Phytoplankton succession affects the composition of Polynucleobacter subtypes in humic lakes

Effects of Phycosphere Bacteria on Their Algal Host Are Host Species-Specific and Not Phylogenetically Conserved

Phytoplankton is fundamental to life on Earth. Their productivity is influenced by the microbial communities residing in the phycosphere surrounding algal cells. Expanding our knowledge on how algal-bacterial interactions affect algal growth to more hosts and bacteria can help elucidate general principles of algal-host interactions. Here, we isolated 368 bacterial strains from phycosphere communities, right after phycosphere recruitment from pond water and after a month of lab cultivation and examined their impacts on growth of five green algal species. We isolated both abundant and rare phycosphere members, representing 18.4% of the source communities. Positive and neutral effects predominated over negative effects on host growth. The proportion of each effect type and whether the day of isolation mattered varied by host species. Bacteria affected algal carrying capacity more than growth rate, suggesting that nutrient remineralization and toxic byproduct metabolism may be a dominant mechanism. Across-host algal fitness assays indicated host-specific growth effects of our isolates. We observed no phylogenetic conservation of the effect on host growth among bacterial isolates. Even isolates with the same ASV had divergent effects on host growth. Our results emphasize highly specific host-bacterial interactions in the phycosphere and raise questions as to which mechanisms mediate these interactions.

Metabolic Functional Community Diversity of Associated Bacteria during the Degradation of Phytoplankton from a Drinking Water Reservoir

In the drinking water reservoir ecosystem, phytoplankton and bacteria play important roles in shaping freshwater health and function. In this work, the associated bacterial community functional diversity during degradation of phytoplankton was determined using the substrate utilization profiling (BIOLOG) technique, meanwhile, the composition and concentration of phytoplankton were examined using a microscope. The results indicated that Euglena decreased 58.33% from 0 to 38 d, while the smallest degradation of Bacillariophyta was 20.19%. Average well color development (AWCD_590nm) increased during the static periods from 0 to 38 d; however, the AWCD_590nm of 18 and 38 d had no significant difference (p < 0.05). The Simpson’s index (D) was in accordance with Shannon’s diversity (H) and species richness(S); it was measured to be18 > 38 > 5 > 0 d. There were significant differences in the pattern and level of carbon sources used by the phytoplankton-associated bacteria. In addition, the principle component analyses (PCA) suggested that the first principle component (PC1) and the second principle component (PC2) explained 46.76% and 21.49% of the total variation for bacterial community, respectively. Redundancy analysis (RDA) revealed that cell abundance of phytoplankton was negatively correlated with the AWCD_590nm, amino acids and other functional indexes. Therefore, the data suggest that there are differences in the phytoplankton-associated bacterial community functional diversity during different static stages of water samples collected from the drinking water reservoir.

Synchrony of Eukaryotic and Prokaryotic Planktonic Communities in Three Seasonally Sampled Austrian Lakes

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.

Fungal community dynamics during a marine dinoflagellate (Noctiluca scintillans) bloom

Contamination and eutrophication have caused serious ecological events (such as algal bloom) in coastal area. During this ecological process, microbial community structure is critical for algal bloom succession. The diversity and composition of bacteria and archaea communities in algal blooms have been widely investigated; however, those of fungi are poorly understood. To fill this gap, we used pyrosequencing and correlation approaches to assess fungal patterns and associations during a dinoflagellate (Noctiluca scintillans) bloom. Phylum level fungal types were predominated by Ascomycota, Chytridiomycota, Mucoromycotina, and Basidiomycota. At the genus level drastic changes were observed with Hysteropatella, Malassezia and Saitoella dominating during the initial bloom stage, while Malassezia was most abundant (>50%) during onset and peak-bloom stages. Saitoella and Lipomyces gradually became more abundant and, in the decline stage, contributed almost 70% of sequences. In the terminal stage of the bloom, Rozella increased rapidly to a maximum of 50-60%. Fungal population structure was significantly influenced by temperature and substrate (N and P) availability (P < 0.05). Inter-specific network analyses demonstrated that Rozella and Saitoella fungi strongly impacted the ecological trajectory of N. scintillans. The functional prediction show that symbiotrophic fungi was dominated in the onset stage; saprotroph type was the primary member present during the exponential growth period; whereas pathogentroph type fungi enriched in decline phase. Overall, fungal communities and functions correlated significantly with N. scintillans processes, suggesting that they may regulate dinoflagellate bloom fates. Our results will facilitate deeper understanding of the ecological importance of marine fungi and their roles in algal bloom formation and collapse.

Competition and niche separation of pelagic bacteria in freshwater habitats

Freshwater bacterioplankton assemblages are composed of sympatric populations that can be delineated, for example, by ribosomal RNA gene relatedness and that differ in key ecophysiological properties. They may be free-living or attached, specialized for particular concentrations or subsets of substrates, or invest a variable amount of their resources in defence traits against protistan predators and viruses. Some may be motile and tactic whereas others are not, with far-reaching implications for their respective life styles and niche partitioning. The co-occurrence of competitors with overlapping growth requirements has profound consequences for the stability of community functions; it can to some extent be explained by habitat factors such as the microscale complexity and spatiotemporal variability of the lacustrine environments. On the other hand, the composition and diversity of freshwater microbial assemblages also reflects non-equilibrium states, dispersal and the stochasticity of community assembly processes. This review synoptically discusses the competition and niche separation of heterotrophic bacterial populations (defined at various levels of phylogenetic resolution) in the pelagic zone of inland surface waters from a variety of angles, focusing on habitat heterogeneity and the resulting biogeographic distribution patterns, the ecophysiological adaptations to the substrate field and the interactions of prokaryotes with predators and viruses.