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RuBisCO (ribulose 1,5-bisphosphate carboxylase/oxygenase) is one the most abundant enzymes on Earth. Virtually all food webs depend on its activity to supply fixed carbon. In aerobic environments, RuBisCO struggles to distinguish efficiently between CO2 and O2. To compensate, organisms have evolved convergent solutions to concentrate CO2 around the active site. The genetic engineering of such inorganic carbon concentrating mechanisms (CCMs) into plants could help facilitate future global food security for humankind. In bacteria, the carboxysome represents one such CCM component, of which two independent forms exist: α and β. Cyanobacteria are important players in the planet’s carbon cycle and the vast majority of the phylum possess a β-carboxysome, including most cyanobacteria used as laboratory models. The exceptions are the exclusively marine Prochlorococcus and Synechococcus that numerically dominate open ocean systems. However, the reason why marine systems favor an α-form is currently unknown. Here, we report the genomes of 58 cyanobacteria, closely related to marine Synechococcus that were isolated from freshwater lakes across the globe. We find all these isolates possess α-carboxysomes accompanied by a form 1A RuBisCO. Moreover, we demonstrate α-cyanobacteria dominate freshwater lakes worldwide. Hence, the paradigm of a separation in carboxysome type across the salinity divide does not hold true, and instead the α-form dominates all aquatic systems. We thus question the relevance of β-cyanobacteria as models for aquatic systems at large and pose a hypothesis for the reason for the success of the α-form in nature.

Aims To characterize the fermentation process and bacterial diversity of sorghum silage inoculated with Lactiplantibacillus plantarum LpAv, Pediococcus pentosaceus PpM and Lacticaseibacillus paracasei LcAv. Methods and Results Chopped sorghum was ensiled using the selected strains. Physicochemical parameters (Ammonia Nitrogen/Total Nitrogen, Dry Matter, Crude Protein, Acid Detergent Fibre, Neutral Detergent Fibre, Acid Detergent Lignin, Ether Extract and Ashes), bacterial counts, cell cytometry and 16sRNA sequencing were performed to characterize the ensiling process and an animal trial (BALB/c mice) was conducted in order to preliminary explore the potential of sorghum silage to promote animal gut health. After 30 days of ensiling, the genus Lactobacillus comprised 68.4 ± 2.3% and 73.5 ± 1.8% of relative abundance, in control and inoculated silages respectively. Richness (Chao1 index) in inoculated samples, but not in control silages, diminished along ensiling, suggesting the domination of fermentation by the inoculated LAB. A trend in conferring enhanced protection against Salmonella infection was observed in the mouse model used to explore the potential to promote gut health of sorghum silage. Conclusions The LAB strains used in this study were able to dominate sorghum fermentation. Significance and Impact of the Study This is the first report using metaprofiling of 16sRNA to characterize sorghum silage, showing a microbiological insight where resident and inoculated LAB strains overwhelmed the epiphytic microbiota, inhibiting potential pathogens of the genus Klebsiella.

The Salado River basin, Buenos Aires, Argentina, contains numerous permanent shallow lakes under anthropic pressure due to different land uses/land covers (LULC) in their catchments. Here, we explored the relation of LULC in the surrounding areas and the physical and chemical variables of six shallow lakes of the aforementioned basin with phytoplankton structure and morpho-functional groups (MFGs) along one year and evaluated whether LULC or physical and chemical variables were more relevant in shaping phytoplankton structure. We hypothesized that: (1) shallow lakes with higher proportion of land committed to agriculture exhibit more eutrophic characteristics; (2) the phytoplankton structure and MFGs differ in shallow lakes with different LULC in their surrounding areas; and (3) LULC effects decrease with increase in buffer width. Landsat satellite images were used to extract LULC composition in three buffer zones surrounding each lake: 1, 2.5 and 5 km. In all the lakes studied, LULC had greater influence on phytoplankton MFGs than physical and chemical variables. This influence was highest for the 2.5 km buffer. The phytoplankton structure and some physical and chemical characteristics were also different between the shallow lakes studied. Cyanobacteria were widely detected, probably related to the high proportion of land dedicated to agriculture in the region. Chlorococcales, typical of these eutrophic systems, were also well represented. Our study evidenced the influence of LULC on the shallow lakes studied, particularly on the phytoplankton structure. The response of the planktonic microalgal communities to human activities in catchments evidences their value as sentinels of aquatic ecosystems.

