Deep, cold, and dark hypolimnia represent the largest volume of water in freshwater lakes with limited occurrences of phototrophs. However, the presence of prokaryotes supports populations of bacterivorous ciliates and heterotrophic nanoflagellates (HNF). Nevertheless, protistan bacterivory rates and the major hypolimnetic ciliate bacterivores are poorly documented. We conducted a high frequency sampling (three-times a week) in the oxic hypolimnion of a stratified mesoeutrophic reservoir during summer, characterized by stable physicochemical conditions and low water temperature. Using fluorescently labeled bacteria we estimated that ciliates and HNF contributed, on average, 30% and 70% to aggregated protistan bacterivory, respectively, and collectively removed about two thirds of daily hypolimnetic prokaryotic production. The ciliate community was analyzed by the quantitative protargol staining method. One scuticociliate morphotype dominated the hypolimnetic ciliate community, accounting for 82% of total ciliates and over 98% of total ciliate bacterivory, with average cell-specific uptake rate of 202 prokaryotes per hour. Moreover, long-amplicon sequencing revealed that the scuticociliate belongs to an unidentified clade closely related to the Ctedoctematidae and Eurystomatellidae families. The high-resolution sampling, microscopic, and sequencing methods allowed uncovering indigenous microbial food webs in the hypolimnetic environment and revealed a functional simplification of ciliate communities, dominated by a new bacterivorous scuticociliate lineage.

• Klíčová slova
• bacterivorous protists, cold hypolimnetic layer, freshwater reservoir, new lineage of scuticociliates, protistan bacterivory rates,
• MeSH
• Bacteria * klasifikace   izolace a purifikace   genetika   MeSH
• Ciliophora * klasifikace   genetika   MeSH
• fylogeneze MeSH
• jezera mikrobiologie   MeSH
• Oligohymenophorea * klasifikace   genetika   izolace a purifikace   MeSH
• sekvenční analýza DNA MeSH
• sladká voda * mikrobiologie   parazitologie   MeSH
• Publikační typ
• časopisecké články MeSH

Freshwater systems are characterized by an enormous diversity of eukaryotic protists and prokaryotic taxa. The community structures in different lakes are thereby influenced by factors such as habitat size, lake chemistry, biotic interactions, and seasonality. In our study, we used high throughput 454 sequencing to study the diversity and temporal changes of prokaryotic and eukaryotic planktonic communities in three Austrian lakes during the ice-free season. In the following year, one lake was sampled again with a reduced set of sampling dates to observe reoccurring patterns. Cluster analyses (based on SSU V9 (eukaryotic) and V4 (prokaryotic) OTU composition) grouped samples according to their origin followed by separation into seasonal clusters, indicating that each lake has a unique signature based on OTU composition. These results suggest a strong habitat-specificity of microbial communities and in particular of community patterns at the OTU level. A comparison of the prokaryotic and eukaryotic datasets via co-inertia analysis (CIA) showed a consistent clustering of prokaryotic and eukaryotic samples, probably reacting to the same environmental forces (e.g., pH, conductivity). In addition, the shifts in eukaryotic and bacterioplanktonic communities generally occurred at the same time and on the same scale. Regression analyses revealed a linear relationship between an increase in Bray-Curtis dissimilarities and elapsed time. Our study shows a pronounced coupling between bacteria and eukaryotes in seasonal samplings of the three analyzed lakes. However, our temporal resolution (biweekly sampling) and data on abiotic factors were insufficient to determine if this was caused by direct biotic interactions or by reacting to the same seasonally changing environmental forces.

• Klíčová slova
• Seasonality, diversity, eukaryotic plankton, freshwater, prokaryotic plankton, protist,
• Publikační typ
• časopisecké články MeSH

While mechanisms of different carbon dioxide (CO2 ) assimilation pathways in chemolithoautotrohic prokaryotes are well understood for many isolates under laboratory conditions, the ecological significance of diverse CO2 fixation strategies in the environment is mostly unexplored. Six stratified freshwater lakes were chosen to study the distribution and diversity of the Calvin-Benson-Bassham (CBB) cycle, the reductive tricarboxylic acid (rTCA) cycle, and the recently discovered archaeal 3-hydroxypropionate/4-hydroxybutyrate (HP/HB) pathway. Eleven primer sets were used to amplify and sequence genes coding for selected key enzymes in the three pathways. Whereas the CBB pathway with different forms of RubisCO (IA, IC and II) was ubiquitous and related to diverse bacterial taxa, encompassing a wide range of potential physiologies, the rTCA cycle in Epsilonproteobacteria and Chloribi was exclusively detected in anoxic water layers. Nitrifiying Nitrosospira and Thaumarchaeota, using the rTCA and HP/HB cycle respectively, are important residents in the aphotic and (micro-)oxic zone of deep lakes. Both taxa were of minor importance in surface waters and in smaller lakes characterized by an anoxic hypolimnion. Overall, this study provides a first insight on how different CO2 fixation strategies and chemical gradients in lakes are associated to the distribution of chemoautotrophic prokaryotes with different functional traits.

• MeSH
• Archaea metabolismus   MeSH
• chemoautotrofní růst fyziologie   MeSH
• Chlorobi genetika   metabolismus   MeSH
• citrátový cyklus fyziologie   MeSH
• Epsilonproteobacteria genetika   metabolismus   MeSH
• fotosyntéza fyziologie   MeSH
• hydroxybutyráty metabolismus   MeSH
• jezera chemie   mikrobiologie   MeSH
• koloběh uhlíku fyziologie   MeSH
• kyselina mléčná analogy a deriváty   metabolismus   MeSH
• oxid uhličitý metabolismus   MeSH
• ribulosa-1,5-bisfosfát-karboxylasa genetika   metabolismus   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• 4-hydroxybutyric acid MeSH Prohlížeč
• hydracrylic acid MeSH Prohlížeč
• hydroxybutyráty MeSH
• kyselina mléčná MeSH
• oxid uhličitý MeSH
• ribulosa-1,5-bisfosfát-karboxylasa MeSH