caution in the estimate of diversity from 16S sequences only, given that different subjective choices in the method to delimit units could provide different results

Dinamica stagionale dei batteri e studio della loro distribuzione orizzontale

Not availableDinamica stagionale dei batteri e studio della loro distribuzione orizzontal

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

Single culture and co-culture of two Synechococcus phylotypes respond differently to nanoflagellate grazing

Cyanobacteria belonging to the genus Synechococcus are found in lake waters typically as planktonic single cells and monospecific microcolonies. In oligotrophic lakes, single cells dominate in spring, while microcolonies are mostly found in late summer-autumn when the large colonial cyanobacteria increase in number. Since grazing activity is known as one of the major factors inducing microbial phenotypical changes, the formation of Synechococcus microcolonies was proposed as an efficient defence strategy against sizeselective predators. To better understand this ecological interaction, we explored the effect of grazing by the mixotrophic nanoflagellate Poteriochromonas sp. on the aggregation of two freshwater Synechococcus strains belonging to different phylogenetic clades (phycoerythrin-rich cells, PE, Group A; phycocyanin-rich cells, PC, Group I). During four days of incubation, we followed the dynamics of single-cells, microcolonies, and flagellates in semicontinuous cultures under different treatments (single culture and co-culture, with and without predators) by flow cytometry, epifluorescence microscopy and PhytoPAM. In single culture with the addition of Poteriochromonas, we observed the formation of grazing-induced monoclonal PE microcolonies, conversely limited in PC. In co-culture, there was an interaction between PE and PC, with an active microcolony formation by both PE and PC, and an increase of PC photosynthetic fitness (Fv/Fm). In co-culture, the microenvironment, generated by the formation of PE microcolonies, PC cells, bacteria and Poteriochromonas, can be the site of a beneficial "communication signalling" among Synechococcus cells for attaining the best spatial distribution for the fitness of the group