Protists play a fundamental role in all ecosystems, but we are still far from estimating the total diversity of many lineages, in particular in highly diverse environments, such as freshwater. Here, we survey the protist diversity of the Parana River using metabarcoding, and we applied an approach that includes sequence similarity and phylogeny to evaluate the degree of genetic novelty of the protists' communities against the sequences described in the reference database PR2 . We observed that ~28% of the amplicon sequence variants were classified as novel according to their similarity with sequences from the reference database; most of them were related to heterotrophic groups traditionally overlooked in freshwater systems. This lack of knowledge extended to those groups within the green algae (Archaeplastida) that are well documented such as Mamiellophyceae, and also to the less studied Pedinophyceae, for which we found sequences representing novel deep-branching clusters. Among the groups with potential novel protists, Bicosoecida (Stramenopiles) were the best represented, followed by Codosiga (Opisthokonta), and the Perkinsea (Alveolata). This illustrates the lack of knowledge on freshwater planktonic protists and also the need for isolation and/or cultivation of new organisms to better understand their role in ecosystem functioning.

In aquatic systems, an interplay between bottom-up and top-down processes determines the dynamic of picocyanobacteria (Pcy) abundance and community structure. Here, we analyzed a 10-year time series (sampled fortnightly) from a hypereutrophic turbid shallow lake located within the Pampa Region of South America, generating the first long-term record of freshwater Pcy from the Southern Hemisphere. We used a cytometric approach to study Pcy community, and focused on its relations with nutrient and light conditions (bottom-up) and potential grazers (top-down). A novel Pcy abundance seasonality with winter maximums was observed for years with relatively stable hydrological levels, related with decreased abundance of seasonal rotifers during colder seasons. Pcy showed lower abundance and higher cytometric alpha diversity during summer, probably due to a strong predation exerted by rotifers. In turn, a direct effect of the non-seasonal small cladocerans Bosmina spp. decreased Pcy abundance and induced a shift from single-cell Pcy into aggregated forms. This structuring effect of Bosmina spp. was further confirmed by Pcy cytometric (dis)similarity analyses from the time series and in situ experimental data. Remarkably, Pcy showed acclimatization to underwater light variations, resembling the relevance of light in this turbid system.

Obesity and overweight, and their concomitant metabolic diseases, emerge as one of the most severe health problems in the world. Prevention and management of obesity are proposed to begin early in childhood, when probiotics may have a role. The Simulator of the Human Intestinal Microbial Ecosystem (SHIME®), in a dynamic validated in vitro system able to simulate the different parts of the gastrointestinal tract, has proven to be useful in analyzing the human intestinal microbial community. L. plantarum 73a and B. animalis subsp. lactis INL1, two strains isolated from breast milk, were assayed in the SHIME® using the fecal microbiota of an obese child. L. plantarum 73a alone or in combination with B. animalis subsp. lactis INL1 demonstrated survival capacity in the SHIME® system. The administration of both strains increased the alpha diversity of the microbiota and reduced the levels of the phylum Proteobacteria. In particular, the genera Escherichia, Shigella, and Clostridium_sensu_stricto_1 were significantly reduced when both strains were administered. The increase of Proteobacteria phylum is generally associated with the microbiota of obese people. Escherichia and Shigella can be involved in inflammation-dependent adiposity and insulin resistance. L. plantarum 73a supplementation reduced ammonia production. L. plantarum 73a alone or in combination with B. animalis subsp. lactis INL1 are potential probiotic candidates for the management of infant obesity. Fil: Oddi, Sofia Lorena. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Lactología Industrial. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Instituto de Lactología Industrial; Argentina Fil: Huber, Maria Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Lactología Industrial. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Instituto de Lactología Industrial; Argentina Fil: Duque, Ana Luiza Rocha Faria. Universidade Estadual Paulista Julio de Mesquita Filho; Brasil Fil: Vinderola, Celso Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Lactología Industrial. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Instituto de Lactología Industrial; Argentina Fil: Sivieri, Katia. Universidade Estadual Paulista Julio de Mesquita Filho; Brasil

Modern agricultural practices heavily rely on the use of agrochemicals. Glyphosate based herbicides are among the most widespread agricultural supplies. The massive application of glyphosate in conventional farming, provides continuous inputs of the herbicide into the environment, unintentionally placing non-target organisms at risk. Shallow lakes immersed in agroecosystems serve as integrators of human interventions in their watersheds. Autotrophic plankton, which are key components of biogeochemical cycles and food-webs, respond quickly to anthropogenic disturbances. Here, we assessed the environmental implications and ecological consequences of agricultural land use and glyphosate contamination on phytoplankton from 52 shallow lakes scattered over 180,000 km2 across the central pampas in Argentina, which lay on areas differentially impacted by glyphosate. Indicators of glyphosate usage positively correlated with lake turbidity and Cyanobacteria abundance, and were negatively associated with Chlorophyceae diversity. Glyphosate impact indicators were also associated with higher abundance of the small-sized representatives within each major algal class, which are assumed to be better adapted to poor light conditions. Detection of the presence of GLY-transporter genes (phnD) in seston samples was associated with higher picocyanobacteria biomass and lower pico-eukaryotic algae. Our results illustrate that the observed distribution patterns of phytoplankton can be explained by the combination of the watershed’s agricultural profile, conductivity, and light quality.

How diversity is structured has been a central goal of microbial ecology. In freshwater ecosystems, selection has been found to be the main driver shaping bacterial communities. However, its relative importance compared with other processes (dispersal, drift, diversification) may depend on spatial heterogeneity and the dispersal rates within a metacommunity. Still, a decrease in the role of selection is expected with increasing dispersal homogenization. Here, we investigate the main ecological processes modulating bacterial assembly in contrasting scenarios of environmental heterogeneity. We carried out a spatiotemporal survey in the floodplain system of the Parana River. The bacterioplankton metacommunity was studied using both statistical inferences based on phylogenetic and taxa turnover as well as co-occurrence networks. We found that selection was the main process determining community assembly even at both extremes of environmental heterogeneity and homogeneity, challenging the general view that the strength of selection is weakened due to dispersal homogenization. The ecological processes acting on the community also determined the connectedness of bacterial networks associations. Heterogeneous selection promoted more interconnected networks increasing β-diversity. Finally, spatiotemporal heterogeneity was an important factor determining the number and identity of the most highly connected taxa in the system. Integrating all these empirical evidences, we propose a new conceptual model that elucidates how the environmental heterogeneity determines the action of the ecological processes shaping the bacterial metacommunity.

The aim of this work was to assess the temporal patterns in the community composition of phytoplankton (PCC) and bacterioplankton (BCC) in two interconnected and hypertrophic Pampean shallow lakes in Argentina. Factors shaping their community dynamics and community temporal covariations were also analysed. We performed 4 years of seasonal samplings (2012–2016) and communities were studied by the Utermöhl approach (PCC) and Illumina MiSeq sequencing (BCC). We found marked seasonal variations in both communities and inter-annual variations with decreasing microbial community similarities during the study. We also observed covariation in community-level dynamics among PCC and BCC within and between shallow lakes. The within-lake covariations remained positive and significant, while controlling for the effects of intrinsic (environmental) and extrinsic (temporal and meteorological) factors, suggesting a community coupling mediated by intrinsic biotic interactions. Algal-bacterial associations between different taxa of phytoplankton and bacterioplankton within each lake were also found. PCC was mainly explained by pure regional extrinsic (17–21%) and intrinsic environmental (8–9%) factors, while BCC was explained by environmental (8–10%) and biotic interactions with phytoplankton (7–8%). Our results reveal that the influence of extrinsic regional factors can be channeled to bacterioplankton through both environmental (i.e. water temperature) and phytoplankton effects.

Picocyanobacteria (cells

Ceratium furcoides is an invasive freshwater dinoflagellate that in the last three decades has expanded its geographic distribution in South America, being recently found in Paraná River floodplain (Argentina). Despite growing concern about the presence and impacts of this invader, information regarding genetic diversity in the Southern Hemisphere is missing. This work constitutes the first phylogenetic characterization of Ceratium populations of South America, particularly, from the Paraná system. After taxonomic identification as C. furcoides based on morphological traits, two sequencing-based approaches were applied using the ribosomal 18S gene: Sanger sequencing to isolated individuals and high-throughput amplicon sequencing (HTS) to environmental DNA. The sequence of C. furcoides obtained shared 100% identity to Asian sequences, and formed a highly supported clade in the constructed reference phylogenetic tree. HTS helped to recover low-frequency genetic variants suggesting the presence of different population of C. furcoides, and to alert potential invasion in its early stages. Fil: Accattatis, Victoria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentina Fil: Piccini, Claudia. Instituto de Investigaciones Biológicas "Clemente Estable"; Uruguay Fil: Huber, Maria Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentina Fil: Metz, Sebastián Darío. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús). Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús); Argentina Fil: Rueda, Eva Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional del Litoral. Facultad de Humanidades y Ciencias; Argentina Fil: Devercelli, Melina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; Argentina

Determining which processes take place in the spatial distribution of bacterioplankton metacommunities has been a central goal of microbial ecology. In freshwater ecosystems, selection has been spotted as the main driver shaping bacterial community. However, its relative importance compared with others processes (dispersal, drift, diversification) may depend on spatial heterogeneity and dispersal rates within a metacommunity. Still, a decrease in the role of selection is expected with increasing dispersal homogenization. Here, we investigate the main ecological processes modulating the bacterial assembly in contrasting scenarios of environmental heterogeneity. We carried out a spatiotemporal survey in the floodplain system of Paraná River. The bacterioplankton metacommunity was studied by a step-forward analysis combining different statistical inferences based on the phylogenetic and taxa turnover as well as co-occurrence networks. We found that selection was the main process even at both extremes of environmental heterogeneity and homogeneity, challenging the general view that the strength of selection is weakened due to dispersal homogenization. The ecological processes acting on the community also determined the complexity and randomness of bacterial networks. The heterogeneous selection promoted greater network complexity increasing the β-diversity, and bacterial associations were more random with the increasing influence of stochasticity. Finally, the spatiotemporal heterogeneity was an important factor determining the number and identity of keystone taxa in the system. Integrating all these empirical evidences we propose a new conceptual model that elucidates how the environmental heterogeneity guides the action of ecological processes shaping the bacterial metacommunity.

The ecological status of Pampean shallow lakes is evidenced by Cyanobacteria Harmful Blooms impairing these nutrient enriched, turbid and polymictic water bodies spread along the Central Plains of Argentina. Under the premise that shallow lakes are sentinels of global climate and eutrophication, a 3-year research in ten lakes located across a climatic gradient explored which factors drove the dynamics of cyanobacterial assemblages frequently driving to bloom prevalence. Contrarily to what is expected, the effect of seasonal temperature on cyanobacteria was subordinated to both the light environment of the water column, which was on turn highly affected by water level conditions, and to nutrient concentrations. Monthly samplings evidenced that cyanobacterial assemblages presented a broad-scale temporal dynamics mostly reflecting inter-annual growth patterns driven by water level fluctuations. Both species composition and biovolume gradually changed across a gradient of resources and conditions and hence, the scenario in each individual lake was unique with patterns at different temporal and spatial scales. More than 35 filamentous and colonial morphospecies constituted the assemblages of Pampean lakes: nostocaleans and chroococcaleans were inversely correlated in the prevailing interannual 3-cycled patterns.

Fil: Huber, Maria Paula. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - La Plata. Instituto de Investigaciones Biotecnologicas. Instituto de Investigaciones Biotecnologicas "Dr. Raul Alfonsin" (sede Chascomus). Universidad Nacional de San Martin. Instituto de Investigaciones Biotecnologicas. Instituto de Investigaciones Biotecnologicas "Dr. Raul Alfonsin" (sede Chascomus); Argentina

17 pages, 5 figures, 3 tables, supplemental material https://doi.org/10.1128/AEM.02659-18.-- Data availability.All sequences generated in this study can be accessed through European Nucleotide Archive (ENA) under the study accession number PRJEB27291 and accession numbers ERR2639355 to ERR2639364, High-throughput sequencing (HTS) of the 16S rRNA gene has been used successfully to describe the structure and dynamics of microbial communities. Picocyanobacteria are important members of bacterioplankton communities, and, so far, they have predominantly been targeted using universal bacterial primers, providing a limited resolution of the picocyanobacterial community structure and dynamics. To increase such resolution, the study of a particular target group is best approached with the use of specific primers. Here, we aimed to design and evaluate specific primers for aquatic picocyanobacterial genera to be used with high-throughput sequencing. Since the various regions of the 16S rRNA gene have different degrees of conservation in different bacterial groups, we therefore first determined which hypervariable region of the 16S rRNA gene provides the highest taxonomic and phylogenetic resolution for the genera Synechococcus, Prochlorococcus, and Cyanobium. An in silico analysis showed that the V5, V6, and V7 hypervariable regions appear to be the most informative for this group. We then designed primers flanking these hypervariable regions and tested them in natural marine and freshwater communities. We successfully detected that most (97%) of the obtained reads could be assigned to picocyanobacterial genera. We defined operational taxonomic units as exact sequence variants (zero-radius operational taxonomic units [zOTUs]), which allowed us to detect higher genetic diversity and infer ecologically relevant information about picocyanobacterial community composition and dynamics in different aquatic systems. Our results open the door to future studies investigating picocyanobacterial diversity in aquatic systems, This study was supported by the Argentinean Network for the Assessment and Monitoring of Pampean Shallow Lakes (PAMPA2–CONICET), the ANPCyT (PICT-2014-1290 and PICT-2016-1079), and by MAGGY (grant CTM2017-87736-R) to S. G. Acinas and REMEI (grant CTM2015-70340-R) to J. M. Gasol from the Spanish Ministry of Economy and Competitiveness (MINECO)

We analyzed the interplay between neutral and deterministic processes in maintaining contrasting alternative bacterioplankton communities through time in highly productive shallow lakes and evaluated the relevance of these processes when a regime shift from a clear to a turbid state occurred. We observed that local assembly is ruled primary deterministically, via local habitat filtering, with a secondary role of stochastic processes. We also found a hierarchy in the environmental sorting: while an unusual Verrucomicrobia dominance characterizes the three systems, local conditions limit within-bacterial community membership to closely phylogenetically related and ecologically similar taxa. These results indicate that bacterial abilities to establish in these lakes are strongly determined by their traits, and point toward special physiological adaptations to persist when these systems undergo a regime shift. Altogether, these results hint to a divergence in function among these alternative communities, mediated by major shifts in bacterial community trait structure, particularly regarding carbon use. Fil: Llames, Maria Eugenia del Rosario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús). Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús); Argentina Fil: Huber, Maria Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús). Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús); Argentina Fil: Metz, Sebastián Darío. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús). Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús); Argentina Fil: Unrein, Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús). Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. Instituto de Investigaciones Biotecnológicas "Dr. Raúl Alfonsín" (sede Chascomús); Argentina

Dominant primary producer in macrophyte- or phytoplankton-dominated shallow lakes might imply differences in dissolved organic carbon (DOC) composition. We compared chromophoric dissolved organic matter (CDOM), plankton respiration (R), and bacterial (BP) and primary production (PP), in two contrasting shallow lakes. We hypothesized that DOC from the macrophyte-dominated lake would be qualitatively inferior, so that it can support a lower yield than DOC from the phytoplankton-dominated one. Macrophyte-dominated lake had more humic and aromatic CDOM, though molecular weight was similar in both lakes. A clear synchronism between lakes was observed in mean depth and several CDOM absorption coefficients, suggesting an external driver of the variation in DOC concentration and CDOM quality. The positive BP-PP and BP-Chl-a correlations in the macrophyte-dominated lake point out to a dependence of bacteria on phytoplankton for a supply of labile DOC. In turn, BP in the phytoplankton-dominated lake was balanced with grazing by HF (heterotrophic flagellates). The significantly higher HB:DOC and HF:DOC carbon ratios in the phytoplankton-dominated lake also suggest that better DOC quality would mean relatively more efficient C transfer to higher trophic levels. According to PP:BP and PP:R ratios both lakes should be considered autotrophic, although the macrophyte-dominated lake would be comparatively more heterotrophic. Fil: Torremorell, Ana María. Universidad Nacional de Luján. Departamento de Ciencias Básicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Pérez, Gonzalo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Patagonia Norte. Instituto de Investigación en Biodiversidad y Medioambiente; Argentina. Universidad Nacional del Comahue; Argentina Fil: Lagomarsino, Leonardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Investigaciones Biotecnológicas - Instituto Tecnológico Chascomús. Instituto de Investigaciones Biotecnológicas (sede Chascomús); Argentina. Universidad Nacional de San Martín; Argentina Fil: Huber, Maria Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Investigaciones Biotecnológicas - Instituto Tecnológico Chascomús. Instituto de Investigaciones Biotecnológicas (sede Chascomús); Argentina. Universidad Nacional de San Martín; Argentina Fil: Queimaliños, Claudia Patricia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Patagonia Norte. Instituto de Investigación en Biodiversidad y Medioambiente; Argentina. Universidad Nacional del Comahue; Argentina Fil: Bustingorry, Jose Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Investigaciones Biotecnológicas - Instituto Tecnológico Chascomús. Instituto de Investigaciones Biotecnológicas (sede Chascomús); Argentina. Universidad Nacional de San Martín; Argentina Fil: Fermani, Paulina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Investigaciones Biotecnológicas - Instituto Tecnológico Chascomús. Instituto de Investigaciones Biotecnológicas (sede Chascomús); Argentina. Universidad Nacional de San Martín; Argentina Fil: Llames, Maria Eugenia del Rosario. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Investigaciones Biotecnológicas - Instituto Tecnológico Chascomús. Instituto de Investigaciones Biotecnológicas (sede Chascomús); Argentina. Universidad Nacional de San Martín; Argentina Fil: Unrein, Fernando. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Investigaciones Biotecnológicas - Instituto Tecnológico Chascomús. Instituto de Investigaciones Biotecnológicas (sede Chascomús); Argentina. Universidad Nacional de San Martín; Argentina

Picocyanobacteria can occur as single-cell (Pcy) or as colonies (CPcy). Published evidence suggests that some Pcy strains have the capability to aggregate under certain culture conditions, however this has not been demonstrated to occur in natural environments. We investigated whether the Pcy and CPcy belong to the same species (i.e. phylotype), and the factors that determine their morphological and genetic variability in a hypertrophic shallow lake dominated by picocyanobacteria. Six main different morphologies and30 phylotypes were observed. All sequences retrieved belonged to the 'Anathece + Cyanobium' clade (Synechococcales) that are known to have the capability of aggregation/disaggregation. The temporal variation of picocyanobacteria morphotype composition was weakly correlated with the DGGE temporal pattern, and could be explained by the composition of the zooplankton assemblage. Laboratory experiments confirmed that the small cladoceran Bosmina favoured the dominance of CPcy, i.e. Cyanodictyon doubled the size of the colonies when present, most likely through the aggregation of single-cell picocyanobacteria into colonies. Flow cytometry cell sorting and 16S rRNA + ITS sequencing of the Pcy and CPcy cytometrically-defined populations revealed that some phylotypes could be found in both sorted populations, suggesting phenotypic plasticity in which various Synechococcales phylotypes could be found in situ either as single-cells or as colonies.

Background Cyanobacteria are the major prokaryotic primary producers occupying a range of aquatic habitats worldwide that differ in levels of salinity, making them a group of interest to study one of the major unresolved conundrums in aquatic microbiology which is what distinguishes a marine microbe from a freshwater one? We address this question using ecogenomics of a group of picocyanobacteria (cluster 5) that have recently evolved to inhabit geographically disparate salinity niches. Our analysis is made possible by the sequencing of 58 new genomes from freshwater representatives of this group that are presented here, representing a 6-fold increase in the available genomic data. Results Overall, freshwater strains had larger genomes (≈2.9 Mb) and %GC content (≈64%) compared to brackish (2.69 Mb and 64%) and marine (2.5 Mb and 58.5%) isolates. Genomic novelties/differences across the salinity divide highlighted acidic proteomes and specific salt adaptation pathways in marine isolates (e.g., osmolytes/compatible solutes - glycine betaine/ggp/gpg/gmg clusters and glycerolipids glpK/glpA), while freshwater strains possessed distinct ion/potassium channels, permeases (aquaporin Z), fatty acid desaturases, and more neutral/basic proteomes. Sulfur, nitrogen, phosphorus, carbon (photosynthesis), or stress tolerance metabolism while showing distinct genomic footprints between habitats, e.g., different types of transporters, did not obviously translate into major functionality differences between environments. Brackish microbes show a mixture of marine (salt adaptation pathways) and freshwater features, highlighting their transitional nature. Conclusions The plethora of freshwater isolates provided here, in terms of trophic status preference and genetic diversity, exemplifies their ability to colonize ecologically diverse waters across the globe. Moreover, a trend towards larger and more flexible/adaptive genomes in freshwater picocyanobacteria may hint at a wider number of ecological niches in this environment compared to the relatively homogeneous marine system.